Entropy Calculations (College Board AP® Chemistry) : Study Guide

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

Updated on

Entropy Calculations

  • Entropy changes are much smaller in magnitude compared to enthalpy changes, leading to measurements in joules rather than kilojoules.

    • The complete unit for entropy is J C-1 mol-1 

  • The standard entropy change (ΔS°) for a reaction can be determined by utilizing the standard entropies (S°) of the reactants and products.

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

    • ΔS°reaction = ΣS°products - ΣS°reactants      (where Σ signifies the sum of).

  • Consider the standard entropy change for the formation of ammonia (NH3) from nitrogen (N2) and hydrogen (H2

N2 (g) + 3H2 (g) ⇋ 2NH3 (g)

  • ΔS°= (2 x ΔS°(NH3)) − (ΔS°(N2) + 3 x ΔS°(H2))

  • Unlike enthalpy of formation for elements, entropy for elements is not zero

Worked Example

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

2Mg (s) + O2 (g) → 2MgO (s)

  • S°[Mg (s)] = 32.60 J C-1 mol-1

  • S°[O2 (g)] = 205.0 J C-1 mol-1

  • S°[MgO (s)] = 38.20 J C-1 mol-1

Answer:

  • ΔS° = ΣΔS°products - ΣΔS°reactants

  • ΔS°= (2 x ΔS°(MgO)) − (2 x ΔS°(Mg) + ΔS°(O2))

  • ΔS° = (2 x 38.20) - (2 x 32.60 + 205.0)

  • ΔS° = -193.8 J C-1 mol-1

    • A negative value indicates that there has been a decrease in entropy

    • We also knew this from the equation by the fact that a solid is produced from a solid and a gas 

Examiner Tips and Tricks

Don't forget to use the stoichiometry of the equation when calculating the entropy change of a reaction!

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry Content Creator

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Stewart Hird

Reviewer: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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