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Predicting Entropy Changes

Predict entropy changes from states, gaseous amounts and accessible arrangements.

  • GCE A-Level H2 Chemistry 9476-2027

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Entropy helps explain why melting, evaporation and reactions that produce gas often occur readily. It describes the ways energy can be distributed among the accessible microscopic arrangements of a system. “Disorder” can be a reminder, but visible untidiness is not a reliable test of entropy.

Compare the states of the same substance

A gas has many more accessible positions and energy distributions than the same amount of liquid or solid. For the same substance, entropy usually increases on melting and increases substantially on vaporisation.

Heating a substance within one phase also increases its entropy: more energy levels and distributions become accessible. Compare the same amount of substance under the stated conditions; do not use the slogan “gas has higher entropy” to compare unrelated samples of different sizes.

Check: What is the sign of the entropy change when water vapour condenses?

Negative. The system changes from gas to liquid, reducing the accessible arrangements. The surroundings may gain entropy as energy is released; this question concerns the water itself.

Use the balanced equation to predict a reaction’s sign

First identify each state symbol. Then compare the gaseous amounts using the balanced coefficients. Producing more gas usually increases the system’s entropy; consuming gas usually decreases it. This is a useful qualitative guide, not a calculation of Δ S.

For CaCO₃(s) → CaO(s) + CO₂(g), a gas is produced from solids, so a positive entropy change is expected. For reactions with equal gaseous amounts, gas counting alone may not decide the sign; use additional information supplied in the question.

Guided practice 1

Predict an entropy-change sign qualitatively

About 5 min

Problem

Predict the sign of Δ S⦵ for N₂(g) + 3H₂(g) → 2NH₃(g). Explain using gaseous-particle evidence; do not calculate from tabulated molar entropies.

Try this before viewing the solution

Hints

Hint 1: count gaseous particles
Compare the total gaseous coefficients on the two sides.
Hint 2: link to dispersal
Decide whether changing from four gaseous particles to two increases or decreases dispersal.
View solution step by step

  1. Compare gaseous amounts

    Method

    Add the gaseous stoichiometric coefficients on each side.

    Reason

    All species are gases, so the change in gaseous-particle amount is useful qualitative evidence.

    Working

    Reactants: 1 + 3 = 4 mol gas; products: 2 mol gas.

  2. Predict the sign

    Method

    State a negative entropy change.

    Reason

    Fewer gaseous particles means less dispersal and fewer accessible arrangements.

    Working

    Δ S⦵ < 0.

Try three changes without a worked method

Predict the sign of the system’s entropy change and explain the decisive evidence:

  1. Ice melts at its melting point.
  2. 2SO₂(g) + O₂(g) → 2SO₃(g).
  3. A fixed amount of an ideal gas expands isothermally into a larger volume.
Compare your reasoning
  1. Positive: liquid water has more accessible microscopic arrangements than ice.
  2. Negative is expected: three moles of gaseous reactants become two moles of gaseous product.
  3. Positive: the particles have more accessible positions in the larger volume, even though temperature is unchanged.

A system can lose entropy in a spontaneous change

A learner says: “Water cannot freeze spontaneously because its entropy decreases.” The missing part is the surroundings. Freezing releases energy to them; their entropy gain can outweigh the water’s entropy loss. Spontaneity concerns the total entropy change of system and surroundings, not the system alone.

Check: Does a positive system entropy change prove a reaction is feasible?

No. The enthalpy change and temperature also matter. An endothermic change can remove energy from the surroundings; a favourable system entropy change alone does not settle the overall balance.

Next: combine enthalpy and entropy

You can now explain and predict entropy changes qualitatively. In Gibbs free energy and feasibility, use supplied entropy changes with Δ G⦵ = Δ H⦵-TΔ S⦵. H2 does not require calculating reaction entropy changes from tabulated standard molar entropies. Use the topic check for practice.

Syllabus and review details

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