CIE A Level Chemistry (9701) exams from 2022

Revision Notes

5.2.3 Gibbs Free Energy Change & Gibbs Equation

The Gibbs Equation

Gibbs free energy

  • The feasibility of a reaction does not only depend on the entropy change of the reaction, but can also be affected by the enthalpy change
  • Therefore, using the entropy change of a reaction only to determine the feasibility of a reaction is inaccurate
  • The Gibbs free energy (G) is the energy change that takes into account both the entropy change of a reaction and the enthalpy change
  • The Gibbs equation is:

ΔG = ΔHreaction – TΔSsystem

  • The units of ΔG are in kJ mol-1
  • The units of ΔHreaction are in kJ mol-1
  • The units of T are in K
  • The units of ΔSsystem are in J K-1 mol-1 (and must therefore be converted to kJ K-1 mol-1 by dividing by 1000)

The Gibbs Equation: Calculations

  • The Gibbs equation can be used to calculate the Gibbs free energy change of a reaction

ΔG = ΔHreaction –  TΔSsystem

  • The equation can also be rearranged to find values of ΔHreaction, ΔSsystem or the temperature, T
  • For example, if for a given reaction, the values of ΔG, ΔHreaction and ΔSsystem are given, the temperature can be found by rearranging the Gibbs equation as follows:

Gibbs equation calcualations eq 1

Worked example: Calculating Gibbs free energy

Entropy - Worked example - Calculating Gibbs free energy, downloadable AS & A Level Chemistry revision notes


  • Step 1 – Calculate ΔSsystem

ΔSsystem= ΣΔSproducts – ΣΔSreactants

ΔSsystem= (ΔS [CH3Br(g)] + ΔS [H2O(l)]) –  (ΔS [CH3OH(l)] + ΔS[HBr(g)])

= (246 + 70.0) – (240 + 99.0)

= -23.0 J K-1 mol-1

  • Step 2 – Convert ΔSinto kJ K-1 mol-1

Gibbs equation calcualations worked example eq 1

= -0.023 kJ K-1 mol-1

  •  Step 3 – Calculate ΔG

ΔG = ΔHreaction – TΔSsystem

= -47 – (298 x -0.023)

= -40.146 kJ mol-1

= -40.1 kJ mol-1

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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