oxidation and reduction - franklychemistry€¦ · the terms oxidant and reductant can be used to...

OXIDATION AND REDUCTIONAS TOPIC 8

OXIDATION NUMBERS I

OXIDATION NUMBERS II

INTRODUCTION TO REDOX

OXIDISING AGENTS AND REDUCING AGENTS

IONIC HALF-EQUATIONS AND THEIR COMBINATION

METALS WITH ACIDS

RELATIONSHIP BETWEEN CHANGE IN OXIDATION NUMBERAND REACTION STOICHIOMETRY

DISPROPORTIONATION

EXTRACTION OF ALUMINIM

EXTRACTION OF METALS BY REDUCTION OF OXIDEOR CHLORIDE

BLAST FURNACE

TEST QUESTION I

TEST QUESTION II

Questionsheet 1

Questionsheet 2

Questionsheet 3

Questionsheet 4

Questionsheet 5

Questionsheet 6

Questionsheet 7

Questionsheet 8

Questionsheet 9

Questionsheet 10

Questionsheet 11

Questionsheet 12

Questionsheet 13

QUESTIONSHEETS AS LevelCHEMISTRY

27 marks

18 marks

17 marks

22 marks

14 marks

18 marks

15 marks

17 marks

18 marks

15 marks

17 marks

16 marks

14 marks

Curriculum Press – Licence Agreement:Paper copies of the A-Level Chemistry Questionsheetsmay be copied free of charge by teaching staff or studentsfor use within their school, provided the PhotocopyMasters have been purchased by their school. No part ofthese Questionsheets may be reproduced or transmitted,in any other form or by any other means, without theprior permission of the publisher. All rights are reserved.This license agreement is covered by the laws of Englandand Wales © Curriculum Press March 2008.

Curriculum Press • Bank House • 105 King Street • Wellington • Shropshire • TF1 1NU

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AuthorsTrevor Birt John BrockingtonDonald E Caddy Kevin FrobisherAndrew Jones Andy ShepherdAdrian Bond

EditorsJohn BrockingtonStuart Barker

Do notwrite inmargin

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OXIDATION NUMBERS I

27

TOPIC 8 Questionsheet 1

a) Work out the oxidation number of nitrogen in each of the following substances.

(i) NO .......................................................... (v) NH3

.................................................................

(ii) NO2

.......................................................... (vi) HNO3

.................................................................

(iii) NF3

.......................................................... (vii) N2

.................................................................

(iv) N2O

5.......................................................... (viii) N

2O .................................................................

[8]b) Work out the oxidation number of chlorine in each of the following substances.

(i) Cl2O ......................................................... (iii) ClO

2...................................................................

(ii) Cl2O

7......................................................... (iv) KCl ...................................................................

[4]

c) Work out the oxidation number of the named elements in the following ions:

(i) Chlorine in ClO - ............................. (vi) Chlorine in ClO4- .............................

(ii) Chromium in CrO4

2- ............................. (vii) Chromium in Cr2O

72- .............................

(iii) Aluminium in AlF63- ............................. (viii) Copper in CuF

42- .............................

(iv) Oxygen in OH - ............................. (ix) Oxygen in O2

2-. .............................

(v) Phosphorus in PO43- ............................. (x) Nitrogen in NO

2- .............................

[10]d) Complete the following names by adding the appropriate Roman numeral:

(i) SO4

2- sulfate( ) ion

(ii) SO3

2- sufate( ) ion

(iii) CuO copper( ) oxide

(iv) Cu2O copper( ) oxide

(v) NaClO3

sodium chlorate( )[5]


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OXIDATION NUMBERS II


a) Calculate the oxidation number of the stated element in each of the following species.

(i) Mn in MnO4

2- ........................................................................................................................................ [1]

(ii) N in HNO3

........................................................................................................................................ [1]

(iii) S in S2O32- ........................................................................................................................................ [1]

(iv) Cr in Cr2O

72- ........................................................................................................................................ [1]

(v) V in VO2+ ........................................................................................................................................ [1]

b) What exceptions are there to the rule that the oxidation number of oxygen in its compounds is -2, while that ofhydrogen is +1?

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

c) Why is the oxidation number of fluorine in its compounds always -1?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

d) NH3 and AsH

3 are related compounds in that they are both covalent hydrides of Group 5 elements.

Why is it that hydrogen has an oxidation number of +1 in NH3 but -1 in AsH3?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

e) Explain why ammonia dissolving in water does not involve a redox change

...........................................................................................................................................................................

....................................................................................................................................................................... [1]

f) When a species is oxidised or reduced, what relationship is there between the change in oxidation number ofthe central atom and the number of electrons lost or gained? Illustrate your answer by referring to the MnO

4-

and Cr2O

72- ions, both in acidic solution.

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [5]

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INTRODUCTION TO REDOX


a) (i) Define oxidation and reduction in terms of electrons.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(ii) Explain why it is that oxidation and reduction always occur together.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

b) (i) Oxidation used to be defined as ‘combination with oxygen’. Explain why, even though the definitionhas now been broadened, it is still generally true to say that combination with oxygen is oxidation.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

(ii) When oxygen reacts with fluorine, converting it into oxygen difluoride, OF2, does the fluorine become

oxidised? Explain your answer.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

c) For each of the following, state whether or not it is a redox reaction (Y/N) and give a reason if it is or a reactiontype if not:I HCl(aq) + NaOH(aq) → NaCl(aq) + H

2O(l)

Redox (Y/N)? ........................................................................................................................................ [1]

Reason/reaction type ........................................................................................................................... [1]

II AgNO3(aq) + KCl(aq) → AgCl(s) + KNO

3(aq)

Redox (Y/N)? ........................................................................................................................................ [1]


III Ca(s) + Cl2(g) → CaCl

2(s)

Redox (Y/N)? ........................................................................................................................................ [1]


IV PCl3(l) + 3H

2O(l) → H

3PO

3(aq) + 3HCl(aq)

Redox (Y/N)? ........................................................................................................................................ [1]


17


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22

OXIDISING AGENTS AND REDUCING AGENTS

a) Oxidation and reduction are brought about by substances referred to as oxidising agents and reducing agents.Define each of these substances in terms of electrons.

Oxidising agent ............................................................................................................................................. [1]

Reducing agent .............................................................................................................................................. [1]

b) Why is it that, during a redox reaction, the oxidising agent always becomes reduced, while the reducing agentbecomes oxidised?

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

c) Some oxidising agents, e.g. potassium dichromate(VI), function much better in acidic solution than in neutralsolution. Explain this.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

d) Explain why hydrochloric acid can be used for titrations using dichromate(VI) but not with manganate(VII)

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

The terms oxidant and reductant can be used to mean ‘oxidising agent’ and ‘reducing agent’ respectively.During a redox reaction, the oxidant becomes reduced and the reductant becomes oxidised.

e) Complete the following table by inserting the formulae of the species to which the given oxidants are reduced,and any colour change, precipitate formation or evolution of gas which is observed.


[8]

Oxidant Species on reduction Observation

MnO4- in acidic solution

Cr2O

72- in acidic solution

Bromine water

Iodine solution

f) Complete the next table to show (by formula) the species to which the given reductants are oxidized.

Reductant Species on oxidation

I- in neutral solution

Zn in acidic solution

Fe2+ in acidic solution[3]


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14

IONIC HALF-EQUATIONS AND THEIR COMBINATION

Throughout this Questionsheet, for simplicity, state symbols may be omitted.

a) Write ionic half-equations for the following oxidation changes.

(i) Ethanedioic acid to carbon dioxide.

....................................................................................................................................................................... [1]

(ii) Sulfite ions to sulfate ions.

....................................................................................................................................................................... [1]

(iii) Chlorine to chlorate(I) during disproportionation.

....................................................................................................................................................................... [1]

(iv) Hydrogen peroxide to oxygen.

....................................................................................................................................................................... [1]

b) Write ionic half-equations for the following reduction changes.

(i) Manganate(VII) ions to manganese(II) ions in acidic solution.

....................................................................................................................................................................... [1]

(ii) Iodate(V) ions to iodine in acid.

....................................................................................................................................................................... [1]

(iii) Dichromate(VI) ions to chromium(III) ions in acidic solution.

....................................................................................................................................................................... [1]

(iv) Aqueous nitrate ion to ammonia.

....................................................................................................................................................................... [1]

c) Combine the ionic half-equations from a)(ii) and b)(i) to obtain the complete ionic equation for the redoxreaction between manganate(VII) ions and sulfite ions in acidic solution.

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

d) Titanium dissolves in concentrated hydrochloric acid to give titanium(III) chloride and hydrogen. Constructan ionic equation for this reaction by writing down two ionic half-equations and then combining them.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]



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Whenever a metal dissolves in an acid, a redox reaction occurs. The metal is always the reductant, but the identityof the oxidant (i.e. the ion or molecule which oxidises the metal) depends on the particular acid which is used.a) This part of the Questionsheet refers to zinc dissolving in dilute sulfuric acid, which you can assume is fully

dissociated into H+ and SO42- ions. The products are zinc sulfate and hydrogen.

(i) How could you test for the evolution of hydrogen?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(ii) Write two ionic half-equations for the change.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(iii) Combine the half-equations into an overall ionic equation.

....................................................................................................................................................................... [1]

(iv) Identify the oxidant in this reaction ...................................................................................................... [1]

b) This part refers to zinc dissolving in concentrated sulfuric acid (98% H2SO

4). For the purposes of this

Questionsheet, you should assume it is undissociated, i.e. it consists entirely of H2SO

4 molecules. The products

are zinc sulfate, sulfur dioxide and water.(i) Describe a chemical test for the evolution of sulfur dioxide.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(ii) Write two ionic half-equations for the change.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(iii) Combine the half-equations into an overall ionic equation.

....................................................................................................................................................................... [1]

(iv) Identify the oxidant in this reaction ...................................................................................................... [1]

c) The reaction between zinc and concentrated nitric acid (which you can assume is dissociated into H+ and NO3-

ions) is given by the following equation: 4Zn(s) + 10HNO3(aq) → 4Zn(NO

3)

2(aq) + NH

4NO

3(aq) + 3H

2O(l)

(i) Rewrite this equation, first as a complete ionic equation, and then as two ionic half-equations.

Complete ionic equation ....................................................................................................................... [1]

Ionic half-equations ..................................................................................................................................

............................................................................................................................................................... [2]

(ii) Identify the oxidant in this reaction ...................................................................................................... [1]

d) When copper dissolves in concentrated nitric acid, the principal nitrogen-containing product is NO2 rather

than NH4NO

3. How do you account for this?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

METALS WITH ACIDS

18



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15

RELATIONSHIP BETWEEN CHANGE IN OXIDATION NUMBERAND REACTION STOICHIOMETRY

Carbon monoxide can be prepared by reacting a metal carbonate with hydrazine, N2H

4, as shown by the ‘unbalanced

equation’ referred to as Scheme X.

Scheme X N2H

4 + CO

32- → N

2 + CO + OH-

a) Assign an oxidation number to each element (i.e. N, H, C & O) in all species of Scheme X.

N2H

4+ CO

32- → N

2+ CO + OH-

N = .............. C = ............... N = ................ C = ............... O =..............

H = .............. O = .............. O = .............. H =............. [5]

b) Hence state which species becomes oxidised and which is reduced.

Species oxidised ............................................................................................................................................. [1]

Species reduced ............................................................................................................................................. [1]

c) For hydrazine, write down the following.

(i) The increase in oxidation number of nitrogen ...................................................................................... [1]

(ii) The number of N atoms which are oxidised in one molecule of N2H

4................................................. [1]

(iii) Hence, the number of electrons which are lost by one molecule of N2H

4............................................ [1]

d) For the carbonate ion, write down the following.

(i) The decrease in oxidation number of carbon ........................................................................................ [1]

(ii) The number of C atoms which are reduced in one CO32- ion ................................................................ [1]

(iii) Hence, the number of electrons which are gained by one CO32- ion ..................................................... [1]

e) Given that the number of electrons which are lost on oxidation must be equal to the number of electrons whichare gained on reduction, state how many carbonate ions are reduced for each molecule of hydrazine which isoxidised.

....................................................................................................................................................................... [1]

f) Use this information to convert Scheme X into a balanced equation.

....................................................................................................................................................................... [1]



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17

DISPROPORTIONATIONa) Define the term disproportionation.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

b) (i) Write an ionic equation for the reaction that occurs when chlorine dissolves in a cold aqueous solutionof sodium hydroxide.

....................................................................................................................................................................... [1]

(ii) Explain this reaction in terms of disproportionation. Support your answer by referring to the changesthat occur in the oxidation number of chlorine.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

c) One of the reaction products of b)(i) in turn disproportionates when heated to temperatures above 70 oC.Explain why, when this happens, the oxidation number of chlorine increases by 4 but decreases by only 2.

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [4]

d) Copper(I) compounds (except for complexes) are unstable in aqueous solution and decompose bydisproportionation. Suggest what you would expect to observe when the white soluble salt, copper(I) sulfate,dissolves in water and write a chemical equation for the reaction.

Observations .................................................................................................................................................. [2]

Equation ........................................................................................................................................................ [1]

e) Manganate(x) disproportionates into manganese(IV) oxide and manganate(VII) in acid, the manganese productsbeing in the ratio 1:2

(i) Deduce a value for x and give a formula for the original ion

....................................................................................................................................................................... [2]

(ii) Write an ionic equation for the reaction.

....................................................................................................................................................................... [2]



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18

EXTRACTION OF ALUMINIUM

c) Draw a diagram to show the electrolysis cell used to extract aluminium from purified bauxite, and label it to show:(i) the composition and temperature of the electrolyte, [2](ii) the anode and cathode, and the materials from which they are made, [2](iii) how aluminium is removed. [1]

d) Write an ionic equation for the reaction occurring at each electrode.

Anode reaction............................................................................................................................................... [1]

Cathode reaction ........................................................................................................................................... [1]

e) Why are these plants often situated in mountainous regions?

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

f) Give three major environmental concerns associated with the overall process of extracting aluminium frombauxite.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

g) List four main variable costs associated with the process.

...........................................................................................................................................................................

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [4]


a) State one general advantage and one disadvantage of the method of electrolytic reduction over the reductionof ores by carbon.

Advantage ..................................................................................................................................................... [1]

Disadvantage ............................................................................................................................................... [1]

b) Why can aluminium not be obtained by reducing the oxide with carbon?

....................................................................................................................................................................... [1]


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15

EXTRACTION OF METALS BY REDUCTION OF OXIDE OR CHLORIDE

a) In the extraction of lead, galena, PbS, is first converted into lead(II) oxide, PbO, and then reduced by carbon togive the metal and carbon dioxide

(i) How is a sulfide ore converted to the corresponding oxide?

....................................................................................................................................................................... [1]

(ii) Write an equation for the conversion of lead(II) sulfide to lead(II) oxide.

....................................................................................................................................................................... [1]

(ii) Write an equation for the reduction of the oxide.

....................................................................................................................................................................... [1]

b) Although titanium can be extracted by reducing TiO2 with carbon, the method is unsatisfactory. Explain why.

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

c) In the Kroll process, TiO2 is first converted into TiCl

4, which is then reduced to the metal.

(i) How is TiO2 converted into TiCl

4?

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [3]

(ii) How is the TiCl4 purified?

....................................................................................................................................................................... [1]

(iii) What reducing agent is used to convert TiCl4 to titanium?

....................................................................................................................................................................... [1]

(iv) Why must the reduction be carried out in the absence of air?

....................................................................................................................................................................... [1]

d) Tungsten (VI) oxide is obtained from the treatment of tungsten ores with alkali.

(i) Suggest an equation for the reduction of the oxide with hydrogen

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

(ii) Hydrogen is less economic than using carbon. Suggest why hydrogen is employed as the reducing agent

....................................................................................................................................................................... [1]

(iii) Suggest a risk associated with the process

....................................................................................................................................................................... [1]



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a) (i) Name the three materials that are fed into the top of a blast furnace.

....................................................................................................................................................................... [3]

(ii) What other material is essential to the operation of the furnace?

....................................................................................................................................................................... [1]

b) (i) State the two reducing agents which take part in reactions in the blast furnace.

....................................................................................................................................................................... [2]

(ii) Only one of these reducing agents is fed into the furnace: the second is formed from it. Write twochemical equations to show how this happens.

Equation 1 ............................................................................................................................................. [1]

Equation 2 ............................................................................................................................................. [1]

c) Write chemical equations to show the formation of iron from its ore:(i) in the upper, cooler regions of the furnace.

....................................................................................................................................................................... [1]

(ii) in the lower, hotter regions.

....................................................................................................................................................................... [1]

BLAST FURNACE


(Continued...)


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BLAST FURNACE

TOPIC 8 Questionsheet 11 Continued

d) Write two equations to show how one of the materials in the blast furnace removes sand (SiO2) from the iron

ore.

Equation 1 .................................................................................................................................................... [1]

Equation 2 .................................................................................................................................................... [1]

e) Why is it beneficial, at the bottom of the furnace, to have a layer of molten slag floating on molten iron?

....................................................................................................................................................................... [1]

f) State two features of a blast furnace which improve its energy efficiency.

Feature 1 ....................................................................................................................................................... [1]

Feature 2 ....................................................................................................................................................... [1]

g) Suggest why a blast furnace is wider at the base than it is at the top.

...........................................................................................................................................................................

...........................................................................................................................................................................

....................................................................................................................................................................... [2]

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TEST QUESTION I

16

a) Nitric acid is manufactured by the Ostwald process, which involves the following three stages.Stage 1 Oxidation of ammonia with oxygen over a hot platinum-iridium catalyst:

4NH3(g) + 5O

2(g) → 4NO(g) + 6H

2O(g)

Stage 2 Conversion of nitrogen monoxide to nitrogen dioxide with oxygen at a lower temperature:

2NO(g) + O2(g)→ 2NO

2(g)

Stage 3 Dissolving nitrogen dioxide in water in the presence of oxygen:

4NO2(g) + 2H

2O(l) + O

2(g) → 4HNO

3(aq)

(i) Write down the change in oxidation number of nitrogen at each of the three stages.

Change at Stage 1 ................................................................................................................................. [1]

Change at Stage 2 ................................................................................................................................. [1]

Change at Stage 3 ................................................................................................................................. [1]

(ii) If nitrogen dioxide were to be dissolved in water in the absence of oxygen, it would give a mixture ofnitric acid, HNO

3, and nitrous acid, HNO

2. Write an equation for this reaction, state the oxidation number

of nitrogen in NO2, HNO

3 and HNO

2, and hence deduce the type of reaction that occurs.

Equation ................................................................................................................................................ [1]

Oxidation number of N in : NO2

....................... HNO3

......................... HNO2

.......................... [2]

Type of reaction ..................................................................................................................................... [1]

b) As a powerful oxidising agent, concentrated nitric acid presents serious health and safety hazards whenever itis used in the laboratory. Name three major hazards and state what precautions you would take to minimise thedanger in each case. (Do not mention ‘goggles’ and ‘laboratory overalls’, which must always be worn in thelaboratory.)

Hazard 1 ........................................................................................................................................................ [1]

Precaution ..................................................................................................................................................... [1]

Hazard 2 ........................................................................................................................................................ [1]

Precaution ..................................................................................................................................................... [1]

Hazard 3 ........................................................................................................................................................ [1]

Precaution ..................................................................................................................................................... [1]

c) ‘Starch-iodide’ papers are commonly used in the laboratory as a test for oxidising agents. (They are strips ofpaper impregnated with potassium iodide and starch.) State what is observed when starch-iodide paper isdipped into a solution of an oxidising agent and explain the chemistry of the test.

Observation ................................................................................................................................................... [1]

Chemistry ...................................................................................................................................................... [2]



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TEST QUESTION II

14

a) Chromium occurs naturally as a mineral called chromite or chrome ironstone, which can be regarded as amixed oxide of iron and chromium, FeO.Cr

2O

3. Pure chromium is obtained from this by a four stage process

summarised by the following diagram:

FeO.Cr2O

3 K

2CrO

4 K

2Cr

2O

7 Cr

2O

3 Cr

Complete the table below to show, for each stage, the change in oxidation number of chromium and hencestate which stage does not involve oxidation-reduction.

[4]

Stage which does not involve oxidation-reduction ........................................................................................ [1]

b) Magnesium chloride, which is electrolysed to produce magnesium, is manufactured by passing chlorine overa heated mixture of magnesium oxide and carbon :

MgO(s) + C(s) + Cl2(g) → MgCl

2(s) + CO(g)

State which element is oxidised and which is reduced, and hence identify the oxidising agent and reducingagent in this reaction.

Element oxidised ........................................................................................................................................... [1]

Element reduced ............................................................................................................................................ [1]

Oxidising agent ............................................................................................................................................. [1]

Reducing agent .............................................................................................................................................. [1]

c) TiO2, known in industry as ‘titanium dioxide’, dissolves in concentrated sulfuric acid to give an orange solution

of ‘titanyl sulfate’, TiO2+ SO4

2-. It also reacts with fused potassium hydroxide to give potassium titanate,(K+)

2TiO

32-.

(i) Write down the oxidation number of titanium in each of the following :

TiO2

........................................................................................................................................................ [1]

TiO2+ ........................................................................................................................................................ [1]

TiO3

2- ........................................................................................................................................................ [1]

(ii) Hence state whether each of these conversions involves ‘oxidation’, ‘reduction’ or ‘neither’.

TiO2 → TiO2+ ........................................................................................................................................... [1]

TiO2 → TiO

32- ........................................................................................................................................ [1]

Change in oxidation number of Cr

From To

Stage 1

Stage 2

Stage 3

Stage 4

Stage 1

Heat in airwith K

2CO

3

Stage 2 Stage 3 Stage 4

H2SO

4 Heat withcarbon

Al


oxidation and reduction - franklychemistry€¦ · the terms oxidant and reductant can be used to...

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