Chemical Equilibria: Reversible Reactions and Dynamic Equilibrium

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. (a) Understand what is meant by a reversible reaction
    (b) Understand what is meant by dynamic equilibrium in terms of the rate of forward and reverse reactions being equal and the concentration of reactants and products remaining constant
    (c) Understand the need for a closed system in order to establish dynamic equilibrium

  2. Define Le Chatelier's principle as: if a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise this change

  3. Use Le Chatelier's principle to deduce qualitatively (from appropriate information) the effects of changes in temperature, concentration, pressure or presence of a catalyst on a system at equilibrium

  4. Deduce expressions for equilibrium constants in terms of concentrations, Kc

  5. Use the terms mole fraction and partial pressure

  6. Deduce expressions for equilibrium constants in terms of partial pressures, Kp (use of the relationship between Kp and Kc is not required)

  7. Use the Kc and Kp expressions to carry out calculations (such calculations will not require the solving of quadratic equations)

  8. Calculate the quantities present at equilibrium, given appropriate data

  9. State whether changes in temperature, concentration or pressure or the presence of a catalyst affect the value of the equilibrium constant for a reaction

  10. Describe and explain the conditions used in the Haber process and the Contact process, as examples of the importance of an understanding of dynamic equilibrium in the chemical industry and the application of Le Chatelier's principle

1. Reversible Reactions

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What is a reversible reaction?

Most chemical reactions you've studied so far go to completion. This means the reactants are completely used up to form products, and the reaction stops when all the reactants are gone.

However, reversible reactions are different. In a reversible reaction, the products can react together to reform the original reactants.

To show a reversible reaction, we use a special symbol: ⇌ (two opposing half arrows).

Example:

The hydration and dehydration of copper(II) sulfate is a reversible reaction:

CuSO₄·5H₂O (s) ⇌ CuSO₄ (s) + 5H₂O (l)

  • Forward reaction: Hydrated copper(II) sulfate (blue crystals) loses water to form anhydrous copper(II) sulfate (white powder)
  • Backward reaction: Anhydrous copper(II) sulfate gains water to reform hydrated copper(II) sulfate

Both reactions can happen at the same time.

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