If a reaction is exothermic, shouldn’t that be enough to determine favorability? Actually, no!

Even if a reaction is highly exothermic, the level of order it requires may make it statistically improbable.

Entropy (ΔS) is the tendency of a system to take its most probable form.

  • Negative values (-) indicate less probable = Unfavored
  • Positive values (+) indicate more probable = Favored

There are 3 common ways to make reactions more probable (increase ΔS)

  • They all become more likely as we add heat to the reaction (increase T)

1. Increasing the Number of Molecules

Reactions that create extra molecules are more probable since there are more ways to arrange them.

2. Phase Transition

Transformation of solid to liquid or liquid to gas is more probable since the molecules will have a greater vibrational freedom.

3. Increasing Molecular Freedom of Motion

Converting cyclic molecules to acyclic molecules are more probable since it increases freedom of rotation.