IBChemistryHLRevision NotesWhat Drives Chemical Reactions?Entropy & SpontaneityCalculating Standard Entropy Changes (HL)

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First teaching 2023

First exams 2025

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Calculating Standard Entropy Changes (HL) (HL IB Chemistry)

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Philippa

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Chemistry

Calculating Standard Entropy Changes

  • The standard molar enthalpy values, Sꝋ, relate to standard conditions of temperature and pressure
  • The entropy change, ΔSꝋ, can be calculated from thermodynamic data using the following equation:

ΔSꝋ298(reaction) = ΣSꝋ298(products) - ΣSꝋ298(reactants)

    • This equation is provided in the data booklet
  • The units of ΔSsystemꝋ are in J K-1 mol–1
  • Entropy will change depending on the state of the matter
    • Taking water as an example the values for Sꝋ will be different for the liquid and gaseous phases
      • Sꝋ298(H2O (l)) = 70.0 J K-1 mol–1
      • Sꝋ298(H2O (g)) = 188.8 J K-1 mol–1
  • When calculating ΔSꝋ, the coefficients used to balance the equation must be applied when calculating the overall entropy change
  • For example, when calculating the ΔSꝋ for the reaction below we need to double the value for Sꝋ (NO (g))
    • N2O4 (g) → 2NO2 (g)
    • ΔSꝋ298(reaction) = ΣSꝋ298(products) - ΣSꝋ298(reactants)
    • ΔSꝋ = [(2 x Sꝋ298(NO2)] - Sꝋ298(N2O4)

Worked example

What is the entropy change when calcium carbonate decomposes?

CaCO3 (s) → CaO (s) + CO2 (g)

  • Sꝋ298(CaCO3 (s)) = 92.9 J K-1 mol–1
  • Sꝋ298(CaO (s)) = 39.7 J K-1 mol–1
  • Sꝋ298(CO2 (g)) = 213.6 J K-1 mol–1

 

Answer:

Step 1: Write out the equation to calculate ΔSꝋ298(reaction)

  • ΔSꝋ298(reaction) = ΣSꝋ298(products) - ΣSꝋ298(reactants)

Step 2: Substitute in formulas and then values for Sꝋ

  • ΔSꝋ298(reaction) = [Sꝋ298(CaO) + Sꝋ298(CO2)] - Sꝋ298(CaCO3)
  • ΔSꝋ(reaction) = (39.7 + 213.6) - 92.9
  • ΔSꝋ(reaction) = +160.4 J K-1 mol–1 

Worked example

What is the entropy change when ammonia is formed from nitrogen and hydrogen?

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

  • Sꝋ298(N2 (g)) = 191.6 J K-1 mol–1
  • Sꝋ298(H2 (g)) = 131 J K-1 mol–1
  • Sꝋ298(NH3) = 192.3 J K-1 mol–1

 

Answer:

Step 1: Write out the equation to calculate ΔSꝋ298(reaction)

  • ΔSꝋ298(reaction) = ΣSꝋ298(products) - ΣSꝋ298(reactants)

Step 2: Substitute in formulas and then values for Sꝋ taking into account the coefficients

  • ΔSꝋ298(reaction) = [2 x Sꝋ298(NH3)] - [Sꝋ298(N2)+ (3 x Sꝋ298(H2 ))]
  • ΔSꝋ298(reaction) = [2 x 192.3] - [191.6 + (3 x 131)]
  • ΔSꝋ298(reaction) = 384.6 - 584.6
  • ΔSꝋ298(reaction) = -200 J K-1 mol–1
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Philippa

Author: Philippa

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.