Chemical Equilibria (Oxford AQA International A Level Chemistry)

Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Stewart Hird

Reversible Reactions & Dynamic Equilibrium

Reversible reaction

  • Some reactions go to completion where the reactants are used up to form the products

    • The reaction stops when all of the reactants are used up

  • In reversible reactions the products can react to reform the original reactants

  • To show a reversible reaction, two opposing half arrows are used: ⇌

Reversible reaction example

Equilibria Reversible Reactions, downloadable AS & A Level Chemistry revision notes

Dynamic equilibrium

  • In a dynamic equilibrium the reactants and products are dynamic (they are constantly moving)

  • In a dynamic equilibrium:

    • The rate of the forward reaction is the same as the rate of the backward reaction in a closed system

    • The concentrations of the reactants and products are constant

Snapshot of the dynamic equilibrium between hydrogen iodide, hydrogen and iodine
The diagram shows a snapshot of a dynamic equilibrium in which molecules of hydrogen iodide are breaking down to hydrogen and iodine at the same rate as hydrogen and iodine molecules are reacting together to form hydrogen iodide

Sketch graphs of reactant and product concentrations as a reaction approaches equilibrium

Sketch graph of reactant concentrations as a reaction proceeds to equilibrium
The diagram shows that the concentration of the reactants and products does not change anymore once equilibrium has been reached (equilibrium was approached using reactants)
Sketch graph of product concentrations as a reaction proceeds to equilibrium
The diagram shows that the concentration of the reactants and products does not change anymore once equilibrium has been reached (equilibrium was approached using products)

Open and closed systems

  • A closed system is one in which none of the reactants or products escape from the reaction mixture

  • In an open system, matter and energy can be lost to the surroundings

  • When a reaction takes place entirely in solution, equilibrium can be reached in open flasks as a negligible amount of material is lost through evaporation

  • If the reaction involves gases, equilibrium can only be reached in a closed system

Thermal decomposition of calcium carbonate in a closed system

Diagram showing the equilibrium achieved by the thermal decomposition of calcium carbonate in a closed system
The diagram shows a closed system in which no carbon dioxide gas can escape and the calcium carbonate is in equilibrium with the calcium oxide and carbon dioxide

Thermal decomposition of calcium carbonate in an open system

Diagram showing the equilibrium achieved by the thermal decomposition of calcium carbonate in an open system
The diagram shows an open system in which the calcium carbonate is continually decomposing as the carbon dioxide is lost causing the reaction to eventually go to completion

Examiner Tips and Tricks

A common misconception is to think that the concentrations of the reactants and products are equal.

However, they are not equal but they remain constant at dynamic equilibrium (i.e. the concentrations are not changing).

The concentrations will change as the reaction progresses, only until the equilibrium is reached. 

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Richard Boole

Author: Richard Boole

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.