Revision NotesExam QuestionsPast Papers
A LevelChemistryCambridge (CIE)Revision Notes23. Chemical EnergeticsEntropy Change, ΔSCalculating Entropy Changes

Calculating Entropy Changes (Cambridge (CIE) A Level Chemistry): Revision Note

Philippa Platt

Last updated

Entropy Change Calculations

  • The standard entropy change (ΔSsystemθ) for a given reaction can be calculated using the standard entropies (Sθ) of the reactants and products

  • The equation to calculate the standard entropy change of a system is:

ΔSsystemθ = ∑Sproductsθ - ∑Sreactantsθ

(where ∑ = 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)

ΔSsystemθ = (2 x ΔSθ(NH3)) - (ΔSθ(N2) + 3 x ΔSθ(H2))

Worked Example

Calculate the entropy change of the system for the following reaction:

2Mg (s) + O2 (g) → 2MgO (s)

  • Sθ [Mg (s)] = 32.60 J K–1 mol–1

  • Sθ [O2 (s)] = 205.0 J K–1 mol–1

  • Sθ [MgO (s)] = 38.20 J K–1 mol–1

Answer:

  • ΔSsystemθ = ΣΔSproductsθ - ΣΔSreactantsθ

  • ΔSsystemθ = (2 x 38.20) - (2 x 32.60 + 205.0)

  • ΔSsystemθ = –193.8 J K-1 mol-1

Examiner Tips and Tricks

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

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Test yourself

    Did this page help you?

    Previous:Entropy & Entropy ChangeNext:Gibbs Free Energy Change & Gibbs Equation
    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener