Chemistry Test Thermochemistry 3 — Questions and Answers
Question 1: Which statement correctly describes entropy (S)?
- Entropy measures the heat content of a system
- Entropy is a measure of disorder or dispersal of energy (Correct answer)
- Entropy always decreases in spontaneous processes
- Entropy is zero for all ideal gases at 25°C
Correct answer: Entropy is a measure of disorder or dispersal of energy
Entropy quantifies the number of microstates available to a system, reflecting disorder or energy dispersal.
Question 2: For a spontaneous process at constant temperature and pressure, the Gibbs free energy change (ΔG) must be:
- Positive
- Zero
- Negative (Correct answer)
- Equal to ΔH
Correct answer: Negative
A negative ΔG indicates a spontaneous process; ΔG < 0 means the reaction releases free energy.
Question 3: The relationship ΔG = ΔH − TΔS shows that a reaction is spontaneous at all temperatures when:
- ΔH > 0 and ΔS > 0
- ΔH < 0 and ΔS < 0
- ΔH < 0 and ΔS > 0 (Correct answer)
- ΔH > 0 and ΔS < 0
Correct answer: ΔH < 0 and ΔS > 0
When ΔH is negative and ΔS is positive, ΔG is negative regardless of temperature.
Question 4: The heat of solution of NH₄NO₃ in water is endothermic (+25.7 kJ/mol). This means dissolving NH₄NO₃:
- Warms the solution
- Cools the solution (Correct answer)
- Has no effect on temperature
- Releases gas
Correct answer: Cools the solution
An endothermic dissolution absorbs heat from the solution, lowering the temperature.
Question 5: What is the standard enthalpy change (ΔH°rxn) calculated from bond energies if bonds broken require 800 kJ and bonds formed release 950 kJ?
- +150 kJ
- -150 kJ (Correct answer)
- +1750 kJ
- -1750 kJ
Correct answer: -150 kJ
ΔH°rxn = energy in (bonds broken) − energy out (bonds formed) = 800 − 950 = −150 kJ.
Question 6: Which law states that the total energy of an isolated system is constant?
- Hess's Law
- Le Chatelier's Principle
- The First Law of Thermodynamics (Correct answer)
- The Third Law of Thermodynamics
Correct answer: The First Law of Thermodynamics
The First Law of Thermodynamics (conservation of energy) states that energy cannot be created or destroyed in an isolated system.
Question 7: A calorimeter contains 200 g of water at 20.0°C. After a reaction, the temperature rises to 26.5°C. How much heat (q) was absorbed by the water? (c = 4.18 J/g·°C)
- 2.73 kJ
- 5.43 kJ (Correct answer)
- 10.86 kJ
- 22.2 kJ
Correct answer: 5.43 kJ
q = mcΔT = 200 × 4.18 × 6.5 = 5434 J ≈ 5.43 kJ.
Chemistry Test Thermochemistry 3
Which statement correctly describes entropy (S)?