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Reduction and oxidation

Reduction is gain of electrons by a reactant (or loss of oxygen).

Oxidation is loss of electrons by a reactant (or gain of oxygen).

Redox reaction

A Redox Reaction is when an oxidation and a reduction reaction takes place at the same time.

Oxidising and reducing agents

Oxidising agent is a substance that accepts electrons (helps another reactant to be oxidised by being reduced itself).

Reducing agent is a substance that donates electrons (helps another reactant to be reduced by being oxidised itself).

Oxidising and reducing agents in reactions

Elements with low electronegativities can form ions by losing electrons so act as reducing agents (elements with high electronegativities can act as oxidising agents).

Strongest reducing agents are in Group 1 of the Periodic Table (strongest oxidising agents are in Group 7).

Oxidising or reducing agents

Oxidising agents are at the bottom of the left-hand side column of the electrochemical series:

• permanganate (MnO₄^-)

• dichromate (Cr₂O₇^2-)

• hydrogen peroxide (H₂O₂)

Reducing agents are at the top right-hand column of the electrochemical series.

Uses of oxidising agents

Oxidising agents can be used to kill fungi and bacteria and also as a bleach.

Balance ion-electron equations

You need to be able to balance ion-electron equations by adding water, Hydrogen ions and electrons eg:

Cr₂O₇^2-(aq) → Cr³⁺(aq)

Step 1

Balance the chromium ions.

Cr₂O₇^2-(aq) → 2Cr³⁺(aq)

Step 2

Balance the oxygen by adding water.

Cr₂O₇^2-(aq) → 2Cr³⁺(aq) + 7H₂O(l)

Step 3

Balance the hydrogen by adding hydrogen ions.

Cr₂O₇^2-(aq) + 14H⁺(aq) → 2Cr³⁺(aq) + 7H₂O(l)

Step 4

Balance the charge. In this example there is a charge of 12+ on the LHS and 6+ on the RHS. This means 6 electrons will need to be added to the LHS to give both sides a charge of 6+.

Cr₂O₇^2-(aq) + 14H⁺(aq) + 6e^- → 2Cr³⁺(aq) + 7H₂O(l)

Combine ion-electron equations to produce redox equations

Combine the following oxidation and reduction reactions to show the overall redox reaction:

1: Cl₂(g) + 2e^- → 2Cl^-(aq)
2: Fe²⁺(aq) → Fe³⁺(aq) + e^-

Step 1

Before combining the equations the number of electrons must be equal in each equation. Remember whatever you multiply e^- by, everything else in the equation must be multiplied by same value.

1: Cl₂(g) + 2e^- → 2Cl^-(aq)
2: Fe²⁺(aq) → Fe³⁺(aq) + e^- × 2
2: 2Fe²⁺(aq) → 2 Fe³⁺(aq) + 2e^-

Step 2

Combine equations.

Cl₂(g) + 2e^- + 2Fe²⁺(aq) → Cl^-(aq) + 2 Fe³⁺(aq) + 2e^-

Step 3

Cancel out electrons (and anything else that appears on both sides).

Cl₂(g) + 2Fe²⁺(aq) → 2Cl^-(aq) + 2Fe³⁺(aq)