Redox

Oxidation is gain of oxygen, loss of electrons, or an increase in oxidation number. Reduction is loss of oxygen, gain of electrons, or a decrease in oxidation number. They always happen together: a redox reaction. The oxidising agent is itself reduced; the reducing agent is itself oxidised.

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Section 1 of 14

Learning objectives

Define oxidation and reduction in terms of oxygen, electrons and oxidation number.
Identify what is oxidised and reduced in a reaction.
Identify oxidising and reducing agents.

Pause and think

Rusting and burning are both oxidation. Why must something else be reduced at the same time?

Section 2 of 14

Core idea

Start hereA blue solution turns colourless

Drop zinc into blue copper(II) sulfate solution. The blue fades and pinky-brown copper coats the zinc. Electrons have jumped from zinc to copper ions.

The big idea

Oxidation and reduction always happen together. Electrons lost by one substance are gained by another: OIL RIG.

1. Three definitions

Oxygen: oxidation is gain of oxygen.
Electrons: oxidation is loss of electrons.
Oxidation number: oxidation is an increase.

Worked exampleChlorine and iron(II)

$\mathrm{2Fe^{2+}+Cl_2\rightarrow2Fe^{3+}+2Cl^-}$

Goal: What is oxidised, what is reduced, and the agents.
Strategy: Track charges.

$\mathrm{Fe^{2+}\rightarrow Fe^{3+}}+e^-$
Loses an electron: oxidised.
$\mathrm{Cl_2}+2e^-\rightarrow\mathrm{2Cl^-}$
Gains electrons: reduced.

Answer

\mathrm{Fe^{2+}}

is oxidised (reducing agent);

\mathrm{Cl_2}

is reduced (oxidising agent).

✓ Check2 electrons lost, 2 gained ✓.

2. Oxidation numbers

Worked exampleChromium in dichromate

$\mathrm{Cr_2O_7^{2-}}$

Goal: Oxidation number of Cr.
Strategy: Sum equals the ion's charge.

$2x+7(-2)=-2$
O is $-2$ .
$x=+6$
Solve.

Answer

+6

✓ CheckNamed dichromate(VI) ✓.

Agents

Drawing diagram…

An agent is named for what it does to the other substance.

Key takeawayTests Reducing agents decolourise purple acidified

\mathrm{KMnO_4}

. Oxidising agents turn colourless KI solution brown.

Pause and think

Is the reaction $\mathrm{NaOH+HCl\rightarrow NaCl+H_2O}$ redox?

Summary

OIL RIG.
Oxidation number up = oxidised.
Oxidising agent is reduced.

Oxidation: Loss of electrons or gain of oxygen.
Reduction: Gain of electrons or loss of oxygen.
Oxidising agent: Substance that oxidises another and is itself reduced.

HistoryPhlogiston. 18th-century chemists thought burning released "phlogiston". Lavoisier showed that burning substances actually gain mass by combining with oxygen, the original meaning of "oxidation".

Fun factStatue of Liberty. The Statue of Liberty was shiny copper-brown in 1886. Oxidation turned its surface green within about 30 years.

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