2.3.8Chemical Bonding, Application Of Chemical Reactions and Organic Chemistry

Oxidation and Reduction Using Electrons

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Section 1 of 7Redox in terms of electrons

Oxidation

Oxidation A substance is oxidised when it loses electrons.
- Its particles become more positive (or less negative).
Earlier you met oxidation as gaining oxygen;
- the electron definition is wider and also covers reactions with no oxygen at all.

Reduction

Reduction A substance is reduced when it gains electrons.
- Its particles become less positive (or more negative).

Redox reactions

Redox reactions Electrons cannot just vanish, so oxidation and reduction always happen together in the same reaction.
- Such a reaction is called a redox reaction.
The electrons lost by one species are exactly the electrons gained by the other.

Memory aids

Memory aids OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons.
Always say 'of electrons' in an answer, because loss and gain of oxygen or hydrogen mean something different.

Section 2 of 7Working out a displacement reaction

Write the word equation: iron + copper sulfate → iron(II) sulfate + copper.

Write the balanced symbol equation: Fe + CuSO₄ → FeSO₄ + Cu.

Split every dissolved ionic compound into ions: Fe + Cu²⁺ + SO₄²⁻ → Fe²⁺ + SO₄²⁻ + Cu.

Cross out the spectator ions, which appear unchanged on both sides. Here the sulfate ions are spectators.

The overall ionic equation is Fe + Cu²⁺ → Fe²⁺ + Cu.

Decide what happened to each species: Fe atoms lose two electrons to become Fe²⁺ (oxidised); Cu²⁺ ions gain two electrons to become Cu atoms (reduced).

Section 3 of 7Half-equations

Change	Half-equation	What it shows
Iron atom to ion	Fe → Fe²⁺ + 2e⁻	Electrons on the product side, so oxidation
Copper ion to atom	Cu²⁺ + 2e⁻ → Cu	Electrons on the reactant side, so reduction
Lead ion at a cathode	Pb²⁺ + 2e⁻ → Pb	Reduction
Bromide ion at an anode	2Br⁻ → Br₂ + 2e⁻	Oxidation

Section 4 of 7Redox during electrolysis

Electrolysis

At the cathode (negative)

At the cathode (negative) Positive ions are attracted to the negative electrode, where they gain electrons.
Gaining electrons is reduction, so the cathode is where reduction happens.

At the anode (positive)

At the anode (positive) Negative ions are attracted to the positive electrode, where they lose electrons.
Losing electrons is oxidation, so the anode is where oxidation happens.

Remembering it

Remembering it RED CAT: REDuction at the CAThode. AN OX: ANode OXidation.
The two electrode reactions together make a redox reaction driven by the electrical supply.

Section 5 of 7Worked example

Question: in V³⁺ + Fe⁴⁺ → V⁴⁺ + Fe³⁺, which species is oxidised?

Look at vanadium: V³⁺ becomes V⁴⁺. The charge has become more positive, so one electron has been lost.

Look at iron: Fe⁴⁺ becomes Fe³⁺. The charge is less positive, so one electron has been gained.

Loss is oxidation, so V³⁺ is oxidised. Gain is reduction, so Fe⁴⁺ is reduced.

Section 6 of 7Key terms

Term	Meaning
Oxidation	The loss of electrons by an atom, ion or molecule.
Reduction	The gain of electrons by an atom, ion or molecule.
Redox reaction	A reaction in which oxidation and reduction occur at the same time.
Spectator ion	An ion present in the mixture that does not change during the reaction and is left out of the ionic equation.
Half-equation	An equation showing only the oxidation or only the reduction, with electrons written as e⁻.
Anode	The positive electrode in electrolysis, where oxidation occurs.
Cathode	The negative electrode in electrolysis, where reduction occurs.

Section 7 of 7Exam technique

Mixing up the electron and oxygen definitions: in the electron definition, oxidation is loss of electrons, not gain.

Writing the electrons on the wrong side of a half-equation: reduction has electrons on the left, oxidation has them on the right.

Leaving spectator ions in an ionic equation instead of cancelling them.

Forgetting that the charges must balance on both sides, so the number of electrons must match the ion charge.

Saying positive ions go to the anode: positive ions are attracted to the negative cathode.