A measure of the disorder of a system, in terms of the random dispersal of particles and energy quanta.

How does entropy change with temperature?

It increases, because the energy quanta can be distributed in more ways.

What is the natural direction of change?

Towards an increase in total entropy.

How is total entropy defined?

The entropy change of the system plus the entropy change of the surroundings.

Why does heat released by a reaction increase the entropy of the surroundings?

The energy is dispersed among the particles of the surroundings, so there are more ways of distributing it.

Why does entropy increase when an ionic solid dissolves?

The ordered lattice breaks up and the ions are randomly dispersed in solution.

Predict the sign of ΔS for N₂(g) + 3H₂(g) → 2NH₃(g).

Negative, because 4 mol of gas become 2 mol of gas.

Why do gases spread out to fill a room?

There are many more ways to arrange the molecules when they are dispersed, so total entropy increases.

Entropy and spontaneous changeEdexcel International A Level Chemistry: Flashcards

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Can an endothermic reaction be spontaneous?

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Can an endothermic reaction be spontaneous?: Yes. Spontaneous change depends on total entropy, not enthalpy change alone.
Define entropy.: A measure of the disorder of a system, in terms of the random dispersal of particles and energy quanta.
What are the units of entropy?: J K⁻¹ mol⁻¹
How does entropy change with temperature?: It increases, because the energy quanta can be distributed in more ways.
What is the order of entropy of the three states of a substance?: Solid < liquid < gas.
What is the entropy of a perfect crystal at 0 K?: Zero.
What is the natural direction of change?: Towards an increase in total entropy.
How is total entropy defined?: The entropy change of the system plus the entropy change of the surroundings.
Why does heat released by a reaction increase the entropy of the surroundings?: The energy is dispersed among the particles of the surroundings, so there are more ways of distributing it.
Why does entropy increase when an ionic solid dissolves?: The ordered lattice breaks up and the ions are randomly dispersed in solution.
Predict the sign of ΔS for N₂(g) + 3H₂(g) → 2NH₃(g).: Negative, because 4 mol of gas become 2 mol of gas.
Why do gases spread out to fill a room?: There are many more ways to arrange the molecules when they are dispersed, so total entropy increases.

Exam questions on Entropy and spontaneous change

A gas jar containing brown nitrogen dioxide is placed beneath a gas jar of colourless air, and the lid between them is removed. After a few minutes both jars contain a uniform pale brown mixture. There is no measurable energy change during mixing.
Explain, in terms of molecules and energy quanta, why the entropy of the system increases when the gases mix.2 marks
A student predicts the sign of the entropy change of the system, ΔS, for several changes at room temperature and pressure.
Explain why the entropy of water increases when liquid water at 100 °C changes to steam at 100 °C.2 marks
A student dissolves citric acid and sodium hydrogencarbonate in water. The mixture fizzes and its temperature falls from 21 °C to 12 °C. The equation for the reaction is C₆H₈O₇(aq) + 3NaHCO₃(aq) → Na₃C₆H₅O₇(aq) + 3H₂O(l) + 3CO₂(g).
Explain why this reaction occurs spontaneously even though it is endothermic.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).