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1.4.1 (HL)—Entropy (S)

Syllabus
First assessment 2025
Objective
1.4.1
Level
HL

Entropy and Dispersal

HL only

S(gas)>S(liquid)>S(solid)S(gas) > S(liquid) > S(solid)

Entropy describes the dispersal of matter and available energy. For the reaction system, calculate ΔS° = ΣS°(products) − ΣS°(reactants), including every coefficient, and report J K⁻¹ mol⁻¹ for the reaction as written. System ΔS° is not the same as total entropy change of system plus surroundings; state the boundary before using entropy to discuss spontaneity.

Use phase and particle count to predict a likely sign before calculating: producing more gas particles usually increases dispersal. Treat that prediction as a check, not a replacement for the standard-entropy sum.

Worked entropy example: for HX2(g)+ClX2(g)→2 HCl(g)\ce{H2(g) + Cl2(g) -> 2HCl(g)}, use S∘(HCl)=187S^\circ(\ce{HCl})=187, S∘(HX2)=131S^\circ(\ce{H2})=131 and S∘(ClX2)=223 J K−1 mol−1S^\circ(\ce{Cl2})=223\,\mathrm{J\,K^{-1}\,mol^{-1}}. ΔS∘=2(187)−[131+223]=+20 J K−1 mol−1\Delta S^\circ=2(187)-[131+223]=+20\,\mathrm{J\,K^{-1}\,mol^{-1}}. The small positive value is plausible because gas moles are unchanged; the tabulated values, not gas count alone, determine the sign.

Calculating Standard Entropy Change

HL only

Assessment in practice

Representative question

Question 1

[Maximum number: 1]

Calculate the standard entropy change, ΔS⊖\Delta S^{\ominus}, of the reaction between carbon monoxide and chlorine to form phosgene. Use section 13 of the data booklet and the following data:

Standard entropy S⊖S^{\ominus}, of chlorine =223 J mol−1 K−1=223 \mathrm{~J} \mathrm{~mol}^{-1} \mathrm{~K}^{-1}
Standard entropy S⊖S^{\ominus}, of phosgene =284 J mol−1 K−1=284 \mathrm{~J} \mathrm{~mol}^{-1} \mathrm{~K}^{-1}

Combustion and Thermodynamics Summary

Retrieve the route: identify complete or incomplete combustion products, compare fuels and biofuels, balance fuel-cell half-equations, calculate ΔS° and ΔG°, then use ΔG, Q and K to reason about spontaneity and equilibrium.

Check products before balancing, evidence before evaluation, oxidation versus reduction, kelvin and unit consistency, the sign of ΔG, and whether Q is below, equal to, or above K.