Oxidation and Reduction

1
Oxidation and Reduction

• Chapter 14

2
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

3
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, SO32-
• Oxidising agents are useful in Sterilising and
  Bleaching.

4
Experiments (a) Halogens as Oxidising agents
(b) Displacement
reactions of Metals. (See Book Pages 184-186)
5
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)

6
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

7
2.6.2Balancing equations using Oxidation
Numbers
8

• 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

9
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

10
1. Assign oxidation numbers

• (a) Assign Oxidation increase in Ox. No
• (b) Assign Reduction decrease in Ox. No

2. Select those which change
5
5
4
8

• MnO4- Fe2 H Mn2 Fe3 H2O
• 7 -2 2 1 2 3
  1 -2

7
2
2
3
Oxidation
Reduction
5. Write electron gains and losses
6. Balance electron transfers
Fe2 Fe3 2 3
5
5
- 1 e-
5
Mn Mn2 7 2
5 e-
7. Put numbers up into the equation
8. Balance equation by inspection - leave H
till last
11
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

12
1. Assign oxidation numbers

• (a) Assign Oxidation increase in Ox. No
• (b) Assign Reduction decrease in Ox. No

2. Select those which change

• C2H5OH Cr2O72- H CH3COOH Cr3
  H2O
• -2 1-21 6 -2 1 0
  1 0 -2-21 3 1-2

3
3
4
2
11
16
-2
3
0
6
0
Reduction
Oxidation
5. Write electron gains and losses
6. Balance electron transfers
3
3
4
12
x3
C2 C2 -2
0
- 2 e-
x2
2
4
Cr2O72- Cr3 6
3
2
3 e-
6
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7. Put numbers up into the equation

• Balance equation by inspection - leave H till
  last - O 3(27) (32) x ? x11
• 
  H ? 3(61) X (34)
  11(21) ? x 16

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Examples

• C2H5OH MnO41- H CH3CHO Mn2 H2O
• C2H5OH MnO41- H CH3COOH Mn2 H2O
• Cr2O72- Fe2 H Cr3 Fe3 H2O
• MnO41- C2O42- H CO2 Mn2 H2O
• Fe2 Cl2 Fe3 Cl-
• MnO41- H2O2 H Mn2 O2 H2O
• Cr2O72- CH3OH H Cr3 HCHO H2O
• Cr2O72- CH3CHO H Cr3 CH3COOH H2O

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Answers

• 5C2H5OH 2MnO41- 6H 5CH3CHO 2Mn2 8H2O
• 5C2H5OH 4MnO41- 12H 5CH3COOH 4Mn2
  11H2O
• Cr2O72- 6Fe2 14H 2Cr3 6Fe3 7H2O
• 2MnO41- 5C2O42- 16H 10CO2 2Mn2 8H2O
• 2Fe2 Cl2 2Fe3 2Cl-
• 2MnO41- 5H2O2 8H 2Mn2 5O2 8H2O
• Cr2O72- CH3OH H Cr3 HCHO H2O
• Cr2O72- CH3CHO H Cr3 CH3COOH H2O