AP Chemistry: Thermodynamics & Thermochemistry 3

Question 1

Which process always has a positive ΔS for the system?

Accepted Answer

Sublimation of solid CO₂

Answer

Sublimation goes from solid to gas, greatly increasing disorder and entropy.

Question 2

For an exothermic reaction with a decrease in entropy, the reaction is spontaneous at what temperatures?

Accepted Answer

Low temperatures

Answer

When ΔH < 0 and ΔS < 0, ΔG is negative only at low temperatures.

Question 3

What is the first law of thermodynamics?

Accepted Answer

Energy is conserved

Answer

The first law states energy cannot be created or destroyed, only transferred or converted.

Question 4

The internal energy change ΔU equals which expression?

Accepted Answer

q + w

Answer

ΔU = q + w, where heat added and work done on the system increase internal energy.

Question 5

A reaction has ΔH = +50 kJ and ΔS = +150 J/K. At what approximate temperature does it become spontaneous?

Accepted Answer

Above 333 K

Answer

Setting ΔG = 0, T = ΔH/ΔS = 50000/150 ≈ 333 K, spontaneous above this.

AP Practice Test