Chemistry • Year 11 • Module 4 • Lesson 11
Entropy — Definition, Modelling & Predicting ΔS
Lock in the vocabulary of entropy, predict the sign of ΔS from first principles, and build the habit of applying the Second Law before attempting any calculation.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: entropy (S), microstate, Second Law of Thermodynamics, ΔS, J K⁻¹ mol⁻¹, standard entropy (S°), Δn(gas), spontaneous process, state function, Third Law of Thermodynamics. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A thermodynamic quantity that measures the dispersal of energy across the available microstates of a system. | |
| 1.2 | One specific arrangement of all the energy distributed among all the particles of a system. | |
| 1.3 | The entropy units used for standard entropy values; note — not kJ. | |
| 1.4 | The change in entropy of a reaction, calculated as S(products) − S(reactants). | |
| 1.5 | The total entropy of the universe increases for any spontaneous process. | |
| 1.6 | A quantity whose value depends only on the current state of a system, not on how that state was reached. | |
| 1.7 | The absolute entropy of a pure substance at 298 K and 100 kPa; unlike ΔH°f, these values are never zero for elements. | |
| 1.8 | The difference between moles of gas in products and moles of gas in reactants; the most reliable predictor of the sign of ΔS. | |
| 1.9 | A process that proceeds without an ongoing external input of energy; determined by ΔS(universe) > 0. | |
| 1.10 | The entropy of a perfect crystal at absolute zero (0 K) is zero; this provides the reference point for absolute entropy values. |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line provided. 10 marks (1 T/F + 1 correction each)
2.1 Entropy is formally defined as a measure of “messiness” or visual disorder in a system. T / F
2.2 Standard entropy values (S°) are measured in J K−1 mol−1. T / F
2.3 When the number of moles of gas increases in a reaction, ΔS is expected to be negative. T / F
2.4 The Second Law of Thermodynamics states that the entropy of the universe always increases for spontaneous processes. T / F
2.5 The standard entropy of a pure element in its most stable form at 298 K is zero, just like its standard enthalpy of formation. T / F
3. Cloze — fill the blanks
Complete the paragraph below using words from the word bank. Each word is used once only. 8 marks
Word bank: microstates • gas • J K−1 mol−1 • increases • dispersal • sublimation • Second • universe
Entropy (S) is a measure of the ________ of energy across the available ________ of a system. The unit of standard entropy is ________. Among the three states of matter, ________ particles have the highest entropy because they have the greatest number of possible positions and speeds. When dry ice (solid CO&sub2;) disappears at an Australian food festival, the process is known as ________, and it results in a positive ΔS because a solid converts to a gas. The ________ Law of Thermodynamics states that for any spontaneous process the entropy of the ________ always ________.
4. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 8 marks, 2 each
4.1 What does a positive value of Δn(gas) tell you about the sign of ΔS for a reaction?
4.2 What is the significance of the Third Law of Thermodynamics for tabulating entropy values?
4.3 Why must you convert S° from J K−1 mol−1 to kJ K−1 mol−1 before using it in the equation ΔG = ΔH − TΔS?
4.4 Why does dissolving an ionic solid such as NaCl in water generally increase entropy?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “measures”, “increases with”, “requires”). Aim for at least 5 labelled arrows. 5 marks
Supplied terms: entropy (S) · microstates · moles of gas · ΔS · spontaneous process · Second Law.
Q1 — Term–definition matches
1.1 entropy (S) • 1.2 microstate • 1.3 J K−1 mol−1 • 1.4 ΔS • 1.5 Second Law of Thermodynamics • 1.6 state function • 1.7 standard entropy (S°) • 1.8 Δn(gas) • 1.9 spontaneous process • 1.10 Third Law of Thermodynamics.
Q2 — True / False with correction
2.1 False. Entropy is formally defined as a measure of the dispersal of energy across available microstates, not visual messiness or disorder. Disorder is a useful intuition but not the correct definition.
2.2 True. S° values are in J K−1 mol−1 (not kJ).
2.3 False. An increase in moles of gas means ΔS is expected to be positive (more gas = more microstates = higher entropy).
2.4 True.
2.5 False. Unlike standard enthalpy of formation, the standard entropy of elements is not zero — e.g. S°(graphite) = 5.7 J K−1 mol−1. The Third Law sets S = 0 only at 0 K for a perfect crystal.
Q3 — Cloze
In order: dispersal • microstates • J K−1 mol−1 • gas • sublimation • Second • universe • increases.
Q4.1 — Δn(gas) and ΔS sign
If Δn(gas) > 0, the number of moles of gas increases from reactants to products, giving more possible arrangements (more microstates), so ΔS is positive. This is the most reliable single predictor of the sign of ΔS.
Q4.2 — Third Law and entropy tables
The Third Law establishes that S = 0 for a perfect crystal at absolute zero, providing an absolute reference point. This means absolute S° values (not just changes) can be measured and tabulated for all substances, unlike enthalpy where only ΔH can be measured.
Q4.3 — Unit conversion for ΔG
ΔH is normally in kJ mol−1 while S° is in J K−1 mol−1. Using S° directly in ΔG = ΔH − TΔS without converting to kJ K−1 mol−1 would make the TΔS term 1000 times too large, giving a completely wrong answer for ΔG.
Q4.4 — Dissolution and entropy
When NaCl dissolves, the ordered crystal lattice breaks apart and Na&sup+; and Cl− ions become free to move throughout the solution in many possible arrangements. This large increase in microstates raises the entropy of the system, so ΔS > 0.
Q5 — Sample concept map
Correct maps should include arrows such as:
- entropy (S) — measures dispersal across → microstates
- moles of gas — increasing gas increases → entropy (S)
- ΔS — is positive when moles of gas increase from → moles of gas
- Second Law — governs → spontaneous process
- ΔS — predicts universe entropy change used in → Second Law
- spontaneous process — requires positive → ΔS (of universe)
Any biologically valid linking phrases are accepted. Award full marks for at least 5 correctly labelled arrows that respect causal direction.