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Calculating Entropy Changes (CIE A Level Chemistry)

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Calculating Entropy Changes

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

ΔSsystemꝋ = ΣSproductsꝋ - ΣSreactantsꝋ

(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)

ΔSsystemꝋ = (2 x ΔSꝋ(NH3)) - (ΔSꝋ(N2) + 3 x ΔSꝋ(H2))

Worked example: Calculating entropy changes

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

Answer

ΔSsystemꝋ = ΣΔSproductsꝋ - ΣΔSreactantsꝋ

ΔSsystemꝋ = (2 x 38.20) - (2 x 32.60 + 205.0)

= -193.8 J K-1 mol-1

Examiner Tip

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

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    Francesca

    Author: Francesca

    Expertise: Head of Science

    Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.