Redox Reactions

Lesson

Part 1 — oxidation and reduction always happen together. We learn to follow the electrons with a bookkeeping number.

In this lesson you will

State the classical and electron-transfer definitions of oxidation and reduction
Assign oxidation numbers using the rules, including fractional values, peroxides and superoxides
Identify the oxidising and reducing agents in a reaction from changes in oxidation number
Use Stock notation and distinguish oxidation number from valence

Table of everyday redox processes, what is oxidised and why each matters
Process	What is oxidised	Why it matters
Rusting of an iron nail	Iron	Corrosion costs bridges and ships
Burning of cooking gas	The fuel	Heat for cooking
A mobile-phone battery working	The metal at the negative terminal	Portable electricity
Chlorine bleaching a stain	The coloured substance	Cleaning and disinfecting

Concept

The classical view

Early chemists defined the terms by what happens to oxygen, hydrogen or other elements. Oxidation and reduction are opposite changes in the same reaction, for example $\ce{H2S + Cl2 -> 2HCl + S}$ , where hydrogen sulfide loses hydrogen.

Table of the classical definitions of oxidation and reduction
Kind of change	Oxidation	Reduction
Oxygen	Addition of oxygen	Removal of oxygen
Hydrogen	Removal of hydrogen	Addition of hydrogen
Electronegative element	Addition (Cl₂ added to a metal)	Removal
Electropositive element	Removal (K removed from KI)	Addition (Mg added to a non-metal)

Concept

The electron-transfer view

Oxidation is loss of electrons; reduction is gain of electrons. The substance that gains electrons (and is itself reduced) is the oxidising agent; the one that loses electrons is the reducing agent. Each reaction splits into two half-reactions.

Table splitting the reaction of zinc with copper two ions into oxidation and reduction half-reactions
Half-reaction	Equation	Electrons	Role of the species
Oxidation	$\ce{Zn -> Zn^2+ + 2e-}$	Lost	Zn is the reducing agent
Reduction	$\ce{Cu^2+ + 2e- -> Cu}$	Gained	$\ce{Cu^2+}$ is the oxidising agent
Overall	$\ce{Zn + Cu^2+ -> Zn^2+ + Cu}$	Cancel	Electrons lost equal electrons gained

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