Entropy change
Introduced| English | Meaning |
|---|---|
| disorder/dɪˈsɔːdə/ | disorder |
| entropy/ˈentrəpi/ | A thermodynamic measure related to energy dispersal and the number of microscopic arrangements consistent with a system's macroscopic state. |
Measuring disorder 无序
- Entropy 熵 ($S$) measures the number of ways particles and their energy can be arranged.
- More ways = more disorder.
- A change that increases disorder has a positive $\Delta S$.
Entropy change lab
deltaS = products disorder - reactants disorder
Increase disorder and see entropy change become more positive.
Entropy is a measure of:
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More ways to arrange particles/energy means higher entropy (more disorder).
Entropy decreases when a solid melts or a gas is formed.
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False. Melting and forming a gas increase disorder, so entropy increases.
When entropy rises
| Change | Sign of $\Delta S$ |
|---|---|
| solid → liquid → gas (or dissolving) | positive |
| a rise in temperature | positive |
| a reaction making more gas molecules | positive |
| a reaction making fewer gas molecules | negative |
Entropy rises from solid to liquid to gas
Which change has a positive entropy change?
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Melting increases disorder (solid → liquid), so ΔS is positive; the others decrease disorder.
A reaction that produces more gas molecules than it uses has:
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More gas molecules means more disorder, so ΔS is positive.
Match each entropy idea.
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Each item links the term to its correct meaning.
Calculating ΔS
- Use the standard entropies given in the data.
An instant cold pack cools by an endothermic dissolving process — driven by the increase in entropy.
The standard entropy change of a reaction is:
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ΔS° = ΣS°(products) − ΣS°(reactants).
Predicting entropy change
- Entropy increases when more gas molecules are formed or a solid becomes liquid/gas.
- A reaction making more moles of gas has a positive ΔS.
Worked examples
1. For $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$, $\Delta H = +178\ \text{kJ/mol}$ and $\Delta S = +161\ \text{J/(K mol)}$. Convert $\Delta S$ to $0.161\ \text{kJ/(K mol)}$. At $298\ \text{K}$, $\Delta G = \Delta H - T\Delta S = 178 - 298 \times 0.161 = +130\ \text{kJ/mol}$, so it is not feasible.
2. It becomes feasible when $\Delta G < 0$, that is when $T > \dfrac{\Delta H}{\Delta S} = \dfrac{178}{0.161} = 1106\ \text{K}$.
You've got it
- entropy = number of ways to arrange particles/energy (more = more disorder)
- $\Delta S$ is positive for solid→liquid→gas, dissolving, higher temperature, or more gas molecules
- calculate it: $\Delta S^{\ominus} = \sum S^{\ominus}(\text{products}) - \sum S^{\ominus}(\text{reactants})$