CIE AS Chemistry

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First teaching 2020

Last exams 2024

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1.7.1 Chemical Equilibria

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Reversible Reactions & Dynamic Equilibrium

Reversible reaction

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

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

The diagram shows an example of a forward and backward reaction that can be written as one equation using two half arrows

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, and the concentrations of the reactants and products is constant

Equilibria Dynamic Equilibrium, downloadable AS & A Level Chemistry revision notes

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

Equilibria Dynamic Equilibrium Starting Reactants, downloadable AS & A Level Chemistry revision notes

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)

Equilibria Dynamic Equilibrium Starting Products, downloadable AS & A Level Chemistry revision notes

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)

  • 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

Equilibria Closed System, downloadable AS & A Level Chemistry revision notes

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

 

Equilibria Open System, downloadable AS & A Level Chemistry revision notes

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

Exam Tip

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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Francesca

Author: Francesca

Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.