AP Chemistry: Thermodynamics & Thermochemistry 2 — Questions and Answers
Question 1: What is the sign of ΔH for an endothermic reaction?
- Positive (Correct answer)
- Negative
- Zero
- Always equal to ΔS
Correct answer: Positive
Endothermic reactions absorb heat, so ΔH is positive.
Question 2: A 50.0 g sample of water (c = 4.18 J/g·°C) is heated from 20.0 °C to 30.0 °C. How much heat is absorbed?
- 2090 J (Correct answer)
- 209 J
- 418 J
- 1045 J
Correct answer: 2090 J
q = mcΔT = 50.0 × 4.18 × 10.0 = 2090 J.
Question 3: Which quantity is a state function?
- Enthalpy (Correct answer)
- Heat
- Work
- Both heat and work
Correct answer: Enthalpy
Enthalpy depends only on the state, not the path; heat and work are path-dependent.
Question 4: According to Hess's law, the enthalpy change of a reaction depends on what?
- Only the initial and final states (Correct answer)
- The number of steps taken
- The reaction rate
- The catalyst used
Correct answer: Only the initial and final states
Hess's law states ΔH depends only on initial and final states, regardless of path.
Question 5: What does a negative ΔG indicate about a process?
- It is spontaneous (Correct answer)
- It is nonspontaneous
- It is at equilibrium
- It absorbs heat
Correct answer: It is spontaneous
A negative Gibbs free energy change means the process is spontaneous.
Question 6: In the equation ΔG = ΔH − TΔS, what does T represent?
- Absolute temperature in kelvin (Correct answer)
- Temperature in Celsius
- Time
- Heat capacity
Correct answer: Absolute temperature in kelvin
T is the absolute temperature in kelvin.
Question 7: What is the standard enthalpy of formation of an element in its standard state?
- Zero (Correct answer)
- Positive
- Negative
- Equal to its molar mass
Correct answer: Zero
By definition, the standard enthalpy of formation of a pure element in its standard state is zero.
AP Chemistry: Thermodynamics & Thermochemistry 2
What is the sign of ΔH for an endothermic reaction?