AP Chemistry - 9.3 - Gibbs Free Energy & Thermodynamic favorability

What does Delta G (Gibbs Free Energy Change) describe?

Describes whether a reaction is thermodynamically favorable or unfavorable.

Does a thermodynamically favorable reaction always occur quickly?

No, just because a reaction is favorable doesn’t mean it happens quickly.

When is a process thermodynamically favored

When Delta G is < 0, also indicating that the process is spontaneous.

What does it mean when a Delta G is negative?

It indicates that the reaction is spontaneous and thermodynamically favorable.

What does it mean for a reaction to be thermodynamically favorable?

The reaction to occur without external energy input.

What are the signs of ΔH and ΔS for a reaction that is spontaneous at all temperatures/conditions?

ΔH is negative (−), ΔS is positive (+), so ΔG is always negative.

If ΔH is negative (−) and ΔS is negative (−), under what condition is the reaction spontaneous?

At low temperatures

If ΔH is positive (+) and ΔS is positive (+), under what condition is the reaction spontaneous?

At high temperatures

What are the signs of ΔH and ΔS for a reaction that is never spontaneous (non-spontaneous at all temperatures)?

ΔH is positive (+), ΔS is negative (−), so ΔG is always positive.

If ΔH is positive (+) and ΔS is positive (+), under what condition is the reaction not spontaneous?

At low temperatures

If ΔH is negative (−) and ΔS is negative (−), under what condition is the reaction not spontaneous?

At high temperatures

If ΔG is negative what is the value of K, and what in a reaction is favored?

K is greater than 1 and the products are favored at equilibrium.

If ΔG is positive what is the value of K, and what in a reaction is favored?

K must be less than 1 and the reactants are favored at equilibrium.