AQA A Level chemistry - AS Unit 2: Section 3.2.4 Redox Reactions

3.2.4 Redox Reactions - Redox equations

Students should:

be able to write half-equations identifying the oxidation and reduction processes in redox reactions when the reactants and products are specified
be able to combine half-equations to give an overall redox equation

Oxidation and reduction

For the purposes of oxidation and reduction the oxidation number can be thought of as the apparent ionic charge of an atom within a compound.

For example, in sulfuric acid, the sulfur is in the VI (+6) oxidation state. For the purposes of redox we can consider that it has an ionic charge of +6 (even though it is clearly covalently bonded). This makes it easier to follow any transfer of electrons.

If the sulfur changes to an oxidation state of IV during a chemical reaction then it has gone from an apparent ionic charge of +6 to a charge of +4, i.e. it has gained two electrons (negative charges). It has therefore been reduced (gain of electrons) in the process.

Examples

2FeCl₂ + Cl₂ 2FeCl₃

The iron changes state from +2 to +3 and is therefore oxidised (removal of electrons)

The chlorine gains an electron to go from 0 to -1 and is therefore reduced (addition of electrons)

Examples

Zn + CuSO₄ Cu + ZnSO₄

The zinc changes from oxidation state 0 to +2 (removal of electrons) it is oxidised (animation)

The copper changes from 2+ to 0 and is oxidised and is therefore reduced (addition of electrons)

Examples

Cr₂O₇^2- + 3SO₂ + 2H⁺ 2Cr³⁺ + 3SO₄^2- + H₂O

The chromium changes from +6 to +3 and is therefore reduced (gain of electrons)

The sulfur changes from +4 to +6 and therefore loses electrons = oxidation (loss of electrons)

Examples

2KI + Br₂ 2KBr + I₂

The iodide ions (oxidation number = -1) change to iodine (oxidation number = 0) : oxidation

Bromine (element, oxidation number = 0) changes to bromide ions (oxidation number = -1) : reduction

Examples

5Fe²⁺ + MnO₄^- + 8H⁺ 5Fe³⁺ + Mn²⁺ + 4H₂O

The iron changes from 2+ to 3+ and is oxidised (removal of electrons)

The manganese atom changes from +7 to +2 and is therefore reduced (addition of electrons)

Redox equations

If an element is oxidized, its oxidation number will go up (get more positive). If an element is reduced, its oxidation number will go down. To find out, simply write down the oxidation numbers for each element within the compounds as explained previously.

Example

Iron(II) hydroxide iron(III) hydroxide

As the oxidation state of the iron increases from +2 to +3 then it has been oxidised

sodium dichromate(VI) + sulfuric acid + ethanol chromium(III) sulfate + ethanal + sodium sulfate + Water

The oxidation state of the chromium changes from +6 to +3 and so it has been reduced

top