Electrochemistry

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Cambridge International AS & A Level Chemistry (9701)

Electrochemistry

The big idea: Redox is the transfer of electrons, and oxidation numbers are the bookkeeping that tracks it. Once you can assign oxidation numbers, you can spot what is oxidised and reduced, name the oxidising and reducing agents, and balance even awkward equations.

Summary — What This Chapter Covers

Oxidation is loss of electrons or an increase in oxidation number; reduction is gain of electrons or a decrease in oxidation number.
Redox reactions involve oxidation and reduction happening together.
Oxidation numbers follow fixed rules: elements 0, sum equals overall charge, O usually −2, H usually +1.
In a balanced redox equation, total increase in oxidation number = total decrease.
An oxidising agent is itself reduced; a reducing agent is itself oxidised.
Disproportionation: the same element is simultaneously oxidised and reduced.
Roman numerals give the oxidation number: iron(III), manganate(VII).

1. Redox Processes and Oxidation Numbers

Assigning oxidation numbers

The rules

Rule	Example
An uncombined element has oxidation number 0	Na, O₂, S₈, Cl₂ all 0
Sum of oxidation numbers in a compound = 0; in an ion = its charge	SO₄²⁻: S + 4(−2) = −2, so S = +6
Simple ion: oxidation number = charge	Fe³⁺ = +3, Cl⁻ = −1
Group 1 metals +1, Group 2 metals +2, Al +3 in compounds	NaCl, MgO, Al₂O₃
F is always −1 in compounds	OF₂: F −1, so O = +2
O is usually −2 (peroxides −1; with F positive)	H₂O₂: O = −1
H is usually +1 (−1 in metal hydrides)	NaH: H = −1
Cl is usually −1 (except with O or F)	ClO⁻: Cl = +1

Worked Example

Find the oxidation number of the element in brackets: Cr₂O₇²⁻ (Cr), S₂O₃²⁻ (S), NH₄⁺ (N), KMnO₄ (Mn).

Practice Question

Give the oxidation number of chlorine in Cl₂, HCl, NaClO, NaClO₃ and Cl₂O₇.

Redox definitions

Electrons and oxidation numbers

Term	Electron transfer	Oxidation number
Oxidation	Loss of electrons	Increase
Reduction	Gain of electrons	Decrease
Redox	A reaction in which oxidation and reduction take place at the same time
Oxidising agent	Accepts electrons (is reduced)	Its oxidation number decreases
Reducing agent	Donates electrons (is oxidised)	Its oxidation number increases
Disproportionation	A redox reaction in which the same element is both oxidised and reduced

Worked Example

In Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), identify what is oxidised, what is reduced, and the oxidising and reducing agents.

Worked Example

Show that Cl₂ + 2NaOH → NaCl + NaClO + H₂O is a disproportionation.

Practice Question

Is 2H₂O₂ → 2H₂O + O₂ a disproportionation? Explain with oxidation numbers.

Balancing with oxidation numbers

Make the changes cancel

Assign oxidation numbers and find the elements that change.
Work out the change per atom (and per formula, if there are two atoms of that element).
Choose multipliers so the total increase = total decrease.
Balance the remaining atoms: O with H₂O, H with H⁺ (in acid), then check the charges.

Worked Example

Roman numerals

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Naming by oxidation number

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Electrochemistry