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OCR (A) A-Level
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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

5.2.2 Enthalpy and entropy

Entropy Free energy

Entropy

Specification Reference 5.2.2 (a)–(c)

Quick Notes

  • Entropy (symbol S) is a measure of the dispersal energy in a system.
  • Positive entropy change (ΔS > 0) = system becomes more disordered, dispersal energy increases.
  • Negative entropy change (ΔS < 0) = system becomes more ordered, dispersal energy decreases.
  • Entropy increases when:
    • Solids melt or boil
    • Gases form
    • More gas molecules are produced in a reaction
  • We can calculate entropy change using standard entropies (S°) of reactants and products in a reaction:
    • AQA A-Level Chemistry calculating entropy change ΔS° = Σ(entropies of products) − Σ(entropies of reactants).

Full Notes

Entropy and Gibbs Free Energy are covered in more detail here.
This page is just what you need to know for OCR (A) A-level :)

Entropy (S) is a measure of the dispersal energy in a system.

Simplified, this means the greater the level of disorder in a system the higher the entropy.

Entropy has units of J mol−1 K−1

Predicting Entropy Changes

You can often predict the sign of entropy change for a process, ΔS, without needing values.

Changes of State

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

OCR (A) A-Level Chemistry diagram showing entropy increasing from solid to liquid to gas with +ΔS markers.

Examples:

Temperature Changes

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

OCR (A) A-Level Chemistry diagram showing particles vibrating more at higher temperature and annotation that entropy increases with temperature.

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:

OCR (A) A-Level Chemistry entropy example showing Mg(s) + Cl2(g) forming MgCl2(s) with a decrease in number of moles and negative ΔS.

Mg(s) + Cl2(g) → MgCl2(s)
Reactants: 2 moles (1 mole solid, 1 mole gas) ; Products: 1 mole (solid) → ΔS is negative

Calculating Entropy Change (ΔS) 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 for the reacting system (ΔSsystem).

Formula:

AQA A-Level Chemistry calculating entropy change ΔS° = Σ(entropies of products) − Σ(entropies of reactants).
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 substance’s S° value for the correct state (for example, H₂O(l) has a different S° to H₂O(g)).


Worked Example

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

  1. Write the expression:
    ΔS = ΣS°(products) – ΣS°(reactants)
  2. Substitute values:
    ΔS = [2 × 70] – [2 × 131 + 1 × 205]
  3. Work out the totals:
    ΔS = 140 – (262 + 205)
  4. Final calculation:
    ΔS = 140 – 467 = −327 J mol⁻¹ K⁻¹

Answer: Entropy decreases in this reaction.

Summary