Entropy change
Entropy change, ΔS
- Entropy ($S$) measures the number of ways particles and their energy can be arranged.
- More ways = more disorder.
- A change that increases disorder has a positive $\Delta S$.
Practice
Entropy is a measure of:
More ways to arrange particles/energy means higher entropy (more disorder).
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 |
Practice
Which change has a positive entropy change?
Melting increases disorder (solid → liquid), so ΔS is positive; the others decrease disorder.
Practice
A reaction that produces more gas molecules than it uses has:
More gas molecules means more disorder, so ΔS is positive.
Calculating ΔS
$$\Delta S^{\ominus} = \sum S^{\ominus}(\text{products}) - \sum S^{\ominus}(\text{reactants})$$
- Use the standard entropies given in the data.
Practice
The standard entropy change of a reaction is:
ΔS° = ΣS°(products) − ΣS°(reactants).
You've got it
Key idea
- 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: $\Delta S^{\ominus} = \sum S^{\ominus}(\text{products}) - \sum S^{\ominus}(\text{reactants})$