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REDOX REACTIONS

CHEMISTRY 105 THINK NEET | THINK IIB

REDOX REACTIONS


BASIC UNDERSTANDING OF REDOX REACTIONS

“Redox” is a combination of the words “reduction”and “oxidation”

The word oxidation originally implied reaction withoxygen to form an oxide, as O2(g) was the firstrecognized oxidising agent

The word reduction originally referred to the lossin weight upon heating a metal oxide in metallurgy,the loss of weight was due to loss of oxygen gas.

Later these definitions were modified to includesimilar reactions which do not involve oxygen.

CLASSICAL CONCEPT OF REDOXREACTIONS

(i) On the basis of Oxygen : Addition of oxygen is oxidation while removal of

oxygen is reduction

(a) 2Ca + O2 2CaO

(Addition of Oxygen to Ca means oxidation of Ca)

(b) 2Ag2O 4Ag + O2

(Removal of Oxygen from Zn means reduction of Zn)(ii) On the basis of Hydrogen : Addition of hydrogen is reduction while removal

of hydrogen is oxidation.

(a) H2 + Cl2 2HCl(Addition of hydrogen to Chlorine means reductionof chlorine)

(b) H2S + Cl2 2HCl + S(Removal of Hydrogen from Sulfur meansoxidation of Sulfur)

(iii) On the basis of Electropositive element(EP)(similar to hydrogen)

Addition of an electropositive element is reductionwhile removal of electropositive element isoxidation.

(a) 2K +Cl2 2KCl(Addition of potassium(EP atom) to chlorine meansreduction of chlorine)

(b) 2KI +H2O2 2KOH + I2

(Removal of potassium(EP atom) from Iodinemeans oxidation of Iodine)

(iv) On the basis of Electronegative (EN) element Addition of EN element is oxidation while removal

of EN element is reduction.

(a) 2K + Cl2 2KCl

(Addition of chlorine(EN atom) to potassium meansoxidation of potassium)

(b) PbCl4 PbCl2 + Cl2

(Removal of chlorine(EN atom) from lead meansreduction of lead)

MODERN CONCEPT OF REDOXREACTIONS

(i) On the basis of Electron transfer Process which involves the loss of e– in an

element or species is called oxidation/de-electronation while which involves the gain ofelectrons by an element or species is calledreduction/electronation.

(a) Na Na+ + e–

(Loss of electron from Sodium meansoxidation of Sodium)

(b) Cl2 + 2e– 2Cl–

(Gain of electron by chlorine means reduction ofchlorine)

2Na(s) + Cl2(g) 2Na+Cl-(s)

loss of 2e-

gain of 2e-

2Na(s) + O2(g) (Na+)2 O2-(s)

loss of 2e-

gain of 2e-

2Na(s) + S(s) (Na+)2 S2-(s)

loss of 2e-

gain of 2e-

(ii) On the basis of change in oxidation number

Increase in oxidation no. of an element in a reactionis called oxidation while decrease in oxidation no.of an element in a reaction is called reduction.

OXIDATION NUMBER (O.N.) Oxidation number is the charge which an atom

has in its ion or appears to have when present inthe combined states with other atoms

In case of ionic compound, charge on ions(cations/

REDOX REACTIONS


anions) will be their oxidation number

e.g. As NaCl is composed of Na+ & Cl–,+1 willbe the oxidation no. of Na while -1 will be theoxidation no. of Cl

In case of covalent compounds we will assumenegative charge on more electronegative atom &consequently negative oxidation no & assumepositive charge on less electronegative atom sopositive oxidation no on it.e.g. In HCl , Cl is more electronegative so we willgive -1 to Cl & + 1 to less electronegative H.

–1 oxid. no. should be given to more EN atom foreach bond pair while +1 to less EN atom for eachbond pair.

(a) 2

(oxidation no = 0) (oxidation no = +2)2 Fe Fe e

There is increase in oxidation no of Fe(from 0 to+2) means oxidation of Fe

(b) 2 (oxidation no = -1)(oxidation no = 0)2 2 Cl e Cl

There is decrease in oxidation no of Cl (from 0 to–1) means redution of Cl.

RULES FOR CALCULATING O.N. The O.N. of the element in the free or elementary

state or in an alloy is always zero

O.N. of Ag, He, Na = 0

O.N. of Cl in Cl2 , S in S8 = 0

O.N.of Na & Hg in Na-Hg amalgam (alloy) = 0

Summation of the oxidation number of all the atomsin neutral molecule is zero & in case of ion isequal to charge.

In SO42– = (O.N. of S) + (O.N. of O) × 4 = –2

In H2SO4= (O.N.of H) × 2 + (O.N. of S) + (O.N. of O) × 4= 0

Oxidation number of all alkali metals is +1, ofalkaline earth metals is +2 while of Al is +3 inbonded state

O.N. of Na in NaCl is +1

O.N. of Mg in MgCl2 is +2

O.N. of Al in AlCl3 is +3

Hydrogen : General oxidation no = +1 except inmetal hydride it is assigned -1

In H2O O.N. of H = +1

while in NaH or CaH2 or AlH3 it has -1

Oxygen : General oxid. No = – 2

Exception

(i) With F it can show +1(O2F2) or +2(OF2)

(ii) In peroxide it shows -1 like in H2O2 or Na2O2

(iii) In superoxide it shows -1/2 like in KO2 or CsO2

(iv) In Ozonide it shows -1/3 like in KO3

Halogens: General O.N. = –1

F always shows -1 in bonded state

Exception - Cl, Br ,I can show +1 to +7 if they arebonded with more EN atom

Range of O.N. : Maximum oxidation no ofelement can not exceed no of valence electron &minimum O.N. = No. of valence e– – 8 (exceptmetals)

For S Max O.N. = +6 (6 valence e-)

& min. O.N.= 6–8 = –2.

CALCULATION OF O.N. USINGSTRUCTURE

For each covalent bond more EN atom is assigned–1 & less EN atom is assigned +1 oxidation no.

If EN of atoms are same oxid. no. 0 should begiven.

H - Cl SO O

+1 -1+3+3

-2 -2

SO2HCl

FRACTIONAL O. N. It means element is present in at least 2 different

oxidation number.

(a) C3O2 (Carbon suboxide)

2 2 0 2 2 02

43

O . N . o f one CO C C C O O . N .o f tw o C

A vg . O .N .o f C

(b) Br3O8 (Tribromooctaoxide)

O = Br - Br - Br = O

O O O

O O O

-2 -2 -2

-2 -2 -2

-2 -2

+6 +6+4

. . 4

. . 616. . .3

O N of one BrO N of two Br

Avg O N of Br

(c) Na2S4O6 (Sodium tetrathionate)

+Na-O - S - S - S - S - O-Na+

O O

O O

-2 -2

-2 -2

-2 -2+6 +60 0

. . 0

. . 510. . .4

O N of one SO N of two S

Avg O N of S

REDOX REACTIONS


Answer Key

01. Reduction

02. Oxidation

03. Reduction

04. 2e–, Oxidation

05. 3e–, Reduction















20. Reduction

21. Oxidation

22. Oxidation

23. Oxidation

24. Reduction

25. Oxidation

26. Oxidation

27. Reduction

28. Reduction

Practice Problems

Fill in the blanks with required no. of e– and identifywhether the reaction is oxidation or redution.

01. Cu +2 + 2eCu

02. ZnZn+2 + 2e–

03. Fe+3 + e– Fe+2

04. Ca Ca+2 +......

05. N + .....N3–

06. P + ..... P–2

07. S S+6 + ....

08. S+4 + ..... S–2

09. O2+... + 4H+2H2O

10. H2SS + ...

11. N2+ ... 2N3–

12. NO2 NO 3 + .....

13. Cu+1 Cu+2 + .....

14. K2Cr2O7 + ..... 2Cr+3

15. KMnO4+ .... Mn+2

16. KMnO4 + .... Mn+6

17. Cr2O3 Cr2O7–2 + .....

18. I2 -4IO + .....

19. Cl2 -3ClO + ....

20. 2MnO2 + O2 2Mn2O3

21. H2SO3 H2SO4

22. HNO2 HNO3

23. Na2S2O3 Na2S4O6

24. HClO3 HClO2

25. FeO + O2 Fe2O3

26. SnCl2 + Cl2 SnCl4

27. CrO4–2 CrCl3

28. PbO2 PbO

REDOX REACTIONS


Fractional oxidation no. can be used to identifymixed oxides like Fe3O4, Pb3O4, Mn3O4

(a) 3 4 2 3

( . . 2) ( . . 3)

( )Oxid no of Fe Oxid no of Fe

Fe O FeO Fe O

(b)3 4 2

( . . 2) ( . . 4)

(2 )Oxid no of Pb Oxid no of Pb

Pb O PbO PbO

(c) 3 4 2

( . . 2) ( . . 4)

(2 )Oxid no of Mn Oxid no of Mn

Mn O MnO MnO

CALCULATION OF O. N. OF METALS INCOORDINATION COMPOUNDS

Charges on neutral molecules like NH3, H2O, CO,C2H4 should be considered 0.

Same rules as above should be appliedIIB Example :

3 3 1

2 6 3 4 23 4

1 2Cr(H O) Cl Co(NH ) (H O)Cl SO,

(This concept will be discussed in detail incoordination compound” chapter)

STOCK NOTATIONS The method of representing oxidation no.of a metal

by roman numerals (I, II, III, etc.) is known asstock notation.IIB Example :

Aurous chloride Au(I)Cl

(Au in +1 oxid.no.)

Auric chloride Au(III)Cl3

(Au in +3 oxid.no)

Mercurous chloride Hg2(I)Cl2

(Hg in +1 oxid.no.)

Mercuric chloride Hg(II)Cl2

(Hg in +2 oxid.no.)

IIB Knowledge Enhancer :(i) H2SO5 (Peroxymonosulfuric acid/Caro’s Acid) (ii) H2S2O8 (Peroxydisulfuric Acid/ Marshall’s

Acid)

2

1 26

11 1

2

O||

H O S O O H||O

. . 6

. . 2 1

. . 3 2. 1

O N of SO N of oxygenO N of oxygenNo of peroxidelinkage

2 2

1 2 6 0 0 6 2 1

2 2

|| ||

|| ||

O O

H O S O O S O H

O O

. . 1

. . 2

. . 6. 1

O N of twooxygenO N of sixoxygenO N of Sno of peroxidelinkage

(iii) CrO5(Chromium (VI) oxide peroxide ) (iv) H2S2O3 (Thiosulfuric acid)

O

CrO

O

O

O

–1 –1

–1 –1+6

–2 . . 1. . 2. . 6. 2

O N of four oxygenO N of one oxygenO N of C rno of peroxide linkage

H–O SS–2

O–2

O–H+6+1 –2 –2 +1

26

23

O.N. of one SO.N. of one SSp hybridised S is more ENthan sp hybridised S.

(v) H4P2O8 (Peroxydiphosphoric Acid) (vi) HFO2

H – O – P – O – O – P – O – HO O

H+1 H+1

O O+1 –2 +1–2 –1 –1

–2 –2

–2 –2

+5 +5

125

O.N . of two OO.N . of six OO.N . of two P

1 1 1 1H O O F

. . 1

. . 1

O N of one OO N of one O

(vii) CaOCl2 (Bleaching powder) (viii) HCN (Hydrogen cyanide)

2+

12

Ca Cl

OCl

. . 1

. . 1

O N of one ClO N of one Cl

1 2 3. . 3. . 2. . 1

O N of NH C N O N of C

O N of H

REDOX REACTIONS


PracticeProblems Find out Oxidation number of the underlinedelement:01. 2H O 02. 2O03. 3O 04. 3SO

05. 24SO 06. CO

07. 2CO 08. CHClO09. 2HClO 10. 4 6P O

11. 3NaHSO 12. 4FeSO

13. PbS 14. FeO15. 2MnO 16. 2 7Mn O

17. Cr2(SO4)3 18. K2CrO4

19. 2 2 7K Cr O 20. 4KMnO

21. 2 4K MnO 22. 3 2(CH COO) Pb

23. 22 4H C O 24. 3HNO

25. 2N O 26. NO27. 2 5N O 28. 3 4H PO

29. 3N H 30. 22 3Na S O

31. 2SrO 32. 3 4 2Ca (PO )

33. 23Mg N 34. 23Ca P

35. 2 3Cr O 36. 2H S

37. 3 2Mg(NO ) 38. 2 2S Cl

39. 6SF 40. 6XeF

41. 3XeO 42. 2XeOF

43. 2 5V O 44. NaHS45. 22

Hg Cl 46. 4CuSO

47. 4NH OH 48. 4 6K [Fe(CN) ]

49. 2 6 3[Cr(HO ) ]Cl 50. 3 6 3[Co(NH ) ]Br

51. 2 4 4[Cu(H O) ]SO 52. 4 6Na [Pt(Cl) ]

53. 2 6Na [CrF ] 54. 2 4Na [Zn(CN) ]

55. 4Ca[Ni(Cl) ] 56. 2 3Cl O

57. 2 7Cl O 58. 23CO

59. 3HCO 60. 23SO

61. ClO 62. 3ClO

63. 4HSO 64. 24HPO

65. 22 4C O 66. 4PH

67. 22 3S O 68. 4BF

Answer Key

01. – 2 02. 0

03. 0 04. + 6

05. + 6 06. + 2

07. + 4 08. + 2

09. + 3 10. + 3

11. + 4 12. + 2

13. + 2 14. + 2

15. +4 16. + 7

17. + 3 18. + 6

19. + 6 20. + 7

21. + 6 22. + 2

23. + 3 24. + 5

25. + 1 26. + 2

27. + 5 28. + 5

29. –1/3 30. + 2

31. –1 32. + 5

33. –3 34. – 3

35. + 3 36. – 2

37. + 5 38. + 1

39. + 6 40. + 6

41. + 6 42. + 4

43. + 5 44. – 2

45. + 1 46. + 2

47. – 3 48. + 2

49. + 3 50. + 3

51. + 2 52. + 2

53. + 4 54. + 2

55. +2 56. + 3

57. + 7 58. + 4

59. + 4 60. + 461. + 1 62. + 563. + 6 64. + 565. + 3 66. – 367. + 2 68. + 3

REDOX REACTIONS


TYPES OF REDOX REACTIONS Redox Reactions are those reactions in which

reduction and oxidation take place simultaneously.These reactions are classified as below :

(i) Combination reactions: At least two different species react to form a third

compound with change in oxidation no.A + B C

For a combination reaction to be a redox reactioneither A & B or both A & B must be in elementalform .

C + O2 CO2

CH4 + O2 CO2 + H2O(ii) Decomposition reactions: A compound breaks down to form more than one

product with change in oxidation number.A B + C

For a decomposition reaction to be a redox reactionout of B & C one must be in elemental form

2Ag2O 4Ag + O2

2KClO3 2KCl + 3O2

All decomposition reaction cannot be a redoxreaction

MgCO3 MgO + CO2

(iii) Displacement reactions: An atom or an ion in a compound is replaced by

another atom or ion.

A + BC AB + CTypes of displacement reactions:(i) Metal displacement reaction(ii) Non-metal displacement reaction

(i) Metal Displacement Reaction:- A metal in a compound is displaced by another

metal.

CuSO4(aq) + Zn (s) Cu(s) + ZnSO4 (aq)

Cr2O3 (s) + 2 Al (s) Al2O3 (s) + 2Cr(s) Metal displacement reactions are used in

metallurgical processes. The metal which can displace another metal can

be identified using electrochemical series.(ii) Non- Metal displacement reaction: A non-metal is displaced by a metal or non metal

from its compound.(a) Hydrogen displacement

M + H2O Hydroxide + H2

(M=Reactive metals like Alkali metals & Ca, Sr,Ba)

K(s) + 2H2O(l) KOH (aq) + H2(g)

OXIDISING & REDUCING AGENTS

IIB Knowledge Enhancer : Elements which are present in their intermediate oxidation no. can behave as both oxidising as

well as reducing agent. (H2O2, SO2, HNO2 etc.)

S. No. Oxidising Agent /Oxidant Reducing Agent/Reductant1. Species which oxidise other elements in a Species which reduce other elements in a reaction

reaction2. They are reduced in reaction They are oxidised in reaction3. They extract electrons from other elements They lose electrons4. Elements with high electronegative Elements with high electropositive nature

nature(F, O, N, Cl etc.) (Li, K, Zn)5. If elements are present in their highest If elements are present in their lowest oxidation

oxidation state, they can act as oxidixing state, they can act as reducing agent onlyagent only (KMnO4, K2Cr2O7, HClO4, (NH3,CH4 etc.)H2SO4, HNO3 etc.)

6. Compounds which can easily donate Compounds which can easily donate hydrogenoxygen(H2O2, Na2O2 etc.) (NaH, CaH2, LiAlH4, NaBH4)

7. All metallic oxides like Li2, Na2O, CaO All metallic hydrides like NaH, CaH2, LiAlH4 etc8. Some nonmetallic oxides like CO2, SO2 All hydrocaids like HF, HCl, HBr, H2S, etc.

REDOX REACTIONS


M + Steam Hydroxide/oxide + H2

(M=less reactive metals like Al/Mg)

Mg(s) + 2H2O(l) Mg(OH)2(s) + H2(g)

2Fe(s) + 3H2O(l) Fe2O3(s) + 3H2(g) M + Acid Salt + H2

Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g) IIB Knowledge Enhancer : Very less reactive metals(Ag ,Au ) neither react

with steam nor acid. Cd & Sn like metals react with acid not with

cold water. By observing rate of evolution of H2 gas

reactivity of metals can be predicted Layer Test : If we add Chlorine to solution

containing Bromide or Iodide ion & CCl4,chlorine displaces them .Bromine can beidentified by orange yellow colour while Iodinecan be identified by violet colour.(b) Halogen displacement :-

Oxidising power = F2 > Cl2 > Br2> I2

Higher member can displace ion of lower memberfrom its solution.

Cl2 (g) + 2KBr (aq) 2 KCl (aq) + Br2 (l)

Cl2 (g) + 2KI (aq) 2 KCl (aq) + I2 (s) F2 is the strongest oxidising agent ,it even displaces

O2 from water

2H2O (l) + 2F2 (g) 4HF(aq) + O2(g) F–cannot be oxidised to F2 by any element , it can

be done only electrolytically.(iv) Disproportionation reactions : An element in one oxidation state is simultaneously

oxidised & reduced.P4 + 3NaOH + 3H2O PH3 + 3NaH2PO2

S8 + 12NaOH 4Na2S + 2Na2S2O3 + 6H2O X2 with base (Cl, Br,I give similar reactions)-

2 2(Cold & dilute)

2NaOH + Cl NaCl + NaOCl + H O

2 3 2(Hot & conc.)6NaOH + 3Cl 5NaCl + NaClO 3H O

2F2(g) + 2OH– (aq)2F– (aq) + OF2(g) + H2O(l)(v) Comproportionation reactions: It is reverse of disproportionation. 2 2 4 4 2Pb(s) + PbO (s)+2H SO (aq) 2PbSO (s) + 2H O(l)

(vi) Intermolecular redox reactions: Elements participating in oxidation & reduction

both are present in different reactants.

Cl2+ H2S 2HCl + S

(vii) Intramolecular redox reactions: Elements participating in oxidation & reduction

both are present in same reactant.

NH4NO2 N2+2H2O

PracticeProblems

Find the type of Redox Reaction:-

1. 2 2 3Fe + O Fe O

2. 4 4 CuSO Fe FeSO Cu

3. 2 2 Br NaOH NaBr NaOBr H O

4. 3 2 4 4NO H S NH HSO

5. 3 2KClO KCl O

6. 2 2Ag Ag Ag

7. 2 2Na H O NaOH H

8. 4 2 3 212 4 2 24 3 XeF H O Xe XeO HF O

9. 22CuBr CuBr Cu

10. 2 2NaBr Cl NaCl Br

Answer Key

1. Combination

2. Metal displacement

3. Disproportionation

4. Intermolecular

5. Decomposition, Intramolecular

6. Comproportionation.

7. Non-metal displacement

8. Disproportionation.

9. Disproportionation

10. Non-metal displacement.

REDOX REACTIONS


BALANCING OF REDOX REACTIONS Two methods of Balancing Redox reaction (i) Oxidation no method (ii) Half reaction method

Oxidation No. Method (Acidic Medium)

We undustend this concept by taking following examples

Cr2O72– + C2O4

2 – Cr3+ + CO2

Oxidation No. Method (Basic medium) : Follow same step 1 to step no 5, then add OH– on both side equal to no of H+

OH 23 3 4Cr(OH) IO I CrO

6 3 3 42– 2– 3

2 27 4 22– 2– 3

2 7 2 4 2

. .

. .

1 & 2 2

.

2

3

.

.

.

Sr No STEPS REACTION

Calculate O N of elements Cr O C O Cr COBalance all atoms other than O H Cr O C O Cr CO

Make total increase in oxidNo total decrease in ox

2– 2– 32 7 2 4 2

2 2 6 3 6.

2 2( )

{ ( ) }(

)

4.

3 4 2 3

,

3 2 6

Crid No

CMultiply appropriate number if

needed same no should be placedCr O C O Cr CO

before formula of those elementsBalance oxygen by ad

2– 2– 32 7 2 4 2 2

2

2– 2– 32 7 2 4 25

3 2 6 7

7

14 3 2 6 7 .

dingCr O C O Cr CO H O

appropriate no of H O molecule onoxygens are deficient on product side

oxygen deficient sideH Cr O C O Cr CO

Balance hydrogen by adding H

2

14 14 2 2 3 6

3 2 0 0 6

( )

6.

H Ohydrogens are deficient on reactant side

To verify see if charges are Reactant sidesame on both side of equation Product side

– – 2–3 4 23

– – 2–3 4 23

*

2 2 4

5

4 2 2 4 4

7.

8.&

On following sameCr OH IO I CrO H O H

stepup to step noTo make basicmedium add OH

OH Cr OH IO I CrO H O H OHon both side equalto no of H

Convert H OH o – – 2–3 4 23

2

4 2 2 5

n

OH Cr OH IO I CrO H Osame side to H O

REDOX REACTIONS


Half Reaction/Ion Electron Method (Acidic Medium)

Cr2O72– + C2O4

2 – Cr3+ + CO2

2 32 7

22 4 2

2 32 7

22 4 2

1.( )

22.

& 2

.3.

Divide equation in twoCr O Cr

half reactionsC O COOxidation & reduction half

Cr O CrBalance all atom otherthanO H C O CO

Balance oxygenby adding appropriate no ofH

Sr.NO. STEPS REACTION

2 32 7 2

22 2 4 2

2 32 7 2

22 4

6

2 72

14 2 74.

2arg

6.5.

arg

Cr O Cr H OO molecule on C O CO

oxygen deficient sideBalance hydrogen by H Cr O Cr H Oadding H C O CO

Balance ch e by addingesufficient no of

electron on side ofhigher ch e

322 7 214 6 02

22 4 2 22 0

2 32 7 2

22 4 2

2 2 32 7 2 4 2 2

14 2 7

2 2

6 14 2 7.6. & { 2 2 } 3

14 3 2 6 7

H Cr O Cr H O

C O CO e

e Cr O Cr H OMake no of e Sameinboth reaction add C O CO eboth reaction

H Cr O C O Cr CO H O

Half Reaction method (Basic medium) : Follow same step 1 to step no 6, then add OH– on both side equal to no of H+

OH 23 3 4Cr(OH) IO I CrO

22 3 4

3 2

23 3 4 2

3 3

{ ( ) 5 3 } 2

6 6 3.6 2 ( ) 2 4

7. 4 2 ( )

.

OH

OH

OH

OH

H O Cr OH CrO H eOn followingsame stepup to e H IO I H Ostep no Cr OH IO CrO H O H

Tomakebasicmedium add OH

OH Cr OH IOonboth sideequaltono of H

*

24 2

23 3 4 2

2

2 4 4

&8. 4 2 ( ) 2 5OH

I CrO H O H OH

Make H OH onOH Cr OH IO I CrO H O

same sideto H O

REDOX REACTIONS


APPLICATIONS OF REDOX REACTIONS (i) Validity of molecular formula :

Summation of oxidation no of all atoms = 0If Oxid. No. of A = +2, B = +3 & C = –2.Then formula of compound= AB2C4= (+2) + (+3 × 2) + (-2 × 4) = 0

(ii) Acidity:

7 5 3 1

4 3 26 4

3 2

Oxid.no. = Acidity

H ClO > H ClO >H ClO >H ClO

S O > S O

This rule do not work for oxoacid of P. (iii) Calculation of (No. of equivalent /n-factor/

Equivalent weight/Normality)EQUIVALENT CONCEPT

No. of Equivalent / Gram Equivalent : One equivalent is a number that represent amount

of substance responsible for transfer of one moleof electron or ion.

1 mole of KMnO4 = 158g of KMnO4= transfers 5mole of electrons in acidic medium

1mole of electrons transferred by = 15 mole of

KMnO4= 158

5 g KMnO4= 1 equivalent of KMnO4

1 mole of K2Cr2O7 = 294g of K2Cr2O7= transfers6 mole of electrons in acidic medium

1mole of electrons transferred by = 16 mole of

K2Cr2O7 = 294

6 g K2Cr2O7=1 equivalent ofK2Cr2O7

1 mole of H2SO4 = 98 g of H2SO4= transfers 2mole of H+ ion.1 mole of ion transferred by =

12 mole of H2SO4

= 49 g of H2SO4= 1 equivalent of H2SO4.n-factor :

n–factor of compounds depends on the categoryof compounds

n–factor is used to calculate equivalent weight ofcompounds.(a) Elements/Ion :

n–factor = valency/charge

Na n– factor = 1

Mg n– factor = 2

Al n– factor = 3

Al3+ n– factor = 3(b) Salts :

n–factor = total cationic /anionic charge in amolecule of salt.

1 1Na Cl 1n factor

2 2

2Mg Cl 2n factor

1 3

3 4Na PO 3

n factor2 11 1

22.MgCl .6 3

K Cl H O n factor2 2 2

4 4 2 24.(NH ) SO .6 4( ' )Fe SO H O n factorMohr s salt

(c) Acids : n–factor = No. of H+ given by / No. of OH

accepted by 1 molecule of acid.HCl n- factor = 1

H2SO4 + NaOH NaHSO4

(n–factor of H2SO4=1)

H2SO4 + 2NaOH Na2SO4 +2H2O(n–factor of H2SO4= 2)

Practice Problems

Balance the following Redox Reactions Answer Key

1. H 24 2I MnO I Mn

1. 24 2 216H 10I 2MnO 5I 2Mn 8H O

2. H2 3 24Fe MnO Fe Mn

2. 2 3 24 28H 5Fe MnO 5Fe Mn 4H O

3. H2 2 23 4 4SO MnO SO Mn

3. 2 2 23 4 4 26H 5SO 2MnO 5SO 2Mn 3H O

4. weakly basic2 24 2 3 2 4MnO S O MnO SO 4. 2 2

2 4 2 3 2 4H O 8MnO 3S O 8MnO 6SO 2OH

5. H2 2 3 42 7Cr O Sn Cr Sn

5. 2 2 3 42 7 214H Cr O 3Sn 2Cr 3Sn 7H O

REDOX REACTIONS


n–facor of H3PO4 3, 2 or 1 depending onreactionn–facor of H3PO3 2 or 1 depending onreactionn–factor of H3PO2 1n–factor of H3BO3 1 {accepts 1OH– ion}(d) Base :

n–factor = No. of OH– given/no. of H+ acceptedby 1 molecule.n–factor of NaOH =1n–factor of Ca(OH)2 =2 or 1n–factor of Al(OH)3 = 3 or 2 or 1(e) Oxidising/Reducing agent :

n–factor = no of Electrons transferred (gain orloss) by 1 molecule.

It is used to calculate equivalent weight orNormality

n–factor /equivalent weight/normality of asubstance can vary depending on the reaction.

27acide 2

47 4

neutral4 2

67basic 2

4 4126

acide 32 2 7

KMnO Mn n 5

KMnO MnO n 3

KMnO MnO n 1

K Cr O 2Cr n 6

(f) Disproportionating species :

If mole of compound getting oxidised and reducedis same then,

n–factor = no. of e– gained/lost by 1 molecule ifonly oxidation or reduction is assumed.

2 2 21 1 1 2 0

22 2 22H O 2H O O

If 1 H2O2 molecule is oxidised or reduced, it willgain or lose 2e– only.

n–factor of H2O2 = 2.

Equivalent weight (Weq) :

Equivalent weight is weight containing 1 equivalentof substance

Weq(KMnO4) = 158

5 g (in Acidic medium) ,

Weq(KMnO4) = 158

3 g (in neutral or weakly

basic medium)

Weq(K2Cr2O7) = 294

6 g (in acidic medium)

Equivalent weight can be defined by multiple waysfor different category of compounds.

Equivalent weight of oxidizing or reducing agentcan also be defined as amount of substance whichloses or gains 1 mole of e-.

Practice Problems

Calculate Weq of underlined compounds in following reactions if M is their molar mass.

01. 3 4 2 4 2H PO NaOH Na HPO H O

02. 2 2 2Ca(OH) HCl CaCl H O

03. Alkaline2 22 3 4S O SO

04. 2I I

05. 2I I

06. 22 4 2C O CO

ANSWER KEY

01.M2 4.

M1

02.M2 05.

M2

03.M8 06.

M2

Normality : Normality is defined as no. of equivalent of solute

present per litre of solution.

N= ( )eqn

V L = ×n-factor

( )molesn

V L

( )N Molarity M n factor

IIB Example :If 15.8g of KMnO4 is present in 1L solution(inacidic medium) calculate normality

( )N Molarity M n factor

n–factor of KMnO4 in acidic medium is 5

15.8 5Normality = 0.1158 1

N

REDOX REACTIONS


. . .

. .

mole

eq

MolarmassNoof eq n n factor Weqn factor

GivenmassNoof eq N Molarity n factorW

TITRATION A procedure in which a solution of known volume

& concentration is used to determine concentrationof another solution of known volume.

Titration usually involves 3 components-(i) Standard solution : Volume & concentrationboth are known

(ii) Unknown Solution:Volume is known butnot concentration

(iii) Indicators: Substances which indicates endpoint (completion of reaction between substancesin standard & unknown solution).

Law of equivalence is used for calculation intitration.

Law of equivalence : The number of equivalentsof all the reactants(which reacts) will be equaland the same number of gram equivalents of eachproduct will form.For A + B C + D neq(A) = neq(B)

moles molesA Bn ×n-factor = n ×n-factor

(M × V × n-factor)A = (M × V × n-factor )B

(N × V)A = (N × V)B

Basic types of titration:(i) Acid Base Titration (ii) Redox Titration(i) Acid-Base Titration:

When concentration of acid is calculated by usingstandard solution of base or vice versa.

eq (A cid) eq (B ase)n = n

IIB Example :A 25 mL Solution of 0.5 M NaOH is titrated untilneutralized into a 50 mL sample of H2SO4 whatwas the concentration of the H2SO4.

Solution : H2SO4+ NaOH Na2SO4 + H2On–factor of H2SO4 is 2 & NaOH is 1

2 4( ) ( )eq H SO eq NaOHn n

2 4H SO NaOH

M V n factor M V n factor

50 2 0.5 25 10.5 2550 2

0.125

M

M

M M

(ii) Redox titration : When concentration of oxidizing acid is calculated

by using standard solution of reducing agent orvice versa

At end point

eq(oxidizing agent) eq(reducing agent)n n

IIB Example :100 mL of sodium oxalate(Na2C2O4) requires 50mL of 0.2M potassium permanganate to titrate itin acidic medium and cause it to turn pink (theend point). Calculate molarity of sodium oxalatesolution.Solution : Reaction will be

MnO4– + C2O4 2– Mn2+ + CO2

n–factor of MnO4– = 5 & of C2O4

2– = 2

At end point 2 2 4 4(Na C O ) eq(KMnO )n = neq

2 2 4 4Na C O KMnO(M×V×n-factor) = (M×V×n-factor)

M ×100 × 2 = 0.2 × 50 × 5

M = 0.25 M.A type of Redox Titration-Iodometric titration : This is an indirect method to determine the

concentration of oxidizing agent Oxidising agent oxidises I– taken in excess to I2

then I2 is subsequently reduced back to I– bysodium thiosulphate.

(a) Cu2+(aq)+4 I–(aq) Cu2I2(s) + I2(aq) I2 remains in solution as it combine with I– to form

I3–

On addition to starch an intense blue colour isobserved due to I2

(b) I2(aq) + 2S2O32–(aq) 2I–(aq)+S4O6

2–(aq) Blue colour of solution disappears after

consumption by S2O32–, it indicates end point.

At end point

2+ eq(sodium thiosulfate)eq(Cu or oxidising agent)n = n

REDOX REACTIONS &ELECTRODE PROCESS

Oxidising or reducing power of different speciescan be compared using electrochemical serieswhich is prepared using electrochemical cell

REDOX REACTIONS


Redox couple: If oxidized & reduced form of asubstance are present together they are calledredox couple.Example: Zn2+/Zn or Cu2+/Cu

Electrode Potential (E): Potential associatedwith each electrode in an electrochemical cell iscalled electrode potential.

Standard electrode potential (E0) : Electrodepotential of species if measured by connecting withstandard hydrogen electrode when pressure is 1atm, concentration is 1M & temp is 298 K is calledstandard electrode potential.

By convention standard electrode potential ofstandard hydrogen electrode is 0.

ELECTROCHEMICAL CELL / GALVANIC CELL / VOLTAIC CELL

ELECTROCHEMICAL SERIES Series formed by arranging standard electrode

potential of species in decreasing order.1. More positive the E , stronger will be the

oxidizing power2. More negative the E , stronger will be the

reducing agent3. Negative E , means species is stronger

reducing agent than H+/H2 couple.4. Positive E means means species is stronger

oxidising agent than H+/H2 couple.5. Metal present at bottom of series will displace

metal present above it.

F2(g) + 2e– 2F– 2.87H2O2 + 2H+ + 2e– 2H2O 1.78MnO4

– + 8H+ + 5e– Mn2+ + 4H2O 1.51Cl2(g) + 2e– 2Cl– 1.36Cr2O7

2– + 14H+ + 6e– 2Cr3+ + 7H2O 1.33O2(g) + 4H+ + 4e– 2H2O 1.23Br2 + 2e– 2Br– 1.092Hg2+ + 2e– Hg2

2+ 0.92Fe3+ + e– Fe2+ 0.77I2 + 2e– 2I– 0.542H+ + 2e– H2(g) 0.00Zn2+ + 2e– Zn(s) –0.76Al3+ + 3e– Al(s) –1.66Mg2+ + 2e– Mg(s) –2.36Na+ + e– Na(s) –2.71Ca2+ + 2e– Ca(s) –2.87K+ + e– K(s) –2.93Li+ + e– Li(s) –3.05

Reaction (Oxidised form + ne– Reduced form) θE / V

ELECTROCHEMICAL SERIES

Incr

easi

ng s

tren

gth

of o

xidi

sing

age

nt

Incr

easi

ng s

tren

gth

of r

educ

ing

agen

t

REDOX REACTIONS


CLASSICAL & MODERN CONCEPTS OFREDOX REACTIONS

01 Oxidation takes place with -(1) Gain of electrons(2) Loss of electrons(3) Increase in the valency of negative part(4) Decrease in the valency of positive part

02. According to classical concept, oxidationinvolves(1) Addition of oxygen(2) Addition of electronegative radical(3) Removal of either hydrogen or some

electropositive radical(4) All of these

03. According to modern concept, oxidation is(1) Electronation(2) Deelectronation(3) Addition of oxygen(4) Addition of electronegative element

04. The reaction,2K2MnO4 + Cl2 2KMnO4 + 2KClis an example of -(1) Oxidation (2) Reduction(3) Redox (4) Chlorination

05. Fluorine does not show positive oxidationstate due to the(1) Absence of s-orbitals(2) Absence of p-orbitals(3) Absence of d-orbitals(4) Highest electronegativity

06. Oxidation number of C in CH2Cl2 is -(1) +2 (2) + 4(3) – 4 (4) 0

07. Which one of the following is a redoxreaction ?(1) H2 + Br2 2HBr(2) 2NaCl + H2SO4 Na2SO4 + 2HCl(3) HCl + AgNO3 AgCl + HNO3(4) NaOH + HCl NaCl + H2O

08. A redox reaction is -(1) Proton transfer reaction(2) Electron transfer reaction(3) Ion combination reaction(4) A reaction in a solution

09. The reactionCo(s) + Cu2+(aq) Co2+(aq) + Cu(s) is -(1) Oxidation (2) Reduction(3) Redox (4) Hydrolysis

10. The conversion of sugarC12H22O11 CO2 is-(1) Oxidation(2) Reduction(3) Neither oxidation nor reduction(4) Both oxidation and reduction

11. In acidic medium, reactionMnO4

– Mn2+ is an example of(1) Oxidation by three electrons(2) Reduction by three electrons(3) Oxidation by five electrons(4) Reduction by five electrons

12. White P reacts with caustic soda formingPH3 And NaH2PO2. The reaction is -(1) Redox (2) Oxidation(3) Reduction (4) Neutralisation

OXIDATION NUMBER13. The oxidation number of P is + 3 in

(1) H3PO3 (2) H3PO4(3) HPO3 (4) H4P2O7

14. Oxidation state of hydrogen in CaH2 is(1) +1 (2) –1(3) + 2 (4) 0

15. Hydrogen can have oxidation numbers of(1) –1 only (2) +1 only(3) 0 only (4) –1, 0, +1

16. The oxidation number of carbon inC12H22O11 is(1) 0 (2) – 6(3) + 6 (4) + 2

17. In the reaction2Na2S2O3 + I2 Na2S4O6 + 2NaI, theoxidation state of S is(1) Increased(2) Decreased(3) Remains same(4) None

18. The oxidation state of iodine in 4 6H IO is:

IIB CONCEPT BUILDER

REDOX REACTIONS


(1) +7 (2) –1(3) +5 (4) +1

19. In which one of the following compounds theoxidation number of oxygen is positive?(1) H2O2 (2) Na2O2(3) OF2 (4) H2O

20. The oxidation number of Fe in [Fe(CN)6]3–

ion is(1) +2 (2) +3(3) –2 (4) –3

21. In the conversion of K2Cr2O7 to K2CrO4, theoxidation number of chromium(1) Remains same (2) Increases(3) Decreases (4) None

22. In which of the following oxidation numberof chlorine is +5(1) Cl– (2) ClO–

(3) 2ClO (4)

3ClO

23. The oxidation number of Mn is +7 in(1) Manganese dioxide(2) Manganese chloride(3) Manganese sulphate(4) Potassium permanganate

24. The oxidation number of C in sucrose(C12H22O11) is(1) +4 (2) +3(3) +2 (4) zero

25. On the basis of composition, the oxidationnumbers of iron in Fe3O4 is/are(1) +2 and +3(2) +1 and +2(3) +2 only(4) +3 only

26. The change in the oxidation number of Pb andCl in the reaction, PbO2 + Cl– ClO– +

Pb 3OH

respectively are

(1) +2, +2 (2) –2, +2(3) +2, –1 (4) –2, +1

27. The oxidation number of S in 2 4H SO ion is

(1) +5 (2) +6(3) +7 (4) +3

28. Oxidation number of carbon in carbonsuboxide (C3O2) is

(1) +2/3 (2) +4/3(3) +4 (4) –4/3

29. The oxidation number of Cr in CrO5 is(1) +3 (2) +5(3) +6 (4) 0

30. Which atoms have undergone a change in theoxidation number in the reaction,2KClO3 2KCl + 3O2?(1) K and Cl (2) K and O(3) Cl and O (4) K, Cl and O

31. Oxidation number of cobalt in K[Co(CO)4] is(1) +1 (2) +3(3) –1 (4) –3

32. In which of the following compounds, theoxidation number of iodine is fractional?

(1) IF7 (2) 3I

(3) IF5 (4) IF333. The oxidation number of chlorine in HOCI is

(1) – 1 (2) 0(3) + 1 (4) 2

34. In which of the following compounds, nitrogenhas an oxidation state of –1?

(1) N2O (2) 2NO

(3) NH2OH (4) N2H435. Oxidation number of nitrogen in (NH4)2SO4 is

(1) –1/3 (2) –1(3) +1 (4) –3

36. O.N. of hydrogen in KH, MgH2 and NaHrespectively would be-(1) –1, – 1 and –1(2) +1, + 1, and + 1(3) +2, +1 and –2(4) –2, –3 and –1

37. In which of the compounds does manganeseexhibit highest oxidation number?(1) MnO2 (2) Mn3O4(3) K2MnO4 (4) MnSO4

38. One of the following has both positive andnegative oxidation states(1) F (2) Cl(3) He (4) Na

39. Oxygen has the oxidation state of + 2 in-(1) SO2 (2) CO2(3) H2O2 (4) OF2

REDOX REACTIONS


OXIDISING & REDUCING AGENT40. Identify oxidising & Reducing Agent

PbS + 4H2O2 PbSO4 + 4H2O(1) PbS, H2O2 (2) H2O2, PbS(3) H2O2 both (4) PbS both

41. In C + H2O CO + H2, H2O acts as-(1) Oxidising agent (2) Reducing agent(3) Both (4) None

42. Which one can act as oxidising & reducingagent both-(1) HNO2 (2) H2O2(3) H2SO3 (4) all

43. In a reaction, H2O + C CO + H2(1) H2O is the reducing agent(2) H2O is the oxidising agent(3) carbon is the oxidising agent(4) oxidation-reduction does not occur

44. Which compound can not be used asoxidising agent-(1) O3 (2) HNO3(3) KMnO4 (4) NH3

45. Which compound cannot be used asReducing agent-(1) CO2 (2) HNO2(3) H3PO3 (4) H2SO3

46. Nitric oxide acts as a reducing agent in thereaction(1) 4NH3 + 5O2 4NO + 6H2O

(2) 2NO + 3I2 + 4H2O 3NO2 +6I– + 8H+

(3) 2NO + H2SO3 N2O + H2SO4(4) 2NO + H2S N2O + S + H2O

47. Which of the following acts both as anoxidising as well as a reducing agent?(1) HNO3 (2) HNO2(3) H I (4) H2SO4

48. An oxidizing agent is a substance which can(1) Gain electrons(2) Lose an electronegative radical(3) Undergo decrease in the oxidation numberof one of its atoms(4) Undergo any one of the above changes

49. Which of the following acts as an oxidisingagent?(1) HNO3 (2) Cl2(3) FeCl3 (4) All the three

50. The compound that can work both as anoxidising as well as a reducing agent is(1) KMnO4 (2) H2O2(3) Fe2(SO4)3 (4) K2Cr2O7

51. SO2 acts as an oxidant when it reacts with(1) Acidified KMnO4 (2) Acidified K2Cr2O7(3) H2S (4) Acidified C2H5OH

52. Which substance serves as reducing agentin the following reaction?

14H++ 22 7Cr O +3Ni 2Cr3++7H2O+3Ni2+

(1) H2O (2) Ni

(3) H+ (4) 272OCr

53. Which of the following behaves as bothoxidising and reducing agents?(1) H2SO4 (2) SO2(3) H2S (4) HNO3

54. Reducing agent is a substance which can(1) Accept electrons (2) Accept protons(3) Donate electrons (4) Donate protons

55. In the reaction,2FeCl3 + H2S 2FeCl2 + 2HCl + S(1) FeCl3 acts as an oxidising agent(2) Both H2S and FeCl3 are oxidized(3) FeCl3 is oxidised while H2S is reduced(4) H2S acts as an oxidizing agent

56. Which of the following acts as a reducingagent?(1) HNO3 (2) KMnO4(3) H2SO4 (4) (COOH)2

TYPES OF REDOX REACTIONS

57. In the following decomposition reaction

2H2O 2H2 + O2(1) Hydrogen is oxidised(2) Oxygen is reduced(3) Hydrogen is reduced(4) Oxygen remains unaffected

58. Name the type of the redox reaction betweeniron and sulphur giving FeS(1) Combination reaction(2) Displacement reaction(3) Disproportionation reaction(4) Decomposition reaction

59. In the disproportionation reaction, which of

REDOX REACTIONS


the following statement is not true?(1) The same species is simultaneously

oxidised as well as reduced.(2) The reacting species must contain an

element having at least three oxidationstates.

(3) The element in the reacting species ispresent in the lowest oxidation state.

(4) The element in the reacting species ispresent in the intermediate oxidation state.

60. Which of the following is not a redoxreaction?(1) 2BaO + O2 2BaO2(2) BaO2 + H2SO4 BaSO4 + H2O2(3) 2KClO3 2KCl+3O2(4) SO2 + 2H2S 2H2O + 3S

61. The conversion KMnO4 K2MnO4 is anexample of(1) Oxidation half reaction(2) Reduction half reaction(3) Oxidation and reduction(4) Neither oxidation nor reduction

62. MnO2 + 4HCl MnCl2 + Cl2 + 2H2O, Inthe reaction MnO2 acts as(1) Oxidant (2) Reductant(3) Both (1) & (2) (4) None

63. Decomposition of H2O2 is an example of(1) Neutralisation(2) Precipitation(3) Disproportionation(4) Hydrolysis

64. 2CuI Cu+CuI2, the reaction is(1) disproportionation(2) Neutralisation(3) Oxidation(4) Reduction

65. In the reaction

S8 + 12OH– 4S2– + 2S22

3O + 6H2O

S has undergone oxidation and reduction.Which of the following statement is true?(1) In S2–, sulphur has been oxidised and in

232OS , sulphur has been reduced.

(2) In S2–, sulphur has been reduced and in2

32OS , sulphur has been oxidised as wellas reduced

(3) In both S2– and 232OS , sulphur has been

oxidised.

(4) In both S2– and 232OS ions, sulphur has

been reduced.66. Which of the following is decomposition

reaction(1) 2HgO 2Hg + O2(2) CH4 + 2O2 CO2 + 2H2O(3) S + O2 SO2(4) Cl2 + 2KBr 2KCl + Br2

67. Which one of the following reactions does notinvolve either oxidation or reduction

(1) 2 2 3VO VO

(2) NaNa

(3) 272

24 OCrCrO

(4) Zn2+ Zn68. C + O2 CO2 the reaction is

(1) Chemical combination(2) Decomposition reactions(3) Displacement reactions(4) Disproportionation reactions

69. PbS + H2O2 PbSO4 + 4H2O. In thisreaction PbS undergoes(1) Oxidation (2) Reduction(3) Both (1) & (2) (4) None

70. Which of the following is not chemicalcombinations(1) C + O2 CO2(2) S + O2 SO2(3) 2Al + N2 2AlN(4) 2H2O O2 + H2

71. Which of the following is not Decompositionreactions(1) 2HgO 2Hg + O2(2) 2H2O 2H2 + O2(3) 2KClO3 2KCl + 3O2(4) CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

72. Following reaction describes the rusting ofiron 4Fe + 3O2 4Fe3+ + 6O2–. Which oneof the following statement is incorrect(1) This is an example of a redox reaction(2) Metallic iron is reduced to Fe3+

(3) Fe3+ is an oxidising agent

REDOX REACTIONS


(4) Metallic iron is a reducing agent73. Which one of the following is not prepared

from halide by chemical oxidation process(1) F2 (2) Cl2(3) Br2 (4) I2

74. The reaction is Decomposition but it’s notredox reaction(1) 2HgO 2H2 + O2(2) 2H2O 2H2 + O2(3) MgCO3 MgO + CO2(4) 2KClO3 2KCl + 3O2

75. The reaction Br2, Cl2, I2, P4 with NaOHinvolves(1) Decomposition(2) Displacement(3) Combination(4) Disproportionation

76. Which reactions are useful to prepareHydrogen in the laboratory(1) Decomposition (2) Displacement(3) Combination (4) Disproportionation

77. Which of the following is metal displacementreaction(1) Zn + CuSO4 ZnSO4 + Cu(2) 2Na + 2H2O 2NaOH + H2(3) Ca + 2H2O Ca(OH)2 + H2(4) 2HgO 2Hg + O2

78. Which one of the following generally getsdisplaced by more electro positive metals innonmetal displacement reactions.(1) H2 (2) N2(3) F2 (4) Cl2

79. Zn + CuSO4 ZnSO4 + Cu, Zn can act as(1) Oxidising agent (2) Reducing agent(3) Reduced (4) Oxidant

80. Which of the following is not an example ofdisproportionation reaction?(1) Cl2 + 2NaOH NaCl + NaOCl+H2O(2) P4 + 3NaOH + 3H2O PH3 + 3NaH2PO2(3) 2NaOH + 2F2 2NaF + OF2 + H2O(4) 2H2O2 2H2O + O2

81. 2HgO 2Hg + O2, in reactant Hg act as(1) Oxidising agent (2) Reducing agent(3) Oxidised (4) None of the above

82. Which one of the halogn is prepared by onlyelectrolysis method

(1) Cl2 (2) Br2(3) F2 (4) I2

83. Layer test is used for determination of(1) Chalcogens (2) Pnicogens(3) Halogens (4) Noble gases

84. Which of the following is not metaldisplacement(1) Zn + CuSO4 ZnSO4 + Cu(2) 2Na + 2H2O 2NaOH + H2(3) TiCl4 + 2Mg Ti + 2MgCl2(4) 3Fe3O4 + 8Al 4Al2O3 + 9Fe

85. 3Cl2 + 6NaOH 5NaCl + NaClO3 + 3H2O,chlorine gets(1) Oxidised(2) Reduced(3) Both ‘1’ and ‘2’(4) None of the above

86. In the reaction 2Al + N2 2AlN, Al is(1) Reduced (2) Oxidised(3) Oxidising agent (4) None of the above

87. When P reacts with caustic soda. The productsare PH3 and NaH2PO2. This reaction is anexample of(1) Oxidation(2) Reduction(3) Oxidation and reduction (Redox)(4) Neutralization

88. Which of the following is disproportionationreaction(1) 2H2O2 2H2O + O2(2) 2H2O 2H2 + O2(3) 2HgO 2Hg + O2(4) Mg CO3 MgO + CO2

89. Among the following ion the one that cannotundergo disproportionation(1) ClO– (2) ClO2

–

(3) ClO3– (4) ClO4

–

90. Br2, I2 dissolve in(1) C6H6 (2) CO2(3) CCl4 (4) NH3

91. Fluorine does not undergo disproportionationbecause(1) Fluorine always exhibit-1 oxidation state(2) Fluorine exhibit only two oxidation

numbers

REDOX REACTIONS


(3) Fluorine exhibit three oxidation numbers(4) None of the above

92. 2H2O2 2H2O + O2 the reaction is(1) Decomposition(2) Combination(3) Disproportionation reactions(4) ‘1’ and ‘3’

93. Which of the following is a redox reaction?(1) NaCl + KNO3 NaNO3 + KCl(2) CaC2O4 + 2HCl CaCl2 + H2C2O4(3) Mg(OH)2 + 2NH4Cl MgCl2 + 2NH4OH(4) Zn+2AgCN 2Ag + Zn(CN)2

BALANCING OF REDOX REACTIONS

94. The coefficients of I–, IO3– and H+ in the

redox reaction, I– + 3IO + H+ I2 + H2O

in the balanced form respectively are(1) 5, 1, 6 (2) 1, 5, 6(3) 6, 1, 5 (4) 5, 6, 1

95. In the reactionSnO2 + nC Sn + nCOthe value of n is(1) 1 (2) 2(3) 3 (4) 4

96. The value of n in- + - 2+4 2MnO + 8H + ne Mn + 4H O is

(1) 5 (2) 4(3) 3 (4) 2

97. The number of electrons lost or gained duringthe change Fe + H2O Fe3O4 + H2 is(1) 2 (2) 4(3) 6 (4) 8

98. For the redox reaction,

Zn + 3NO Zn2+ + NH4

+

in basic medium, coefficients of Zn, 3NO and

OH– in the balanced equation respectively are(1) 4, 1, 7 (2) 7, 4, 1(3) 4, 1, 10 (4) 1, 4, 10

99. For the reaction, NH3 + OCl– N2H4 + Cl–occurring in basic medium, the coefficient ofN2H4 in the balanced equation will be(1) 1 (2) 2(3) 3 (4) 4

100. C2H6(g) + nO2 (g) CO2(g) + H2O(l)In this equation, ratio of the coefficients ofCO2 and H2O is(1) 1 : 1 (2) 2 : 3(3) 3 : 2 (4) 1 : 3

101. What is the value of n in the following halfequation,

- - 2- -4 24

Cr OH + OH CrO + H O + ne

(1) 3 (2) 6(3) 5 (4) 2

102. The number of electrons required to balancethe following equation,

- + -3 2NO + 4H + e 2H O + NO is

(1) 5 (2) 4(3) 3 (4) 2

APPLICATIONS OF REDOX REACTIONS

103. The correct order of acidity of oxoacids ofhalogens is(1) HClO < HClO2 < HClO3 < HClO4

(2) HClO4 < HClO3 < HClO2< HClO(3) HClO < HClO4 < HClO3< HClO2

(4) HClO4 < HClO2 < HClO3< HClO104. Correct order of acidity of oxoacids of P is :

(1) H3PO2< H3PO3< H3PO4

(2) H3PO4< H3PO3< H3PO2

(3) H3PO2< H3PO4< H3PO3

(4) None of these105. The correct order for the increasing acidic

strength of oxides of nitrogen is-(1) N2O < NO < N2O4 < N2O3 < N2O5

(2) N2O < NO < N2O3 < N2O4 < N2O5

(3) N2O < N2O3< N2O4 < N2O5 < NO(4) NO < N2O< N2O3 < N2O4 < N2O

EQUIVALENT CONCEPT

106. If M is the molecular weight of FeC2O4, thenits equivalent weight in the conversion,FeC2O4 Fe3+ + CO2 is(1) M/3 (2) M/6(3) M/2 (4) Equal to M

107. Equivalent weight of NH3 in the change,N2 NH3 is(1) 17/6 (2) 17(3) 17/2 (4) 17/3

REDOX REACTIONS


108. Equivalent weight of N2 in the changeN2 2NH3 is(1) 28/6 (2) 28(3) 28/2 (4) 28/3

109. In alkaline medium, KMnO4 reacts asfollows:

2KMnO4 + 2KOH 2K2MnO4 + H2O + OTherefore, the equivalent weight of KMnO4

will be(1) 31.6 (2) 52.7(3) 79.0 (4) 158.0

110. Molecular weight of orthophosphoric acid isM. Its equivalent weight is(1) 3M (2) M(3) M/2 (4) M/3

111. Equivalent weight of KMnO4 when it isconverted into MnSO4 is(1) M/5 (2) M/6(3) M/3 (4) M/2

112. The following is not a fixed quantity(1) Atomic weight of an element(2) Equivalent weight of an element (or)

compound(3) Molecular weight of a compound(4) Formula weight of a substance

113. Equivalent weight of CuSO4 in terms of itsmolecular weight M in the following reaction

is 2CuSO4 + 4KI Cu2I2 + I2 + 2K2SO4(1) M (2) M/2(3) M/4 (4) M/8

114. When Ferrous sulphate acts as reductant, itsequivalent weight is(1) twice that of its molecular weight(2) equal to its molecular weight(3) one-half of its molecular weight(4) one-third of its molecular weight

115. The equivalent weight of KMnO4 (in acidmedium) is (At. wt. of K = 39, Mn = 55)(1) 158 (2) 15.8(3) 31.6 (4) 3.16

116. Equivalent weight of Na2S2O3in its titrationwith iodine solution is equal to(1) Mol wt (2) Mol wt/2(3) Mol wt/4 (4) Mol wt/8

117. The equivalent weight of Hypo in the reaction[M = molecular weight]2Na2S2O3+I22NaI+Na2S4O6 is

(1) M (2) M/2(3) M/3 (4) M/4

118. Equivalent weight of FeS2 (mol wt = M) inthe following reaction isFeS2 + O2 Fe2O3 + SO2(1) M/2 (2) M/4(3) M/11 (4) M/14

119. M is molecular weight of KMnO4 . Theequivalent weight of KMnO4 when it isconverted into K2MnO4 is(1) M (2) M/3(3) M/5 (4) M/7

120. Formula weight divided by the change inoxidation number gives(1) Equivalent weight of an oxidant(2) Equivalent weight of the reductant(3) The number of electrons gained in the reaction(4) The equivalent weight of the oxidant orreductant

121. Which of the following acid has the samemolecular weight and equivalent weight(1) H3PO2 (2) H3PO3(3) H3PO4 (4) H2SO4

122. Equivalent weight of I2 in the reaction,I2 + 5Cl2 + 6H2O 2HIO3 + 10 HCl is(1) M (2) M/2(3) M/5 (4) M/10

123 The equivalent weight of CaCO3(1) 100 (2) 50(3) 33.3 (4) 25

124. Equivalent weight of K2Cr2O7 in acidicmedium is(1) 24.5 (2) 49(3) 147 (4) 296

125. The equivalent weight of MnSO4 is half itsmolecular weight when it is converted into(1) Mn2O3 (2)

4MnO(3) MnO2 (4) 2

4MnO126. In acidic medium Dichromate ion oxidises

Ferrous ion to Ferric ion. If thegrammolecular weight of potassiumdichromate is 294 gm, its equivalent weightis(1) 294 (2) 147(3) 49 (4) 24.5

127. The equivalent weight of CuSO4 when it is

REDOX REACTIONS


converted to Cu2I2 [M = mol.wt](1) M/1 (2) M/2(3) M/3 (4) 2M

128. The equivalent weight of glucose in thereaction C6H12O6 + 6O2 6CO2 + 6H2O is[M = mol. wt](1) M/4 (2) M/12(3) M/24 (4) M/48

129. The equivalent weight of Iodine in thereaction 2Na2S2O3 + I2 2NaI + Na2S4O6is [M = mol. wt](1) M (2) M/2(3) M/3 (4) 2M

TITRATION

130. The strength of an aqueous solution of I2 canbe determined by titrating the solution withstandard solution of:(1) Oxalic acid(2) Sodium thiosulphate(3) Sodium hydroxide(4) Mohr’s salt

131. The volume of 0.05 M KMnO4 solutionrequired to oxidize completely 2.70g of oxalicacid (H2C2O4) in acidic medium will be(1) 120 cm3 (2) 240 cm3

(3) 360 cm3 (4) 480 cm3

132. Equivalent weight of As2O3 in the followingequation As2O3 + 2I2 + 2H2O As2O5 +4HI [at. wt of arsenic = 75](1) 49.5 (2) 156.6(3) 94 (4) None

133. A solution of 10ml M10

FeSO4 was titrated

with KMnO4 solution in acidic medium. Theamount of KMnO4 used will be:(1) 5 ml of 0.1 M (2) 10 ml of 0.1 M(3) 10 m of 0.5 M (4) 10 ml of 0.02 M

134. In the reaction,

2-2 3 22S O aq + X s or l

2- -4 6S O aq + 2X aq

X will be(1) Both Br and I (2) Only I(3) Only Br (4) F only

135. Excess of KI reacts with CuSO4 solution andthen Na2S2O3 solution is added to it. Which

of the statements is incorrect for thisreaction?(1) Na2S2O3 is oxidised(2) CuI2 is formed(3) Cu2I2 is formed(4) Evolved I2 is reduced

DETERMINING OXIDISING & REDUCINGPOWER

136. In Cu – Zn cell

(1) Reduction occurs at the copper cathode(2) Oxidation occurs at the copper cathode(3) Reduction occurs at the anode(4) Chemical energy is converted to light

energy137. Correct order of tendency to lose electrons

(1) Zn > Cu > Ag(2) Zn < Cu < Ag(3) Zn > Cu < Ag(4) Cu > Zn > Ag

138. In an electrochemical cell(1) Potential energy changes into kinetic

energy(2) Kinetic energy changes into potential

energy(3) Chemical energy changes into electrical

energy(4) Electrical energy changes into chemical

energy139. The electrode potential of an electrode is

(1) The potential applied to the electrode(2) The ionization potential of the material

of the electrode(3) The tendency of the electrode to lose

or gain electrons when it is in contactwith its ions

(4) The potential energy of the electrons inan electrode

140. Which of the following metals will not reactwith solution of CuSO4?(1) Fe (2) Zn(3) Mg (4) Ag

141. The standard electrode potential (E0) valuesof Al3+/Al, Ag+/Ag,K+/ K and Cr3+/Cr are –1.66V, 0.80 V, –2.93 V and –0.74 V, respectively.The correct decreasing order of reducingpower of the metal is

(1) Ag > Cr > Al > K

REDOX REACTIONS


(2) K > Al > Cr > Ag(3) K > Al > Ag > Cr(4) Al > K > Ag > Cr

142.. Which one of the following is different fromothers?(1) Daniell cell(2) Voltaic cell(3) Galvanic cell(4) Electrolytic cell

143. The primary reference electrode for themeasurement of electrode potential is:

(1) SHE (Standard hydrogen electrode)(2) Normal calomel electrode(3) Glass electrode(4) None of these

144. Standard electrode potentials of three metalsX, Y and Z are –1.2 V, + 0.5 V and –3.0 Vrespectively. The reducing power of thesemetals will be(1) Y > Z > X(2) Y > X > Z(3) Z > X > Y(4) X > Y > Z

ANSWER KEY

01. 2 02. 4 03. 2 04. 3 05. 4 06. 4 07. 1 08. 2 09. 3 10. 1

11. 4 12. 1 13. 1 14. 2 15. 4 16. 1 17. 1 18. 1 19. 3 20. 2

21. 1 22. 4 23. 4 24. 4 25. 1 26. 2 27. 2 28. 2 29. 3 30. 3

31. 3 32. 2 33. 3 34. 3 35. 4 36. 1 37. 3 38. 2 39. 4 40. 241. 1 42. 4 43. 2 44. 4 45. 1 46. 2 47. 2 48. 4 49. 4 50. 2

51. 3 52. 2 53. 2 54. 4 55. 1 56. 4 57. 3 58. 1 59. 3 60. 2

61. 2 62. 1 63. 3 64. 1 65. 2 66. 1 67. 3 68. 1 69. 1 70. 4

71. 4 72. 2 73. 1 74. 3 75. 4 76. 2 77. 1 78. 1 79. 2 80. 3

81. 1 82. 3 83. 3 84. 2 85. 3 86. 2 87. 3 88. 1 89. 4 90. 3

91. 2 92. 4 93. 4 94. 1 95. 2 96. 1 97. 4 98. 3 99. 1 100. 2

101.1 102. 3 103. 1 104. 1 105. 2 106. 1 107. 4 108. 1 109. 4 110. 4

111. 1 112. 2 113. 1 114. 2 115. 3 116. 1 117. 1 118. 3 119. 1 120. 4

121.1 122. 4 123 2 124. 2 125. 3 126. 3 127. 1 128. 3 129. 2 130. 2

131.2 132. 1 133. 4 134. 2 135. 3 136. 1 137. 1 138. 3 139. 3 140. 4

141.2 142. 4 143. 1 144. 3


REDOX REACTIONS

01. If HNO3 changes into N2O, the oxidationnumber is changed by(1) + 2 (2) – 1(3) 0 (4) + 4

02. Which of the following is not a reducing agent?(1) NaNO2 (2) NaNO3(3) HI (4) SnCl2

03. Oxidation number of sulphur in H2SO5 is(1) + 2 (2) + 4(3) + 8 (4) + 6

04. Which of the following is the most powerfuloxidizing agent?(1) F2 (2) Cl2(3) Br2 (4) I2

05. In which of the following cases is the oxidationstate of N atom wrongly calculated?

Compound Oxidation state(1) NH4Cl – 3(2) (N2H5)2SO4 + 2(3) Mg3N2 – 3(4) NH2OH – 1

06. In which of the following compounds transitionmetal is in oxidation state zero(1) [Co(NH3)6]Cl2(2) [Fe(H2O)6SO4](3) [Ni(CO)4](4) [Fe(H2O)6] (OH)2

07. In which one of the following changes thereare transfer of five electrons?(1) MnO4

– Mn2+

(2) CrO42 Cr3+

(3) MnO42– Mn2+ + 5e–

(4) Cr2O72– 2Cr3+

08. Equation H2S + H2O2 S + 2H2O represents(1) Acidic nature of H2O2

(2) Basic nature of H2O2

(3) Oxidising nature of H2O2

(4) Reducing nature of H2O2

09. The oxidation number of Mn in MnO4–1 is

(1) + 6 (2) – 5(3) + 7 (4) + 5

10. In the reactionC2O4

2– + MnO4 + H+ Mn2+ + CO2 + H2O

the reductant is(1) C2O4

2– (2) MnO4–

(3) Mn2+ (4) H+

11. When Sn2+ changes to Sn4+ in a reaction(1) It loses two protons(2) It gains two electrons(3) It loses two electrons(4) It gains two protons

12. In the chemical reaction Cl2 + H2S 2HCl+ S, the oxidation number of sulphur changesfrom(1) 0 to 2 (2) – 2 to 0(3) 2 to 0 (4) – 2 to – 1

13. Oxidation number of N in (NH4)2SO4 is(1) – 3 (2) – 1(3) + 1 (4) – 1/3

14. Oxidation number of nickel in Ni(CO)4

(1) + 2 (2) + 4(3) – 4 (4) 0

15. In which of the following compounds iron haslowest oxidation state?(1) FeSO4. (NH4)2SO4. 6H2O(2) K4Fe(CN)6

(3) Fe(CO)5

(4) Fe2O16. The oxidation numbers of Fe and S in iron

pyrites are(1) 3, – 1 (2) 2, – 1(3) 3, – 1.5 (4) 4, – 2

17. In which of the following compounds theoxidation number of carbon is maximum(1) HCHO (2) CH3OH(3) CHCl3 (4) C12H22O11

18. A solution of sulphur dioxide in water reactswith H2S precipitating sulphur. Here sulphurdioxide acts as(1) An oxidising agent(2) A reducing agent(3) An acid(4) A catalyst

19. In ferrous ammonium sulphate oxidationnumber of Fe is(1) + 3 (2) + 2(3) + 1 (4) – 2

IIB BASICS (LEVEL - I)


REDOX REACTIONS20. Which one is oxidising substance?

(1) C2H2O2 (2) CO(3) H2S (4) CO2

21. The oxidation number of Cr in K2Cr2O7 is(1) – 2 (2) – 7(3) + 2 (4) + 6

22. In which of the following reactions H2O2 is areducing agent?(1) 2FeCl2 + 2HC1 + H2O2 2FeCl3 + 2H2O(2) Cl2 + H2O2 2HCl + O2

(3) 2HI + H2O2 2H2O + I2

(4) H2SO3 + H2O2 H2SO4 + H2O23. Oxidation numbers of two Cl atoms in

bleaching powder, CaOCl2, are(1) – 1, – 1 (2) + 1, – 1(3) + 1, + 1 (4) 0, – 1

24. Oxidation state of oxygen atom in potassiumsuperoxide is

(1) 0 (2)12

(3) – 1 (4) – 225. Select the compound in which chlorine is

assigned the oxidation number +5(1) HClO4 (2) HClO2

(3) HClO3 (4) HCl26. Oxidation number of osmium (Os) in OsO4 is

(1) + 8 (2) + 6(3) + 7 (4) + 4

27. When KMnO4 acts as an oxidising agent andultimately forms MnO4

–2, MnO2, Mn2O3,Mn+2, then the number of electronstransferred in each case respectively is(1) 4, 3, 1, 5 (2) 1, 5, 3, 7(3) 1 ,3, 4, 5 (4) 3, 5, 7, 1

28. Oxidation number of P in KH2PO2 is(1) – 4 (2) + 3(3) + 5 (4) + 1

29. H2MoO4 MoO2+ in the process

H2MoO4

(1) Acts as a reducing agent(2) Acts as an oxidising agent(3) Acts both as a reducing and oxidising agent(4) None of these

30. The oxidation number of phosphorus inBa(H2PO2)2 is(1) + 1 (2) – 1

(3) + 2 (4) + 331. The compound which could not act both as

oxidising and reducing agent is(1) SO2 (2) MnO2

(3) Al2O3 (4) CrO32. Sulphur has lowest oxidation number in

(1) H2SO3 (2) SO2

(3) H2SO4 (4) H2S33. In XeO3 and XeF6 the oxidation state of Xe is

(1) + 4 (2) + 1(3) + 6 (4) + 3

34. Oxidation number of sulphur in S2O32– is

(1) – 2 (2) + 6(3) + 2 (4) 0

35. Oxidation number of carbon in CH3 – Cl is(1) – 3 (2) – 2(3) – 1 (4) 0

36. In the following balanced reaction,4O2

x + 2H2O 4OH+ + 3O2

(1) x = – 4 and species is oxide(2) x = – 2 and species is superoxide(3) x = 0 and species is oxygen(4) x = – 1 and species is superoxide

37. Which of the following substances acts as anoxidising as well as a reducing agent?(1) Na2O (2) SnCl2(3) Na2O2 (4) NaNO2

38. Amongst the following identify the specieswith an atom in + 6 oxidation state(1) MnO4

– (2) Cr(CN)63–

(3) NiF62– (4) CrO2Cl2

39. The oxidation number of iron in the compoundK4[Fe(CN)6] is(1) + 2 (2) + 4(3) + 3 (4) + 6

40. What is the oxidation number of sulphur inNa2S4O6?

(1)52 (2)

32

(3)35 (4)

23

41. HNO2 acts both as reductant and as oxidant,while HNO3 acts only as oxidant. It is due totheir(1) Solubility


REDOX REACTIONS(2) Maximum oxidation number(3) Minimum oxidation number(4) Minimum number of valence electrons

42. Carbon is in the lowest oxidation state in(1) CH4 (2) CCl4(3) CF4 (4) CO2

43. In which reaction there is a change in valency(1) 2NO2 N2O4

(2) NH4OH NH4+ + OH+

(3) 2NO2 + H2O HNO2 + HNO3(4) CaCO3 CaO + CO2

44. Oxidation number of N in NH3 is(1) + 5 (2) + 3(3) 0 (4) – 3

45. When K2Cr2O7 is converted to K2CrO4, thechange in the oxidation state of chromium is(1) 0 (2) 6(3) 4 (4) 3

46. Which one of the following reactions is not anexample of redox reaction?(1) Cl2 + 2H2O + SO2 4H+ + SO4

2–+2Cl–

(2) Cu+2 + Zn Zn2+ + Cu(3) 2H2 + O2 2H2O(4) HCl + H2O H3O+ + Cl–

47. What is ‘A’ in the following reaction2Fe3+

(aq) + Sn2+(aq) 2 Fe2+

(aq) + AA(1) Sn3+

(aq) (2) Sn2+(aq)

(3) Sn4+(aq) (4) Sn

48. Oxidation numbers of Mn in K2MnO4 andMnSO4 are respectively(1) + 7, + 2 (2) + 5, + 2(3) + 6, + 2 (4) + 2, + 6

49. What is the oxidation number of Co in[Co(NH3)4ClNO2]?(1) + 5 (2) + 3(3) + 4 (4) + 2

50. The oxidation number of sulphur in H2S2O7and iron in K4[Fe(CN)6 ] is respectively(1) + 6 and + 4 (2) + 2 and + 2(3) + 8 and + 2 (4) + 6 and + 2

51. The valency of Cr in the complex[Cr(H2O)4Cl2]+

(1) 1 (2) 3(3) 5 (4) 6

52. Oxidation number of nitrogen in NaNO2 is(1) + 2 (2) + 4(3) + 3 (4) – 3

53. Which of the following reactions involvesoxidation- reduction?(1) H2 + Br2 2HBr(2) HBr + AgNO3 AgBr + HNO3

(3) NaBr + HCl NaCl + HBr(4) 2NaOH + H2SO4 Na2SO4 + 2H2O

54. Oxidation number of carbon in H2C2O4 is(1) + 4 (2) + 3(3) + 2 (4) – 2

55. Which of the following is a redox reaction?(1) Zn + 2AgCN 2Ag + Zn(CN)2

(2) Mg(OH)2 + 2NH4Cl MgCl2 + 2NH4OH(3) CaC2O4 + 2HCl CaCl2 + H2C2O4

(4) NaCl + KNO3 NaNO3 + KCl56. During the disproportionation of I2, to iodide

and iodate ions, the ratio of iodate and iodideions formed in alkaline medium is(1) 1 : 5 (2) 5 : 1(3) 3 : 1 (4) 1 : 3

57. In which of the following compounds, anelement exhibits two different oxidationstates?(1) NH2OH (2) NH4NO3

(3) N2H4 (4) N3H58. In the balanced chemical reaction,

- - +3 2 2IO + aI + bH cH O + dI

a, b, c and d respectively correspond to(1) 5, 6, 5, 5 (2) 5, 3, 6, 3(3) 3, 5, 3, 6 (4) 5, 6, 3, 3

59. For the redox reactionMnO4

– + C2O42– + H+ Mn2+ + CO2 + H2O

the correct coefficients of the reactants forthe balanced reaction are

MnO4– C2O4

2– H+

(1) 2 5 16(2) 16 5 2(3) 5 16 2(4) 2 16 5

60. MnO4– oxidises H2O2 to O2 in acidic medium

xMnO4– + yH2O2 + zH+ Mn2+ + O2 + H2O

Coefficients x, y and z are respectively


REDOX REACTIONSX y z

(1) 1 2 3(2) 3 2 5(3) 2 6 5(4) 2 5 6

61. When ZnS is boiled with strong nitric acid,the products are zinc nitrate, sulphuric acidand nitrogen dioxide. What are the changesin the oxidation numbers of Zn, S and N?(1) + 2, + 4, – 1 (2) + 2, + 6, – 2(3) 0, + 4, – 2 (4) 0, +8, – 1

62. For the redox reactionCr2O7

2 + H+ + Ni Cr3+ + Ni2+ + H2OThe correct coefficients of the reactants forthe balanced reaction are

Cr2O72– Ni H+

(1) 1 3 14(2) 2 3 14(3) 1 1 16(4) 3 3 12

63. For the redox reaction,xFe2+ + yCr2O7

2– + zH+Fe3+ + Cr3+ + H2Ox, y and z are

x y z(1) 3 1 14(2) 6 1 7(3) 6 2 14(4) 6 1 14

64. The number of electrons involved per atomin the reduction of Cr2O7

2– in acidic solutionto Cr3+ is(1) 0 (2) 2(3) 3 (4) 5

65. H2O can be oxidised to(1) H2 and O2 (2) O2

(3) OH– (4) O2–

66. In the following reaction,2I– + Cr2O7

2– + 14H+ I2 + 2Cr3+ + 7H2OUnbalanced parts are(1) H+, H2O(2) Cr2O7

2–, Cr3+

(3) I–, I2

(4) None of them are balanced67. What is the molecular state of sulphur as

reactant in, sulphur + 12OH– 4S2– +2S2O3

2– + 3H2O?

(1) S82– (2) 2S2–

(3) S8 (4) S8–

68. C2H6(g) + nO2 CO2(g) + H2O(l)In this equation, the ratio of the coefficientsof CO2 and H2O is(1) 1 : 1 (2) 2 : 3(3) 3 : 2 (4) 1 : 3

69. When potassium permanganate is titratedagainst ferrous ammonium sulphate inacidic medium, the equivalent weight ofpotassium permanganate is -(1) Molecular weight /10(2) Molecular weight/5(3) Molecular weight/2(4) Molecular weight

70. 2MnO4– + 5H2O2 + 6H+ 2Z + 5O2 + H2O.

In this reaction Z is(1) Mn+2 (2) Mn+4

(3) MnO2 (4) Mn71. In balancing the half-reaction

CN– CNO– (skeletal)the number of electrons that must be added is(1) 0 (2) 1 on the right(3) 1 on the left (4) 2 on the right

72. How many moles of K2Cr2O7, can be reducedby 1 mole of Sn2+?(1) 2/3 (2) 1/6(3) 1/3 (4) 1

73. I– reduces IO3– and I2 and itself oxidised to I2

in acidic medium. Thus, final reaction is(1) I– + IO3

– + 6H+ I2 + 3H2O(2) 5I– + IO3

– + 6H+ 3I2 + 3H2O(3) I– + IO3

– I2 + O3

(4) None of them is oxidised74. Values of p, q, r, s and t are in the following

redox reaction[pBr2 + qOH– rBr– + sBrO–

3 + tH2O]p q r s t

(1) 3 6 1 5 3(2) 3 6 5 3 1(3) 3 6 5 1 3(4) 3 5 1 6 3

75. Oxidation of thiosulphate (S2O32–) ion by

iodine gives(1) SO3

2– (2) SO42–

(3) S4O62– (4) S2O4

2–


REDOX REACTIONS76. Which of the following solutions will exactly

oxidize 25 mL of an acid solution of 0.1 M Fe(II) oxalate?(1) 25 mL of 0.1 M KMnO4

(2) 25 mL of 0.2 M KMnO4

(3) 25 mL of 0.6 M KMnO4

(4) 15 mL of 0.1 M KMnO4

77. Mass of KHC2O4 (potassium acid oxalate)required to reduce 100 ml of 0.02 M KMnO4in acidic medium (to Mn2+) is x g and toneutralise 100 ml of 0.05 M Ca(OH)2 is y g,then(1) x = y (2) 2x = y(3) x = 2y (4) none is correct

78. In the following equation:ClO3

– + 6H+ + X Cl– + 3H2O, then X is(1) O (2) 6e–

(3) O2 (4) 5e–

79. In the reactionCrO5 + H2SO4 Cr2(SO4)3 + H2O + O2 onemole of CrO5 will liberate how many moles ofO2:(1) 5/2 (2) 5/4(3) 9/2 (4) 7/2

80. The brown ring complex compound isformulated as [Fe(H2O)5NO]SO4. Theoxidation state of iron is(1) 1 (2) 2(3) 3 (4) 0

81. When KMnO4 is reduced with oxalic acid inacidic solution, the oxidation number of Mnchanges from(1) 7 to 4 (2) 7 to 2(3) 6 to 4 (4) 4 to 2

82. 28 NO3– + 3AS2S3 + 4H2O

6AsO43– + 28 NO + 9SO4

2– + H+

What will be the equivalent mass of As2S3 inthe above reaction?

(1)M.wt

2 (2)M.wt

4

(3)M.wt

24 (4)M.wt

28

83. 100 ml of a mixture of NaOH and Na2SO4 isneutralised by 10 ml of 0.5 M H2SO4. HenceNaOH in 100 mL solution is(1) 0.2 g (2) 0.4 g(3) 0.6 g (4) None of these

ANSWER KEY

01. 4 02. 2 03. 4 04. 1 05. 2 06. 3 07. 1 08. 3 09. 3 10. 1

11. 3 12. 2 13. 1 14. 4 15. 3 16. 4 17. 3 18. 1 19. 2 20. 4

21. 4 22. 2 23. 2 24. 2 25. 3 26. 1 27. 3 28. 4 29. 2 30. 1

31. 3 32. 4 33. 3 34. 3 35. 2 36. 4 37. 4 38. 4 39. 1 40. 1

41. 2 42. 1 43. 3 44. 4 45. 1 46. 4 47. 3 48. 3 49. 4 50. 4

51. 2 52. 3 53. 1 54. 2 55. 1 56. 1 57. 2 58. 4 59. 1 60. 4

61. 4 62. 1 63. 4 64. 3 65. 2 66. 3 67. 3 68. 2 69. 2 70. 1

71. 4 72. 3 73. 2 74. 3 75. 3 76. 4 77. 1 78. 2 79. 4 80. 1

81. 2 82. 4 83. 2


REDOX REACTIONS

01. Oxidation number of Fe in Fe3O4 isfractional because-

(1) It is a mixed [Fe(+2) – Fe(+4)] oxide

(2 It is a non-stoichiometric compound

(3) It is a mixed [Fe(+2) – Fe(+3)] oxide

(4) None of the above

02. Amongst the following identify the specieswith an atom in +6 oxidation state –

(1) MnO4¯ (2) Cr(CN)63–

(3) NiF62– (4) CrO2Cl2

03. An example of redox process is –

(1) CuSO4 + Fe FeSO4 + Cu

(2) Ca(OH)2 + 2HCl CaCl2 + 2H2O

(3) CaO + 2HCl CaCl2 + H2O

(4) CaCO3 heat CaO + CO2

04. In the reduction of dichromate by Fe(II),the number of electrons involved perchromium atom is –

(1) 3 (2) 1

(3) 2 (4) 4

05. Oxidation number of S in H2SO5 is 6. Thisis observed, because –

(1) There are five oxygen atoms in the molecule

(2) The hydrogen atom is directly linked withnon-metal

(3) There is peroxide linkage in the molecule

(4) The sulphur atom shows co-ordinate linkage

06. Consider the following statement in thereaction –

KIO3 + 5KI + 6HCl 3I2 + 6KCl + 3H2O

(A) KI is oxidised to I2

(B) KIO3 is oxidised to I2

(C) KIO3 is reduced to I2

(D) Oxidation number of I increases from

(–1) in KI to zero in I2

of these statements

(1) A, C and D are correct

(2) A, B and D are correct

(3) B and D are correct

(4) A alone is correct

07. The oxidation state of tungsten inNa2W4O13.10H2O is –

(1) + 7 (2) + 6

(3) + 4 (4) + 4.5

08. H2O2 reduces MnO4– ion to

(1) Mn+ (2) Mn2+

(3) Mn3+ (4) Mn–

09. The oxidation state of molybdenum in itsoxocomplex species

[Mo2O4(C2H4)2(H2O)2]2– is

(1) +2 (2) +3

(3) +4 (4) +5

10. In the reaction, 3Br2 + 6NaOH NaBrO3+ 5NaBr + 3H2O which element loses aswell as gains electrons

(1) Na (2) Br

(3) O (4) H

11. In [Cr(O2)(NH3)4 (H2O)] Cl2 oxidation num-ber of Cr is + 3, then O2 will be in the form :

(1) dioxide (2 ) peroxide

(3) superoxide (4) oxide

12. Oxidation number of S in H2S2O7 is –

(1) + 4 (2) – 6

(3) – 5 (4) + 6

13. The oxidation number of S in Na2S4O6 is -

(1) + 2.5

(2) + 2 and + 3 (two S have + 2 and other two have + 3)

(3) + 2 and + 3 (three S have + 2 and one S has + 3)

(4) + 5 and 0 (two S have + 5 and the other twohave 0)

14. Match list – I (compounds) with list – II(Oxidation state of nitrogen) and select thecorrect answer using the codes given belowthe lists –

List – I List – II

(A) NaN3 (1) +5

(B) N2H4 (2) +2

(C) NO (3) –1/3

(D) N2O5 (4) –2

IIB ACCURACY ACHIEVERS (LEVEL - II)


REDOX REACTIONSCodes :–

A B C D(1) c d b a(2) d c b a(3) c d a b

(4) d c a b

15. Which element will have the maximumoxidation number in K2Cr2O7 and KMnO4

(1) Mn (2) Cr (3) O (4) K

16. Oxidation state of chlorine in perchloric acidis

(1) – 1 (2) 0

(c) – 7 (4) + 7

17. Equivalent weight of FeC2O4 in the change,

FeC2O4 Fe3+ + CO2 is

(1) M/3 (2) M/6

(3) M/2 (4) M/1

18. 2 mole of N2H4 loses 20 moles ofelectrons to form a compound Y. Assumingthat all nitrogen appears in the new com-pound, if there is no change in O.N. of hy-drogen, the O.N. of nitrogen in Y is

(1) + 3 (2) – 3(3) + 1 (4) + 5

19. AB4¯ + C+2 C+3 + AA+2. If the O.N. of Bis –2. Choose the true statement for theabove change –

(1) O.N. of A decreases by –5

(2) O.N. of C decreases by +1

(3) O.N. of A decreases by + 5 and that of Cincreases by +1

(4) O.N. of A decreases by +5 and that of Cdecreases by +1

20. The oxidation number of Pt in[Pt(C2H4)Cl3]¯ is

(1) +1 (2) +2

(3) +3 (4) +4

21. In which of the following compounds ironhas lowest oxidation state –

(1) FeSO4.(NH4)2SO4.6H2O

(2) K4Fe(CN)6

(3) Fe(CO)5

(4) K2FeO4

22. Oxidation numbers of two Cl atoms inbleaching powder, CaOCl2 is –

(1) – 1,– 1 (2) + 1, – 1

(3) + 1,+ 1 (4) 0, – 1

23. Select the pair of oxidation processes,

(A) 2Cu2+ Cu22+

(B) MnO4– Mn2+

(C) [Fe(CN)6]–4 [Fe(CN)6]–3

(D) 2I¯ I2

(1) A, B (2) C, D

(3) A, D (4) B, C24. In the following reaction (unbalanced),

equivalent weight of As2S3 is related tomolecular weight M by :

As2S3 + H+ + NO3– NO + H2O + AsO4

3– + SO42–

(1) 2M (2)

4M

(3) 28M (4)

24M

25. In which of the following reactions the starredelement has undergone decrease in itsoxidation number

(1) Fe* + CuSO4 Cu + FeSO4

(2) H2* + Cl2 2HCl

(3) C* + H2O CO + H2

(4) Mn*O2 + 4HCl MnCl2 + Cl2 + 2H2O

26. Oxygen shows oxidation state of –1 in thecompound –

(1) NO2 (2) MnO2

(3) PbO2 (4) Na2O2

27. How many electrons should X2H4 liberate sothat in the new compound X shows oxidationnumber of –1/2 [E.N. of X. > H]

(1) 10 (2) 4

(3) 3 (4) 2

28. Which one is correctly matched:

Substance O.N. of S

(1) H2S + 2

(2) H2SO5 + 4

(3) H2SO4 + 4

(4) Na2S4O6 + 2.5

29. Eq. wt. of MnO4¯ in acidic, basic, neutral


REDOX REACTIONSmedium are in the ratio of -

(1) 3 : 5 : 15 (2) 5 : 3 : 1

(3) 5 : 1 : 3 (4) 3 : 15 : 5

30. The equivalent mass of MnSO4 is half of itsmolar mass when it is converted to -

(1) Mn2O3 (2) MnO2

(3) MnO4¯ (4) MnO42–

31. In the reaction

VO + Fe2O3 FeO + V2O5, the Eq. Wt.of V2O5 is equal to its -

(1) Mol. Wt. (2) Mol. Wt./8

(3) Mol. Wt./6 (4) None of these

32. Which of the following process represent thegain of 8 electrons per molecule -

(1) Conversion of 23 NOHNO

(2) Conversion of 33 NHHNO

(3) Conversion of 43 NHNH

(4) Conversion of NOON 52

33. The equivalent weight of Mohr's saltFeSO4. (NH4)2SO4. 6H2O in redox changeis equal to its

(1) Molecular weight/2

(2) Atomic weight

(3) Molecular weight/3

(4) Molecular weight

34. The oxidation state of C and N in HCN arerespectively -

(1) +2, –3 (2) –2, +3

(3) –2, +2 (4) +3, –3

35. In the reaction -

2 2 NaH H O NaOH H

(1) H– is oxidised

(2) Na+ is reduced

(3) Both NaH and H2O are reduced

(4) None

36. The number of electrons lost or gained duringthe change - 2 3 4 2Fe H O Fe O H

(1) 2 (2) 4

(3) 6 (4) 8

37. Equivalent weight of an oxidising agent ismolecular weight divided by -

(1) Number of electrons lost

(2) Number of electrons gained

(3) Number of H+ ions furnished by molecule

(4) Number of OH¯ ions that its formulacontains

38. In which of the following compound oxidationnumber of Cl is + 3

(1) ICI (2) ClO3–

(3) ClF3 (4) HClO4

39. A species that cannot be a reducing agentis

(1) SO2 (2) SO32–

(3) H2SO4 (4) S2–

40. Oxidation number of carbon in diamond is -

(1) – 4 (2) + 4

(3) 0 (4) + 2

41. 4 2 2 7 2 3 2 24heat(NH ) Cr O Cr O N H O

In the above reaction the oxidation state of Nand chromium changes respectively as -

(1) – 3 to 0 and + 6 to + 3

(2) + 3 to 0 and + 3 to + 6

(3) – 4 to 0 and + 6 to + 2

(4) Unpredictable

42. The eq. wt. of KMnO4 in the reaction :

MnO4¯ + Mn2+ + H2O MnO2 + H+

(unabalanced) is -

(1) 52.7 (2) 158

(3) 31.6 (4) None of these

43. The stoichiometric coefficients of - 2+4MnO , Pb

and 2H O in the balanced chemical equation ofthe reaction

2 22 4 2Mn PbO H MnO Pb H O ,

respectively are -

(1) 2, 5, 2 (2) 1, 4, 2

(3) 1, 2, 6 (4) 1, 2, 3

44. Nitric oxide acts as a reducing agent in thereaction

(1) OH6NO4O5NH4 223


REDOX REACTIONS

(2) OH4I3NO2 22 H8I6NO2 3

(3) 42232 SOHONSOHNO2

(4) OHSONSHNO2 222

45. The reaction 5H2O2 + XClO2 + 2OH– XCl–+ YO2 + 6H2O is balanced if(1) X = 5, Y = 2 (2) X = 2, Y = 5(3) X = 4, Y = 10 (4) X = 5, Y = 5

46. The number of moles of K2Cr2O7 that will beneeded to react completely with one mole offerric sulphite in acidic medium is(1) 0.4 (2) 0.6(3) 1.0 (4) 0.8

47. The oxidation number and the electronicconfiguration of sulphur in H2SO4 is(1) + 6; 1s22s22p6

(2) + 2; 1s22s22p63s23p2

(3) + 3; 1s22s22p23s23p1

(4) + 4; 1s22s22p63s2

48. The equivalent weight of KIO3 in the reaction2Cr(OH)3 + 4OH + KIO3 2CrO4

2– + 5H2O+ KI is

(1)Mol.wt.

3 (2)Mol.wt.

6

(3)Mol.wt.

2 (4) Molecular weight

49. In alkaline medium, KMnO4 reacts as follows2KMnO4+2KOH 2K2MnO4+H2O+ O2.There fore, the equivalent mass of KMnO4will be(1) 31.6 (2) 52.7(3) 7.0 (4) 158.0

50. The equivalent weight of phosphoric acid(H3PO4) in the reaction

NaOH + H3PO4 NaH2PO4 + H2O is(1) 25 (2) 98(3) 59 (4) 49

51. An element forms an oxide, in which theoxygen is 20% of the oxide by weight, theequivalent weight of the given element willbe(1) 32 (2) 40(3) 60 (4) 128

52. List-1 List-2 (Oxidation state)(A) +3 (1) Nitrogen(B) +1 (2) Nitrous oxide(C) 0 (3) Nitrate ion(D) +5 (4) Hydroxylamine

(5) Nitrite ionThe correct match is

A B C D A B C Da) 1 4 3 2 b) 5 2 4 3c) 4 5 3 1 d) 5 2 1 3

ANSWER KEY

01. 3 02. 4 03. 1 04. 1 05. 3 06. 1 07. 2 08. 2 09. 2 10. 2

11. 3 12. 3 13. 4 14. 1 15. 1 16. 4 17. 1 18. 1 19. 3 20. 2

21. 3 22. 2 23. 2 24. 3 25. 4 26. 4 27. 3 28. 4 29. 4 30. 2

31. 3 32. 2 33. 4 34. 1 35. 1 36. 4 37. 2 38. 3 39. 3 40. 3

41. 1 42. 1 43. 1 44. 2 45. 2 46. 3 47. 1 48. 1 49. 4 50. 2

51. 1 52. 4


REDOX REACTIONS

IIB EFFICIENCY ENHANCERS (LEVEL - III)01. Medium Equivalent weight of KMnO4

(A)Acidic (1) 158(B)Neutral (2) 79(C)Strongly basic (3) 52.6(D)Weakly basic (4) 31.6The correct match is(1) A–4, B–3, C–1, D–3(2) A–4, B–3, C–1, D–2(3) A–4, B–2, C–1, D–3(4) A–4, B–3, C–1, D–1

02. Photosynthesis of carbohydrates in plantstakes place as

Sunlight2 2 6 12 6 2 26CO +12H O C H O + 6O + 6H O

Equivalent weights of CO2 and C6H12O6respectively are(1) 11, 7.5 (2) 44, 90(3) 22, 15 (4) 44, 180

03. What is the equivalent weight of HNO3 in thegiven reaction?4Zn +10HNO3 4Zn(NO3)2 + NH4NO3 + 3H2O

(1)6310 (2)

639

(3)63 108 (4)

63 144

04. In the reaction of chlorine with dry slakedlime, the oxidation number of chlorinechanges(i) from –1 to +1 (ii) from +1 to –1(iii)from zero to –1 (iv) from zero to +1The correct combination is(1) Only ii & iii are correct(2) iii & iv are correct(3) i & ii are correct(4) All are correct

05. In the redox reaction:

4 3 2 22 3xMnO + yPbO + zHNO HMnO Pb(NO ) H O

(1) x = 2, y = 5, z = 10(2) x = 2, y = 7, z = 8(3) x = 2, y = 5, z = 8(4) x = 2, y = 5, z = 5

06. When copper is added to a solution of silvernitrate, silver is precipitated. This is due to

(i) Oxidation of silver(ii) Oxidation of copper(iii)Oxidation of silver(iv)Reduction of silver ionThe correct combination is(1) Only iii & iv are correct(2) i & ii are correct(3) ii, iv are correct(4) All are correct

07. Which of the following acid possessesoxidising, reducing and complex formingproperties?(1) HNO3 (2) H2SO4

(3) HCl (4) HNO2

08. The oxidation number of phosphorus insodium hypophosphite is(1) +3 (2) +2(3) +1 (4) –1

09. If 25.8 mL of 0.101 M K2Cr2O7 is requiredto titrate 10.0 mL of a liquid iron supplement,calculate the concentration of iron in vitaminsolution(1) 0.780 M (2) 0.261 M(3) 4.35 × 10–4 M (4) 1.56 M

10. Equivalent weight of N2 in the changeN2 NH3 is(1) 28/6 (2) 28(3) 28/2 (4) 28/3

11. In the following reaction.xKMnO4 + y NH3 KNO3 +MnO2 + KOH

+ H2O x and y are(1) x = 4, y = 6 (2) x = 8, y = 3(3) x = 8, y = 6 (4) x = 3, y = 8

12. CuS is dissolved in dil. HNO3. Balancedequation with correct products is(1) CuS + 2H+ + 3NO3

– Cu(NO3)2 + H2S + H2O + NO2

(2) 3CuS + 8H+ + 8NO3–

3Cu (NO3)2 + 3S + 4H2O + 2NO(3) CuS + 4NO3

– Cu(NO3)2 + H2S + H2O(4) None of the above is correct

13. In the reactionxHI + yHNO3 NO + I2 + H2O(1) x = 3, y = 2 (2) x = 2, y = 3


REDOX REACTIONS

(3) x = 6, y = 2 (4) x = 6, y = 114. When beryllium carbide is reacted with water,

gas X of characteristic smell is given out. Theoxidation state of heaviest constituentelement of gas X should be(1) – 1 (2) + 1(3) – 4 (4) –2

15. 25 mL of 0.50 M H2O2 solution is added to50 mL of 0.20 M KMnO4 in acid solution.Which of the following statements is true?(1) 0.010 mole of oxygen is liberated(2) 0.005 mole of KMnO4 are left(3) 0.030 g atom of oxygen gas is evolved(4) 0.0025 mole H2O2 does not react with KMnO4

16. Which of the following can act as an acid andas a base?(1) HClO3

– (2) H2PO4

(3) HS– (4) All of these17. One mole of CaOCl2 is dissolved in water

and excess of KI added. Hypo (Na2S2O3)required to react with the oxidised partcompletely is(1) 1 mole (2) 2.0 moles(3) 1.5 moles (4) 2.5 moles

18. Equivalent weight of KMnO4 and its reducedspecies in different mediums are given

Medium Equivalent weight

Reduced species

Acidic M/5 I Neutral M/3 II

Concentrated Alkaline

M/1 III

I, II and III are

I II III(1) MnO2 MnO4

2– Mn2+

(2) MnO2 Mn2+ MnO42–

(3) Mn2+ MnO2 MnO42–

(4) Mn2+ MnO42– MnO2

19. Based on the following reaction,XeO6

4–(aq) + 2F– (aq) + 6H+ (aq)

XeO3(g) + F2(g) + 3H2O( ) (G°< 0)It can be concluded that(1) oxidising power of F– is greater than that of XeO6

4–

(2) it is not a redox reaction(3) it is a disproportionation reaction(4) oxidising power of XeO6

4– is greater than thatof F–

20. 0.80 g of sample of impure potassiumdichromate was dissolved in water and madeup to 500 mL solution. 25 mL of this solutiontreated with excess of KI in acidic mediumand I2 liberated required 24 mL of a sodiumthiosulphate solution. 30 mL of this sodiumthiosulphate solution required 15 mL of N/20 solution of pure potassium dichromate.What was the percentage of K2Cr2O7 in givensample?(1) 73.5 % (2) 75.3 %(3) 36.75 % (4) None of these

21 An element A in a compound ABD hasoxidation number An–. It is oxidised byCr2O7

2– in acid medium. In the experiment1.68 × 10–3 moles of K2Cr2O7 was used for3.36 × 10–3 moles of ABD. The new oxidationnumber of A after oxidation is:(1) 3 (2) 3 – n(3) n – 3 (4) + n

ANSWER KEY

01. 1 02. 1 03. 3 04. 2 05. 1 06. 3 07. 1 08. 3 09. 4 10. 1

11. 2 12. 2,3 13. 3 14. 1 15. 2 16. 4 17. 2 18. 3 19. 4 20. 1

21 2


REDOX REACTIONS

IIB WORLD OF COMPETITION[NEET & AIPMT]

01. What is the change in oxidation number ofcarbon in the following reaction? [2020]

4 24(g ) (g) 4(l)CH Cl CCl +4HCl(g)

(1) +4 to + 4 (2) 0 to + 4(3) – 4 to + 4 (4) 0 to – 4

02. The correct structure of tribromooctaoxideis: [2019]

(1)

–

–

–

OO

O OO

O

O OBr Br Br (2)

OO

O OO

O

O OBr Br Br

(3) –

––

OO

O OO

O

O OBr Br Br (4)

–

––

–O

OO O

OO

O OBr Br Br

03. Which of the following reactions aredisproportionation reactions? [2019]

(i) 2 02Cu Cu Cu

(ii) 24 4 2 23MnO 4H 2MnO MnO 2H O

(iii) 4 2 4 2 22KMnO K MnO MnO O

(iv) 24 2 22MnO 3Mn 2H O 5MnO 4H

Select the correct option from the follwoing.(1) (i) and (iv) only(2) (i) and (ii) only(3) (i), (ii) and (iii)(4) (i), (iii) and (iv)

04. The oxidation state of Cr in CrO5 is [2019]

(1) –6 (2) + 12

(3) + 6 (4) + 4

05. The correct order of N-compounds in itsdecreasing order of oxidation states is[2018]

(1) HNO3, NO, N2, NH4Cl

(2) NHO3, NO, NH4 Cl, N2

(3) HNO3, NH4 Cl, NO, N2

(4) NH4Cl, N2, NO, HNO3

06. For the redox reaction,

24 2 4

2+2 2MnO C O H Mn +CO +H O

The correct coefficients of the reactants forthe balanced equation are : [2018]

4MnO 2-2 4C O H

(1) 16 5 2(2) 2 5 16(3) 2 16 5(4) 5 16 2

07. Hot concentrated sulphuric acid is amoderately strong oxidizing agent. Which ofthe following reactions does not showoxidizing behaviour? [2016](1) Cu + 2H2SO4 CuSO4 + SO2 +2H2O(2) S + 2H2 SO4 3SO2 + 2H2O(3) C + 2H2SO4 CO2 + 2SO2 + 2H4O(4) CaF2 + H2SO4 CaSO4 + 2HF

08. 24MnO in neutral aqueous medium is

disproportionate to [2015]

(1) 2/3 mole of 24MnO and 1/3 of MnO2

(2) 1/3 mole of 24MnO and 2/3 mole of MnO2

(3) 1/3 mole of Mn2O7 and 1/3 mole of MnO2

(4) 2/3 mole of Mn2O7 and 1/3 mole of MnO2

09. (I) H2O2 + O3 H2O + 2O2

(II)H2O2+AgO2Ag + H2O + O2

Role of hydrogen peroxide in the above reactions is respectively. [2014](1) Oxidizing in (I) and readueing in (II)(2) Reducing in (I) and oxidizing in (II)(3) Reducing in (I) and (II)(4) Oxidizing in (I) and (II)

10. The oxidation number of sulphur in H2S2O7and iron in K4Fe(CN)6 is respectively [2014](1) +6 and +2 (2) +2 and +2(3) +8 and +2 (4) +6 and +4

11. The pair of compounds that can exist togetheris: [2014](1) FeCl3, SnCl2

(2) HgCl2, SnCl2

(3) FeCl2, SnCl2

(4) FeCl3, KI12. The reaction of aqueous KMnO4 with H2O2

in acidic conditions gives [2014](1) Mn4+ and O2 (2) Mn2+ and O2

(3) Mn2+ and O3 (4) Mn4+ and MnO2


REDOX REACTIONS

13. In acidic medium, H2O2 changes Cr22-7O to

CrO5 which has two (– O – O –) bonds.Oxidation state of Cr in CrO5 is [2014](1) +5 (2) +3(3) +6 (4) -10

14. For decolourization of 1 mole of KMnO4, themole of H2O2 required is [2013](1) 1/2 (2) 3/2(3) 5/2 (4) 7/2

15. A mixture of potassium chlorate, oxalic acidand sulphuric acid is heated. During thereaction which element undergoes maximumchange in the oxidation number ? [2012](1) S (2) H(3) Cl (4) C

16. When Cl2 gas reacts with hot andconcentrated sodium hydroxide solution, theoxidation number of chlorine changes from

[2012](1) Zero to –1 and zero to +3(2) Zero to +1 and zero to –3(3) Zero to +1 and zero to –5(4) Zero to –1 and zero to +5

17. In which of the following compounds,nitrogen exhibits the highest oxidation state?

[2012](1) N3H (2) NH2OH(3) N2H4 (4) NH3

18. Standard reduction potentials of the halfreactions are given below [2012]F2(g) + 2e- 2F-(aq); E0 = +2.85 VCl2(g) + 2e- 2Cl-(aq); E0 = +1.36 VBr2(l) + 2e 2Br-(aq); E0 = +1.06 VI2(s) + 2e- 2I-(aq); E0 = +0.53 VThe strongest oxidation and reducing agents

respectively are(1) F2 and I- (2) Br2 and Cl-

(3) Cl2 and Br- (4) Cl2 and I2

19. Which of the following oxidation state is themost common among the lanthanoides?

[2010](1) 4 (2) 2(3) 5 (4) 3

20. Oxidatini no. of P in H4P2O5, H4P2O6, andH4P2O7 are respectively [2010]

(1) +3, +4, +5 (2) +4, +3, +5(3) +3, +5, +4 (4) +5, +3, +4

21. Which of the following reactions involvesoxidation reduction? [2010](1) NaBr + HCl NaCl + HBr(2) HBr + AgNO3 AgBr + HNO3

(3) H2 + Br2 2HBr(4) 2NaOH + H2SO4 Na2SO4 + 2H2O

22. Oxidation numbers of P in PO43–, of S in

SO42– and that of Cr in Cr2O7

–2 arerespectively. [2009](1) +3, + 6 and + 5(2) + 5, + 3 and + 6(3) –3, + 6 and + 6(4) +5, + 6 and + 6

23. What is the net charge on ferrous ion[2009](1) +2 (2) +3(3) +4 (4) +5

24. Identify the elements which can have highestoxidation numbers [2009](1) N (2) O(3) Cl (4) C

25. What is the oxidation number of sulphur inNa2S4O6 ? [2008]

(1)23 (2)

32

(3)35 (4)

52

26. The brown ring complex compound isformulated as [Fe(H2O)5NO]SO4. Theoxidation state of iron is [2007](1) 1 (2) 2(3) 3 (4) 0

27. The oxidation number of iron in the compoundK4[Fe(CN)6] is [2007](1) +6 (2) +4(3) +3 (4) +2

28. If HNO3 changes into N2O, the oxidationnumber is changed by [2005](1) +2 (2) -1(3) 0 (4) +4

29. An element which never a positive oxidationnumber in any of its compounds [2005](1) Boron (2) Oxygen(3) Chlorine (4) Fluorine


REDOX REACTIONS30. Which is the best description of the

behaviour of bromine in the reaction givenbelow? H2O + Br2 HOBr + HBr [2004](1) Proton acceptor only(2) Both oxidised and reduced(3) Oxidised only(4) Reduced only

31. Which of the following statements is correct? [2004]

(1) Hydrogen has oxidation number –1 and +1(2) Hydrogen has same electronegativity as

halogens(3) Hydrogen will not be liberated at anode(4) Hydrogen has same ionization potential as alkali

metals32. The oxidation number of carbon in CH2Cl2 is

[2004](1) 0 (2) +2(3) –2 (4) +4

33. Oxidation number of nickel in Ni(CO)4 is [2003]

(1) 0 (2) +4(3) –4 (4) +2

34. HNO2 acts both as reductant and oxidant,while HNO3 acts only as oxidant. It is due totheir [2003](1) solubility ability(2) maximum oxidation number(3) minimum oxidation number(4) minimum number of valence electrons

35. The oxidation states of sulphur in the anions23SO , S2O4

2– and S2O62– follow the order

[2003]

(1) 2 2 22 4 3 2 6S O SO S O

(2) 2 2 23 2 4 2 6SO S O S O

(3) 2 2 22 4 2 6 3S O S O SO

(4) 2 2 22 6 2 4 3S O S O SO

36. Which substance is serving as a reducingagent in the following reaction : [2002]

+ 2- 3+ 2+2 7 214H + Cr O + 3Ni 2Cr + 7H O + 3Ni

(1) H2O (2) Ni

(3) H+ (4) 22 7Cr O

37. In C + H2OCO + H2, H2O acts as [2002](1) oxidising agent (2) reducing agent(3) both (a) and (b) (4) none of these

38. Zn gives H2 gas with H2SO4 and HCl but notwith HNO3 because [2002](1) Zn acts as an oxidising agent when it reacts

with HNO3

(2) HNO3 is weaker acid than H2SO4 and HCl(3) In electrochemical series, Zn is above

hydrogen

(4) 3NO is reduced in preference to hydroniumion.

39. Identify the correct statement about H2O2

[2001](1) It acts as reducing agent only(2) It acts as both oxidising and reducing agent(3) It is neither an oxidiser nor reducer(4) It acts as oxidising agent only

40. Following reaction describes the rusting ofiron

4Fe + 3O2 4Fe3+ + 6O2-

Which one of the following statements isincorrect? [2001](1)This is an example of a redox reaction(2) Metallic iron is reduced to Fe3+

(3) Fe3+ is an oxidising agent(4) Metallic iron is a reducing agent

[JEE-Main]

41. The compound that cannot act both asoxidising and reducing agent is [2020](1) H2O2 (2) H2SO4

(3) HNO2 (4) H3PO4

42. The redox reaction among the following is [2020]

(1) Combination of dinitrogen with dioxygen at2000 K

(2) Formation of ozone from atmospheric oxygenin the presence of sunlight

(3) Reaction of H2SO4 with NaOH(4) Reaction of [Co(H12O)6]Cl3 with AgNO3

43. Oxidation number of potassium in K2O, K2O2and KO2, respectively, is [2020](1) +1, +4 and +2 (2) +1, +2 and +4

(3) +1, +1 and +1 (4) +2, +1 and +12


REDOX REACTIONS44. An example of a disproportionation reaction

is [2019](1) 2KMnO4 K2MnO4 + MnO2 + O2

(2) 2KMn 4O + 10I- + 16H+ 2Mn2+ + 5I2

+ 8H2O(3) 2CuBr CuBr2 + Cu(4) 2NaBr + Cl2 2NaCl + Br2

45. In order to oxidise a mixture one mole of eachof FeC2O4, Fe2(C2O4)3, FeSO4 and Fe2(SO4)3in acidic medium, the number of moles ofKMnO4 required is [2019](1) 3 (2) 2(3) 1 (4) 1.5

46. Iodine reacts with concentrated HNO3 to yieldY along with other products. The oxidationstate of iodine in Y, is [2019](1) 5 (2) 3(3) 1 (4) 7

47. In the reaction of oxalate with permaganatein acidic medium, the number of electronsinvolved in producing on molecule of CO2 is

[2019](1) 10 (2) 2(3) 1 (4) 5

48. In which of the following reaction H2O2 actsas a reducing agent [2014](a) H2O2 + 2H+ + 2e- 2H2O](b) H2O2 - 2e- O2 + 2H+

(c) H2O2 + 2e- 2HO-

(d) H2O2 + 2OH- - 2e- O2 + 2H2O(1) (1), (2) (2) (3), (4)(3) (1), (3) (4) (2), (4)

49. The reaction of white phosphorus withaqueous NaOH gives phosphine along withanother phosphorus containing compound.The oxidation states of phosphorus inphosphine and the other product arerespectively [2012](1) Redox reaction; -3 and -5(2) Redox reaction; +3 and +5(3) Disproportionation reaction; -3 and +5(4) Disproportionation reaction; -3 and +3

50. Oxidation states of the metal in the mineralshaematite and magnetite, respectively, are

[2011](1) II, III in haematite and III in magnetite(2) II, III in haematite and II in magnetite(3) II in haematite and II, III in magnetite(4) III in haematite and II, III in magnetite

ANSWER KEY

01. 3 02. 2 03. 2 04. 3 05. 1 06. 2 07. 4 08. 1 09. 3 10. 1

11. 3 12. 2 13. 3 14. 3 15. 3 16. 4 17. 1 18. 1 19. 4 20. 1

21. 3 22. 4 23. 1 24. 3 25. 4 26. 2 27. 4 28. 4 29. 4 30. 2

31. 1 32. 1 33. 1 34. 2 35. 1 36. 2 37. 1 38. 4 39. 2 40. 2

41. 4 42. 1 43. 3 44. 3 45. 2 46. 1 47. 3 48. 4 49. 3 50. 4

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