Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 7

Big Idea #5: Laws of Thermodynamics and Changes in Matter

G, ∆H, AND ∆S

In general, nature likes to move toward two different and seemingly contradictory states—low energy and high disorder, so thermodynamically favored processes must result in decreasing enthalpy or increasing entropy or both.

There is an important equation that relates favorability (∆G), enthalpy (∆H), and entropy (∆S) to one another.

G° = ∆H° − TS°

T = absolute temperature (K)

Note that you should make sure your units always match up here. Frequently, ΔS is given in J/mol•K and ΔH is given in kJ/mol. The convention is to convert the TΔS term to kilojoules (kJ), as that is what Gibbs free energy is usually measured in. However, you can also convert the ΔH term to joules instead, so long as you are keeping track and labeling all units appropriately.

The chart below shows how different values of enthalpy and entropy affect spontaneity.

You should note that at low temperature, enthalpy is dominant, while at high temperature, entropy is dominant.