Oxidation and Reduction (Redox)

Electrochemistry is built on redox reactions — reactions involving the transfer of electrons between species. Oxidation and reduction always occur together.

Oxidation

Loss of electrons (OIL — Oxidation Is Loss). The oxidation state increases.

Zn → Zn²⁺ + 2e⁻

Zinc loses 2 electrons → it is oxidised.

Reduction

Gain of electrons (RIG — Reduction Is Gain). The oxidation state decreases.

Cu²⁺ + 2e⁻ → Cu

Copper ions gain 2 electrons → they are reduced.

Memory tool — OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Oxidation States

An oxidation state (or oxidation number) is a number assigned to an atom in a compound that represents the number of electrons it has gained or lost relative to its neutral state. It is a bookkeeping tool for tracking electron transfer.

Key Rules for Assigning Oxidation States:

• Pure elements: oxidation state = 0 (e.g., Fe, O₂, Cl₂)
• Monatomic ions: equals the ion charge (e.g., Na⁺ = +1, Cl⁻ = −1)
• Oxygen is usually −2 (except in peroxides: −1)
• Hydrogen is usually +1 (except in metal hydrides: −1)
• Sum of oxidation states in a neutral compound = 0

Example: Find oxidation state of Mn in KMnO₄.

K = +1, O = −2 (four oxygens = −8). Total must = 0: +1 + Mn + (−8) = 0 → Mn = +7.

Galvanic (Voltaic) Cells

A galvanic cell converts chemical energy into electrical energy using a spontaneous redox reaction. Electrons flow from the anode (oxidation) through an external circuit to the cathode (reduction). A salt bridge completes the circuit by allowing ion flow.

Zinc-Copper Galvanic Cell

Anode (−)

Zinc electrode in ZnSO₄ solution

Zn → Zn²⁺ + 2e⁻

OXIDATION occurs here

Cathode (+)

Copper electrode in CuSO₄ solution

Cu²⁺ + 2e⁻ → Cu

REDUCTION occurs here

Electrons flow: Zn anode → external wire → Cu cathode. Salt bridge allows ion movement.

The cell potential (voltage) measures the driving force for electron flow. It depends on the relative tendency of each half-cell to be reduced (standard reduction potentials).

Electrolysis

Electrolysis uses electrical energy to drive a non-spontaneous redox reaction. An external power source forces electrons in the opposite direction to a galvanic cell. This allows decomposition of compounds and deposition of metals.

Electrolysis of water

A direct current passes through water (with added electrolyte). Hydrogen gas forms at the cathode; oxygen forms at the anode.

2H₂O → 2H₂ + O₂

Electroplating

A metal object is coated with a thin layer of another metal (e.g., silver, gold, chromium) by electrolysis. The object is the cathode; the plating metal is the anode. Used in jewellery, car parts, and electronics.

Industrial electrolysis — aluminium smelting

In Australia, aluminium is extracted from molten bauxite (aluminium oxide) by electrolysis in the Hall-Héroult process. Al³⁺ ions are reduced at the cathode to form liquid aluminium metal.

Key Vocabulary

Term	Definition
Anode	The electrode where oxidation occurs. In a galvanic cell it is negative; in electrolysis it is connected to the positive terminal.
Cathode	The electrode where reduction occurs. In a galvanic cell it is positive; in electrolysis it is connected to the negative terminal.
Electrolyte	A substance that conducts electricity when dissolved in water or melted, due to the presence of free-moving ions.
Salt bridge	A device containing an electrolyte that connects the two half-cells of a galvanic cell, allowing ions to flow and maintaining electrical neutrality.

Worked Examples

1

Identify what is oxidised and reduced: Mg + CuSO₄ → MgSO₄ + Cu

Step 1: Assign oxidation states. Mg starts at 0, becomes Mg²⁺ (+2). Cu²⁺ becomes Cu (0).

Step 2: Mg goes 0 → +2: oxidation state increases → Mg is oxidised (it is the reducing agent).

Step 3: Cu goes +2 → 0: oxidation state decreases → Cu²⁺ is reduced (it is the oxidising agent).

2

Find the oxidation state of sulfur in H₂SO₄.

Step 1: Assign known states: H = +1 (two H atoms = +2), O = −2 (four O atoms = −8).

Step 2: Sum must equal 0: (+2) + S + (−8) = 0.

Step 3: S = +8 − 2 = +6.

Answer: Sulfur has an oxidation state of +6 in sulfuric acid.

3

During electroplating of a spoon with silver, what happens at each electrode?

Cathode (spoon, connected to −): Ag⁺ ions from solution gain electrons and are reduced to silver metal, depositing on the spoon.

Ag⁺ + e⁻ → Ag

Anode (silver bar, connected to +): Silver metal is oxidised, dissolving as Ag⁺ ions to replenish the solution.

Ag → Ag⁺ + e⁻

Knowledge Check

Select the correct answer for each question.

Question 1

In a redox reaction, an atom that loses electrons is said to be:

Oxidised Reduced Neutralised Electrolysed

Question 2

In a galvanic cell, at which electrode does reduction occur?

Anode Cathode Salt bridge Electrolyte solution

Question 3

What is the oxidation state of nitrogen in NH₃?

+3 0 −3 +1

Question 4

How does electrolysis differ from a galvanic cell?

Electrolysis produces electricity from a chemical reaction Electrolysis requires a salt bridge Electrolysis does not involve any redox reactions Electrolysis uses electrical energy to drive a non-spontaneous chemical reaction

Question 5

What is the purpose of a salt bridge in a galvanic cell?

To provide ions that maintain electrical neutrality in both half-cells To allow electrons to flow between the half-cells To increase the voltage of the cell To prevent any chemical reaction from occurring

Electrochemistry