3.2 Enthalpy, Entropy, and Spontaneous Changes
We've looked at the two thermodynamic factors factors that determine whether or not a chemical reaction will actually occur - enthalpy (ΔH) and entropy (ΔS).
Remember that in chemical and physical terms, a spontaneous reaction simply means that a reaction will occur - how fast it occurs is not an issue.
Let's summarize our general findings:
ΔH positive (endothermic) reaction tends not to be spontaneous ΔH negative (exothermic)
reaction tends to be spontaneous ΔS positive - randomness increases reaction tends to be spontaneous ΔS negative - randomness decreases reaction tends not to be spontaneous
What if we consider both enthalpy and entropy together?
As you can see above, there are two situations where it seems we cannot predict in advance whether a reaction will occur or not - when both ΔH and ΔS are either positive or negative. Is there no way we can predict spontaneity in these cases? Of course there is!