AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The concept of increasing disorder (entropy change, ∆S).
Full specification wording
Gibbs free-energy change, ∆G, and entropy change, ∆S (A-level only): The concept of increasing disorder (entropy change, ∆S).
Entropy, S, measures how spread out the energy and matter of a system are, often described as disorder.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Entropy, S, measures how spread out the energy and matter of a system are, often described as disorder. A perfect crystal at absolute zero has essentially no freedom of arrangement and its entropy is taken as zero, so every substance above that has a positive absolute entropy, quoted in J K⁻¹ mol⁻¹. Entropy rises sharply with freedom of movement: solids are lowest, liquids higher, and gases much higher again, because a gas particle can be almost anywhere with almost any share of the energy. Within a state, entropy rises with temperature, with the number of particles and with molecular complexity. The entropy change of a reaction is ∆S = ΣS(products) − ΣS(reactants), so it is the difference in dispersal between the two sides.
What earns the marks
- Describing entropy as a measure of disorder or of the dispersal of energy and matter.
- Ranking solid, liquid and gas in order of increasing entropy.
- Using ∆S = ΣS(products) − ΣS(reactants) with the correct units J K⁻¹ mol⁻¹.
- Predicting the sign of ∆S from the change in the number of moles of gas.
Common mistakes and how to put them right
Quoting entropy in kJ mol⁻¹, the units of an enthalpy change.
Assuming an element in its standard state has an absolute entropy of zero, as its enthalpy of formation does.
Predicting a negative ∆S when more moles of gas are produced.
Counting the total moles rather than the moles of gas when judging the sign of ∆S.
Calling entropy the energy of a system rather than a measure of dispersal.
Exam tips
- Look at the state symbols first; a change in the number of moles of gas dominates the sign of ∆S.
- Keep the units straight: entropies are in J K⁻¹ mol⁻¹, enthalpies in kJ mol⁻¹.
- Absolute entropies are positive, so subtracting reactants from products is the only sign decision you need.
Try the lesson
The start of the Gibbs free-energy change, ∆G, and entropy change, ∆S (A-level only): The concept of increasing disorder (entropy change, ∆S). lesson, made from checked teaching notes for this specification point.
Have a go first
Which change represents an increase in entropy?
Listen
Entropy, S, measures how spread out the energy and matter of a system are, often described as disorder. A perfect crystal at absolute zero has essentially no freedom of arrangement and its entropy is taken as zero, so every substance above that has a positive absolute entropy, quoted in J K⁻¹ mol⁻¹. Entropy rises sharply with freedom of movement: solids are lowest, liquids higher, and gases much higher again, because a gas particle can be almost anywhere with almost any share of the energy. Within a state, entropy rises with temperature, with the number of particles and with molecular complexity. The entropy change of a reaction is ∆S = ΣS(products) − ΣS(reactants), so it is the difference in dispersal between the two sides. A strong answer earns marks for these points. describing entropy as a measure of disorder or of the dispersal of energy and matter. ranking solid, liquid and gas in order of increasing entropy. using ∆S = ΣS(products) − ΣS(reactants) with the correct units J K⁻¹ mol⁻¹. predicting the sign of ∆S from the change in the number of moles of gas. Exam tip: Look at the state symbols first; a change in the number of moles of gas dominates the sign of ∆S.
Key words
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entropy
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absolute entropy
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solids, liquids and gases
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products minus reactants
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See every definition
- entropy
- A measure of how spread out the energy and matter of a system are, often described as disorder.
- absolute entropy
- Every substance above absolute zero has a positive value of this, quoted in J K⁻¹ mol⁻¹.
- solids, liquids and gases
- Entropy rises sharply with freedom of movement in this order, with gases much higher again.
- products minus reactants
- The entropy change of a reaction is the difference in dispersal between the two sides, calculated as ΣS(products) − ΣS(reactants).
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