3.1.7 Oxidation, reduction and redox equations

Question 1

What is oxidation?

Answer 1

Oxidation is the process of electron loss:
Zn → Zn^2+ + 2e^-
It involves an increase in oxidation number (more positive).

Question 2

What is reduction?

Answer 2

Reduction is the process of electron gain:
Cl2 + 2e^- → 2Cl^-
It involves a decrease in oxidation number (more negative).

Question 3

What are the rules for assigning oxidation numbers?

Answer 3

1. All uncombined elements have an oxidation number of zero
2. The oxidation numbers of the elements in a compound add up to zero
3. The oxidation number of a monoatomic ion is equal to the ionic charge
4. In a polyatomic ion (CO3^2-) the sum of the individual oxidation numbers of the elements adds up to the charge on the ion
5. Several elements have invariable oxidation numbers in their common compounds:
   Group 1 metals = +1
   Group 2 metals = +2
   Al = +3
   H = +1 (except in metal hydrides where it is -1 e.g. NaH)
   F = -1
   Cl, Br, I = -1 except in compounds with oxygen and fluorine
   O = -2 except in peroxides (H2O2) where it is -1 and in compounds with fluorine.

Question 4

What is a reducing agent?
What is an oxidising agent?

Answer 4

Reducing agents - electron donors
Oxidising agents - electron acceptors

Question 5

In the equation

Br2(aq) + 2l^-(aq) → I2(aq) + 2Br^-(aq)

Which element has been oxidised, which has been reduced?

Answer 5

Br has reduced as it has gained electrons.
I has oxidised as it has lost electrons.

The oxidising agent is Bromine water. It is an electron acceptor.
The reducing agent is the lodide ion. It is an electron donor

Question 6

What are half equations?

Answer 6

A reduction half equation only shows the parts of a chemical equation involved in reduction. The electrons are on the left.
An oxidation half equation only shows the parts of a chemical equation involved in oxidation. The electrons are on the right.

Question 7

What are the steps to writing half equations?

Answer 7

1. Work out oxidation numbers for element being oxidised/reduced on each side of equation.
2. Balance equation (changes oxidation numbers)
3. Add electrons equal to the change in oxidation number
4. Sort out Os (Add H2Os)
5. Sort out Hs (Add H^+s)
6. Check the charges are balanced.

Question 8

How do you combine half equations to form a full equation?

Answer 8

To make a full redox equation combine a reduction half equation with an oxidation half equation.
To combine two half equations there must be equal numbers of electrons in the two half equations so that the electrons cancel out.