Calculating Standard Entropy Changes (HL) (DP IB Chemistry): Revision Note

Philippa Platt

Written by: Philippa Platt

Reviewed by: Richard Boole

Updated on

Calculating Standard Entropy Changes

How to calculate ΔS

  • The standard molar enthalpy values, S, refer to to substances in their standard states at 298.15 K and 100 kPa

  • The entropy change, ΔS, can be calculated from thermodynamic data using the following equation:

ΔS298(reaction) = ΣS298(products) - ΣS298(reactants)

Examiner Tips and Tricks

  • This equation is not provided in the data booklet and should be learned

  • Standard entropy values (S) for selected substances are listed in the IB Chemistry data booklet (Section 13)

  • The units of ΔSsystem are in J K-1 mol–1

  • Entropy changes depending on the state of the matter

    • For example, water has different S values for the liquid and gaseous phases

      • S298(H2O (l)) = 70 J K-1 mol–1

      • S298(H2O (g)) = 189 J K-1 mol–1

  • When calculating ΔS, 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 S(NO2 (g))

N2O4 (g) → 2NO2 (g)

ΔS298(reaction) = ΣS298(products) - ΣS298(reactants)

ΔS= [(2 x S298(NO2)] - S298(N2O4)

Worked Example

What is the entropy change when calcium carbonate decomposes?

CaCO3 (s) → CaO (s) + CO2 (g)

  • S298(CaCO3 (s)) = 93 J K-1 mol–1

  • S298(CaO (s)) = 40 J K-1 mol–1

  • S298(CO2 (g)) = 214 J K-1 mol–1

 

Answer:

Step 1: Write out the equation to calculate ΔS298(reaction)

  • ΔS298(reaction) = ΣS298(products) - ΣS298(reactants)

Step 2: Substitute in formulas and then values for S

  • ΔS298(reaction) = [S298(CaO) + S298(CO2)] - S298(CaCO3)

  • ΔS(reaction) = (40 + 214) - 93

  • ΔS(reaction) = +161 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)

  • S298(N2 (g)) = 192 J K-1 mol–1

  • S298(H2 (g)) = 131 J K-1 mol–1

  • S298(NH3) = 192 J K-1 mol–1

 

Answer:

Step 1: Write out the equation to calculate ΔS298(reaction)

  • ΔS298(reaction) = ΣS298(products) - ΣS298(reactants)

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

  • ΔS298(reaction) = [2 x S298(NH3)] - [S298(N2)+ (3 x S298(H2 ))]

  • ΔS298(reaction) = [2 x 192] - [192 + (3 x 131)]

  • ΔS298(reaction) = 384 - 584

  • ΔS298(reaction) = -200 J K-1 mol–1

  • A negative ΔS means the system becomes more ordered

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

Unlock more, it's free!

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

the (exam) results speak for themselves:

Did this page help you?

Philippa Platt

Author: Philippa Platt

Expertise: Chemistry Content Creator

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

Richard Boole

Reviewer: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.