CIE A Level Chemistry (9701) exams from 2022

Revision Notes

5.2.2 Calculating Entropy Changes

Calculating Entropy Changes

  • The standard entropy change (ΔSsystem) for a given reaction can be calculated using the standard entropies (S) of the reactants and products
  • The equation to calculate the standard entropy change of a system is:

ΔSsystem= ΣΔSproducts – ΣΔSreactants

(where Σ = sum of)

  • For example, the standard entropy change for the formation of ammonia (NH3) from nitrogen (N2) and hydrogen (H2) can be calculated using this equation

            N2(g) + 3H2(g) ⇋ 2NH3(g)

ΔSsystem = (2 x ΔS(NH3)) – (ΔS(N2) + 3 x ΔS(H2))

Worked example: Calculating entropy changes

Entropy - Worked example - Calculating entropy changes, downloadable AS & A Level Chemistry revision notes


ΔSsystem= ΣΔSproducts – ΣΔSreactants

ΔSsystem= (2 x 38.20) – (2 x 32.60 + 205.0)

= -193.8 J K-1 mol-1

Exam Tip

Use the stoichiometry  of the equation and the correct state of the compounds when calculating the entropy change of a reaction.

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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