5.5: Gibbs Free Energy
If something happens (to a significant amount), it was thermodynamically favorable.
(though you can make an unfavorable thing happen by putting in enough energy)
For a reaction to work, it must have either a favorable ΔH, favorable ΔS, or both. But if one is favorable and the other not, you need a way to combine them to determine which wins, which is Gibbs Free Energy:
ΔG = ΔH - TΔS
When ΔG is negative, the reaction is favorable
When ΔG is positive, the reaction is unfavorable (or favorable the other direction)
When ΔG=0, the reaction has no desire to go either way (it's at equilibrium).
Depending on the signs (aka favorability) of ΔH and ΔS, the ΔG may be always favorable, never favorable, or favorable only above or below certain temperatures.
You will see positive ΔG referred to as 'spontaneous', but College Board has officially decommissioned this term for AP chem.