Chapter 14

Oxidation and Reduction

Oxidation - addition of Oxygen.Reduction - removal of Oxygen.

In terms of electrons Oxidation is the loss of electrons.

(OILRIG)

Reduction is the gain of electrons

Oxidising and reducing agents

An oxidising agent causes another substance to become oxidised and, in the process, the oxidising agent is reduced. Common oxidising agents include O2, Cl2, ClO-, KMnO4 and H2O2

A reducing agent causes another substance to become reduced and, in the process, the reducing agent is oxidised. Common reducing agents include CO, SO2, SO3

2-

Oxidising agents are useful in Sterilising and Bleaching.

Experiments: (a) Halogens as Oxidising agents

(b) Displacement

reactions of Metals. (See Book Pages 184-186)

Oxidation Numbers

The oxidation number of an atom is the charge that the atom has or appears to have when electrons are distributed according to certain rule.

Rules

The ON of an element is 0.

The ON of an ion is the charge that it has.

The ON of F is always -1.

The ON of O IS -2 except OF2 (+2) and H2O2 (-1).

The ON of H is +1 except with metal hydrides (-1)

Oxidation and reduction in terms of Oxidation numbers

Oxidation is an increase in Oxidation number.

Reduction is a decrease in Oxidation number.

Balancing Redox reactions (See Book Page 187-188) + HANDOUT

2.6.2Balancing equations using

Oxidation Numbers

Chemical reactions are all about the movement of electrons

When an equation is balanced the electron movements must balance

Sometimes it is possible to balance an equation in more than one way

It is therefore possible to make the numbers of atoms balance but not the electron movement

Using oxidation numbers prevents this

Sequence of events

Write out the formulae of all reactants and products

Assign oxidation numbers to all atoms Pick out the atoms whose oxidation numbers

change Write down the electron movements Balance the electron movements Enter the numbers from balanced electron

movement into the overall equation Balance equation by inspection - [do H last]

MnO4- + Fe2+ + H+ = Mn2+ + Fe3+ + H2O

+7 -2 +2 +1 +2 +3 +1 -2+7 +2+2 +3

OxidationReduction

Mn = Mn2+

+7 +2

Fe2+ = Fe3+

+2 +3

5 5

55 48

+ 5 e- - 1 e- 5

1. Assign oxidation numbers2. Select those which change

3. (a) Assign Oxidation – increase in Ox. No

(b) Assign Reduction – decrease in Ox. No

5. Write electron gains and losses

6. Balance electron transfers

7. Put numbers up into the equation8. Balance equation by inspection - leave H+ till last

Sequence of events

Write down reactants and products Assign oxidation numbers Identify oxidation and reduction Write down changes and electrons

involves Balance e- change Put changes into equation Balance equation by inspection [H

last]

C2H5OH + Cr2O72-+ H+ = CH3COOH + Cr3++ H2O

-2 +1-2+1 +6 -2 +1 0 +1 0 -2-2+1 +3 +1-2

-2

0+6 +3

OxidationReduction

C2 = C2

-2 0Cr2O7

2- = Cr3+

+6 +3

2

33 42

- 2 e-

+ 3 e-

0

3 3 4

6 2

12

12

x3

x24

11 16

1. Assign oxidation numbers2. Select those which change

3. (a) Assign Oxidation – increase in Ox. No

(b) Assign Reduction – decrease in Ox. No

5. Write electron gains and losses

6. Balance electron transfers

7. Put numbers up into the equation8. Balance equation by inspection - leave H+ till last :- O 3+(2*7) =

(3*2) + x x=11 H 3(6*1) + X = (3*4) +

11(2*1) x= 16

Examples

1. C2H5OH + MnO41- + H+ = CH3CHO + Mn2+ + H2O

2. C2H5OH + MnO41- + H+ = CH3COOH + Mn2+ + H2O

3. Cr2O72- + Fe2+ + H+ = Cr3+ + Fe3+ + H2O

4. MnO41- + C2O4

2- + H+ = CO2 + Mn2+ + H2O

5. Fe2+ + Cl2 = Fe3+ + Cl-

6. MnO41- + H2O2 + H+ = Mn2+ + O2 +H2O

7. Cr2O72- + CH3OH + H+ = Cr3+ + HCHO + H2O

8. Cr2O72- + CH3CHO + H+ = Cr3+ + CH3COOH + H2O

Answers

1. 5C2H5OH + 2MnO41- + 6H+ = 5CH3CHO + 2Mn2+ +

8H2O

2. 5C2H5OH + 4MnO41- + 12H+ = 5CH3COOH + 4Mn2+

+ 11H2O

3. Cr2O72- + 6Fe2+ + 14H+ = 2Cr3+ + 6Fe3+ + 7H2O

4. 2MnO41- + 5C2O4

2- + 16H+ = 10CO2 + 2Mn2+ + 8H2O

5. 2Fe2+ + Cl2 = 2Fe3+ + 2Cl-

6. 2MnO41- + 5H2O2 + 8H+ = 2Mn2+ + 5O2 +8H2O

7. Cr2O72- + CH3OH + H+ = Cr3+ + HCHO + H2O

8. Cr2O72- + CH3CHO + H+ = Cr3+ + CH3COOH + H2O