Redox I: Oxidation, Reduction, and Electron Transfer

Full Notes

Defining Redox Processes

Redox reactions involve the transfer of electrons between species. We can define redox in terms of electron transfer and oxidation number change.

For electron transfer:

• Oxidation is loss of electrons
• Reduction is gain of electrons

("OIL RIG" — Oxidation Is Loss, Reduction Is Gain)

For change in oxidation number:

• Oxidation = increase in oxidation number
• Reduction = decrease in oxidation number

Both definitions are valid and often used together.

Oxidising and Reducing Agents

In any redox reaction, one species donates electrons (reducing agent), and another accepts them (oxidising agent):

Oxidising agent:

• Accepts electrons (is reduced)
• Causes another species to be oxidised

Reducing agent:

• Donates electrons (is oxidised)
• Causes another species to be reduced

Example Zinc and copper ions

Zn + Cu²⁺ → Zn²⁺ + Cu
Zn is oxidised (loses electrons) = reducing agent
Cu²⁺ is reduced (gains electrons) = oxidising agent

Oxidation and Reduction by Electron Transfer

We can write half-equations to show the movement of electrons:

Example Magnesium and chlorine

Mg + Cl₂ → MgCl₂

• Oxidation: Mg → Mg²⁺ + 2e⁻ (Mg loses electrons = oxidation)
• Reduction: Cl₂ + 2e⁻ → 2Cl⁻ (Cl₂ gains electrons = reduction)

These changes also match the oxidation number changes:

• Mg: 0 → +2 (oxidation)
• Cl: 0 → −1 (reduction)

General Behaviour of Metals and Non-Metals

Most redox reactions involving elements fall into clear patterns:

Example Metals

Metals tend to lose electrons and form positively charged ions. They are usually oxidised in reactions.
e.g. Na → Na⁺ + e⁻

Example Non-metals

Non-metals tend to gain electrons and form negatively charged ions. They are usually reduced in reactions.
e.g. Cl₂ + 2e⁻ → 2Cl⁻

Summary

• Oxidation = loss of electrons or increase in oxidation number
• Reduction = gain of electrons or decrease in oxidation number
• Redox reactions always involve both oxidation and reduction
• Oxidising agents accept electrons and are reduced
• Reducing agents donate electrons and are oxidised
• Metals are generally oxidised, non-metals are generally reduced