AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Reversible reactions: A + B C + D
This equation is a general model of a reversible reaction.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This equation is a general model of a reversible reaction. A and B are the reactants on the left; C and D are the products on the right. The symbol ⇌ between them shows that the forward reaction (A + B → C + D) and the reverse reaction (C + D → A + B) can both happen. The letters are placeholders, so you can replace them with real substances. For example, in the Haber process, nitrogen + hydrogen ⇌ ammonia, and in the hydration of copper(II) sulfate, anhydrous copper(II) sulfate + water ⇌ hydrated copper(II) sulfate. The equation does not tell you how fast either direction goes or how far the reaction shifts; those depend on temperature, pressure and concentration.
What earns the marks
- A and B represent the reactants, and C and D represent the products.
- The ⇌ symbol shows the reaction is reversible, so products can react to reform reactants.
- The forward reaction is A + B → C + D; the reverse reaction is C + D → A + B.
- The equation is a general template that can be replaced by real formulae or names.
- The position of ⇌ does not by itself show which direction is favoured under given conditions.
- A balanced symbol equation for a reversible reaction must have the same number of each type of atom on both sides.
Common mistakes and how to put them right
Treating A, B, C and D as fixed chemicals rather than placeholders.
Put it right: They stand for any reactants and products in a reversible change.
Reading the equation as only going left to right.
Put it right: The ⇌ shows both directions are possible.
Forgetting to balance a real equation written from this template.
Put it right: Check that atoms are conserved on both sides.
Exam tips
- When a question gives a general equation, translate it into a named example to show you understand the pattern.
- Label the forward and reverse reactions clearly if asked to explain the equation.
- Check that any real equation you write from the template is balanced and uses ⇌, not →.
Try the lesson
The start of the Reversible reactions: A + B C + D lesson, made from checked teaching notes for this specification point.
Have a go first
In a reversible reaction shown as A + B ⇌ C + D, what does the symbol ⇌ between the two sets of substances tell you?
Listen
This equation is a general model of a reversible reaction. A and B are the reactants on the left; C and D are the products on the right. The symbol ⇌ between them shows that the forward reaction (A + B → C + D) and the reverse reaction (C + D → A + B) can both happen. The letters are placeholders, so you can replace them with real substances. For example, in the Haber process, nitrogen + hydrogen ⇌ ammonia, and in the hydration of copper(II) sulfate, anhydrous copper(II) sulfate + water ⇌ hydrated copper(II) sulfate. The equation does not tell you how fast either direction goes or how far the reaction shifts; those depend on temperature, pressure and concentration. A strong answer earns marks for these points. A and B represent the reactants, and C and D represent the products. The ⇌ symbol shows the reaction is reversible, so products can react to reform reactants. The forward reaction is A + B → C + D; the reverse reaction is C + D → A + B. The equation is a general template that can be replaced by real formulae or names. A common slip: Treating A, B, C and D as fixed chemicals rather than placeholders. Instead, they stand for any reactants and products in a reversible change. Exam tip: When a question gives a general equation, translate it into a named example to show you understand the pattern.
See it
Key words
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Reactants and products
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Forward and reverse reactions
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General equation for reversible reactions
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Conservation of atoms in equations
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See every definition
- Reactants and products
- A and B are the reactants on the left, and C and D are the products on the right.
- Forward and reverse reactions
- The forward reaction is A + B → C + D, and the reverse reaction is C + D → A + B.
- General equation for reversible reactions
- A + B ⇌ C + D is a general model of a reversible reaction, and the letters are placeholders that can be replaced with real substances.
- Conservation of atoms in equations
- A balanced symbol equation for a reversible reaction must have the same number of each type of atom on both sides.
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