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*Revision Materials* 1 Atomic Structure 2 Atoms, molecules and stoichiometry 3 Chemical Bonding 4 States of matter 5 Chemical energetics 6 Electrochemistry 7 Equilibria 8 Reaction kinetics 9 The Periodic Table, chemical periodicity 10 Group 2 11 Group 17 12 Nitrogen and sulfur 13 Organic 14 Hydrocarbons 15 Halogen compounds 16 Hydroxy compounds 17 Carbonyl compounds 18 Carboxylic acids and derivatives 19 Nitrogen compounds 20 Polymerisation 21 Organic synthesis 22 Analytical techniques 23 Chemical energetics 24 Electrochemistry 25 Equilibria 26 Reaction kinetics 27 Group 2 28 Chemistry of transition elements 29 Organic 30 Hydrocarbons 31 Halogen compounds 32 Hydroxy compounds 33 Carboxylic acids and derivatives 34 Nitrogen compounds 35 Polymerisation 36 Organic synthesis 37 Analytical techniques

23 Chemical energetics

23.1 Lattice energy and Born-Haber cycles 23.2 Enthalpies of solution and hydration 23.3 Entropy change, ΔS 23.4 Gibbs free energy change, ΔG

Entropy change, ΔS

Specification Reference Physical Chemistry, Chemical energetics 23.3

Quick Notes

  • Entropy (S) is a measure of disorder or the number of ways particles and their energy can be arranged.
  • Positive entropy change (ΔS > 0) = system becomes more disordered.
  • Negative entropy change (ΔS < 0) = system becomes more ordered.
  • Entropy increases when:
    • Solids melt or boil
    • Gases form
    • More gas molecules are produced in a reaction
  • Can calculate entropy change using: ΔS = ΣS°(products) – ΣS°(reactants)

Full Notes

Definition of Entropy

Entropy (symbol S) is a measure of the amount of disorder in a system, or the number of ways particles and their energy can be arranged.

A system with more possible arrangements (e.g. gases) has higher entropy than one with fewer (e.g. solids).

It has units of J mol⁻¹ K⁻¹.

Predicting Entropy Changes

You can often predict the sign of entropy change (positive or negative ΔS) for a process, without needing values.

Changes of State

Entropy increases when a substance changes from solid to liquid to gas.
It decreases when going in the reverse direction.

CIE A-Level Chemistry diagram showing entropy increasing from solid to liquid to gas.

Examples:

Temperature Changes

Raising temperature increases kinetic energy and disorder, so entropy increases.

CIE A-Level Chemistry diagram showing entropy increasing as temperature rises.

Change in Number of Gaseous Molecules

If the number of gas molecules increases in a chemical reaction, entropy increases and if number of gas molecules decreases, entropy decreases.

For Example:

CIE A-Level Chemistry example showing entropy decrease when Mg and Cl2 react to form MgCl2.

Reactants: 2 moles (1 mole solid, 1 mole gas)
Products: 1 mole (1 mole solid) → ΔS is negative

Calculating Entropy Change for a Reaction

Every substance, in a given state, has a standard entropy value (S°).

We can use the standard entropy values (S°) for each substance in a reaction (usually given in tables) to determine the entropy change, ΔS, that occurs.

Formula:
ΔS = ΣS°(products) – ΣS°(reactants)

Step-by-step:

Worked Example: Calculating ΔS

Calculate ΔSsystem for the following reaction: 2H2(g) + O2(g) → 2H2O(l)

  • Given: S°(H2O(l)) = 70 J mol⁻¹ K⁻¹
  • S°(H2(g)) = 131 J mol⁻¹ K⁻¹
  • S°(O2(g)) = 205 J mol⁻¹ K⁻¹

ΔS = [2 × 70] – [2 × 131 + 1 × 205]
ΔS = 140 – (262 + 205)
ΔS = 140 – 467 = –327 J mol⁻¹ K⁻¹
Entropy decreases in this reaction.


Photo of Matt
Matt’s Exam Tip

Don’t forget the molar ratios of everything in the equation and keep workings very clear. Also, make sure you are using a substances S° value for the correct state (for example, H2O(l) has a different S° to H2O(g).

Summary