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Year 11
AQA
Higher

Half equations: electrolysis

I can write half–equations and balanced ionic equations.

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Year 11
AQA
Higher

Half equations: electrolysis

I can write half–equations and balanced ionic equations.

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Lesson details

Key learning points

  1. A half–equation describes what happens to one reactant in a reaction, such as the process at an individual electrode.
  2. Half–equations show balanced charges as well as balanced atoms.
  3. A balanced ionic equation shows all reacting ions involved in a reaction.
  4. Ions not involved in a reaction do not appear in a balanced ionic equation.

Keywords

  • Reduction - a type of reaction in which a substance gains electrons or loses oxygen

  • Oxidation - a type of reaction in which a substance loses electrons or gains oxygen

  • Half–equation - a chemical equation used to show the electrons lost in oxidation or the electrons gained in reduction

  • Ionic equation - a balanced equation showing all the ions involved in a reaction

  • Spectator ions - the ions not involved in a chemical reaction

Common misconception

Getting the electrons confused; how many and which side of the half equations to write them on.

When writing a half equation, electrons should be added and not taken away and so should go on the side of the equation where they are added. All charges must cancel each other out and the atoms on each side of the equation balance.


To help you plan your year 11 chemistry lesson on: Half equations: electrolysis, download all teaching resources for free and adapt to suit your pupils' needs...

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This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
What is a chemical reaction?
A process where substances are mixed but not changed.
Correct answer: A process where substances change into new substances.
A process where substances retain their original properties.
Q2.
What are reactants?
Substances that are formed as the result of a chemical reaction.
Substances that do not change during a reaction.
Correct answer: Substances that take part in and undergo change during a reaction.
Q3.
What are products?
Correct answer: Substances that are formed as the result of a chemical reaction.
Substances that are mixed but not changed.
Substances that take part in and undergo change during a reaction.
Q4.
What is a word equation?
A representation of a chemical reaction using numbers.
A representation of a chemical reaction using symbols.
Correct answer: A representation of a chemical reaction using names of the chemicals involved.
Q5.
What does the conservation of mass mean?
Mass is always decreased in a chemical reaction.
Mass is always increased in a chemical reaction.
Correct answer: Mass is neither created nor destroyed in a chemical reaction.
Q6.
What is a chemical symbol?
A number representing the mass of an element.
Correct answer: A one- or two-letter representation of an element.
A phrase describing an element.

Exit quiz

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6 Questions

Q1.
What does a half-equation describe?
What happens to all reactants in a reaction.
Correct answer: What happens to one reactant in a reaction.
What happens to the products in a reaction.
Q2.
What do half-equations show?
Only the atoms
Only the charges
Correct answer: Balanced charges and balanced atoms
Q3.
When an atom gains electrons we say it has been ...
thermally decomposed.
oxidised.
Correct answer: reduced.
Q4.
In an electrolysis cell, where does oxidation occur? why?
At the cathode, because positive ions gain electrons.
Correct answer: At the anode, because negative ions lose electrons.
At the cathode, because negative ions lose electrons.
At the anode, because positive ions gain electrons.
Q5.
What are spectator ions?
Ions that are formed as a result of the reaction.
Correct answer: Ions that do not take part in the reaction.
Ions that react during the reaction.
Q6.
In the electrolysis of molten zinc chloride, the ionic equation is Zn²⁺(l) + 2Cl⁻(l) → Zn(s) + Cl₂(g). Which of the following is correct?
H⁺ and OH⁻ are spectator ions.
Zn²⁺ is a spectator ion.
Correct answer: There are no spectator ions.