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Entropy change, ΔS
Entropy is simply a scientific measure of messiness or disorder. The universe naturally tends towards more disorder, and this tendency helps drive chemical reactions.
Need to know
What you need to know
- **State of Matter:** Entropy increases significantly from solid to liquid to gas. $S(gas) \gg S(liquid) > S(solid)$.
- **Number of Moles:** For reactions involving gases, an increase in the number of moles of gas leads to a large increase in entropy.
- **Complexity:** More complex molecules with more atoms and bonds tend to have higher entropy than simpler ones because there are more ways for them to vibrate and rotate.
- **Temperature:** Increasing the temperature of a substance increases its entropy as particles have more kinetic energy and move more randomly.
Explanation
The Rule of Randomness
- Entropy (S) measures disorder. More possible arrangements of particles (microstates) mean higher entropy. Gases have much higher entropy than solids.
- A positive entropy change (ΔS > 0) means disorder has increased. This happens in melting, boiling, or when a reaction produces more gas molecules.
- We calculate the standard entropy change for a reaction using data tables: ΔS° = ΣS°(products) − ΣS°(reactants). A key factor is the change in moles of gas.
- Entropy's true power is revealed in the Gibbs free energy equation, ΔG = ΔH − TΔS. A large positive ΔS can make a reaction feasible, even if it's endothermic (ΔH > 0).