AP Chemistry: Thermodynamics & Thermochemistry 3 — Questions and Answers
Question 1: Which process always has a positive ΔS for the system?
- Sublimation of solid CO₂ (Correct answer)
- Freezing of water
- Condensation of steam
- Crystallization of salt
Correct answer: Sublimation of solid CO₂
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?
- Low temperatures (Correct answer)
- High temperatures
- All temperatures
- No temperatures
Correct answer: Low temperatures
When ΔH < 0 and ΔS < 0, ΔG is negative only at low temperatures.
Question 3: What is the first law of thermodynamics?
- Energy is conserved (Correct answer)
- Entropy always increases
- Heat flows from cold to hot
- Absolute zero is unreachable
Correct answer: Energy is conserved
The first law states energy cannot be created or destroyed, only transferred or converted.
Question 4: The internal energy change ΔU equals which expression?
- q + w (Correct answer)
- q − w
- w − q
- q × w
Correct answer: q + w
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?
- Above 333 K (Correct answer)
- Below 333 K
- At 0 K
- Never
Correct answer: Above 333 K
Setting ΔG = 0, T = ΔH/ΔS = 50000/150 ≈ 333 K, spontaneous above this.
Question 6: What does a calorimeter measure?
- Heat transferred in a process (Correct answer)
- Pressure of a gas
- Reaction rate
- pH of a solution
Correct answer: Heat transferred in a process
A calorimeter measures the heat absorbed or released during a physical or chemical process.
Question 7: Bond breaking is what type of process energetically?
- Endothermic (Correct answer)
- Exothermic
- Spontaneous
- Catalytic
Correct answer: Endothermic
Breaking bonds requires energy input, making it endothermic.
AP Chemistry: Thermodynamics & Thermochemistry 3
Which process always has a positive ΔS for the system?