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


Enthalpy change (∆H), entropy change (∆S), and free-energy change (∆G) are state functions. That means they all depend only on the change between the initial and final states of a system, not on the process by which the change occurs. For a chemical reaction, this means that the thermodynamic state functions are independent of the reaction pathway; for instance, the addition of a catalyst to a reaction will have no effect on the overall energy or entropy change of the reaction.

Standard State Conditions

When the values of thermodynamic quantities are given on the test, they are almost always given for standard state conditions. A thermodynamic quantity under standard state conditions is indicated by the little superscript circle, so the following is true under standard state conditions:

H = ∆H°
S = ∆S°
G = ∆G°

Standard State Conditions

• All gases are at 1 atmosphere pressure.

• All liquids are pure.

• All solids are pure.

• All solutions are at 1-molar (1 M) concentration.

• The energy of formation of an element in its normal state is defined as zero.

• The temperature used for standard state values is almost invariably room temperature: 25°C (298 K). Standard state values can be calculated for other temperatures, however.