Chemistry Test Thermochemistry 2 — Questions and Answers
Question 1: Which thermodynamic quantity represents the heat exchanged at constant pressure?
- Internal energy (U)
- Enthalpy (H) (Correct answer)
- Gibbs free energy (G)
- Entropy (S)
Correct answer: Enthalpy (H)
Enthalpy (H = U + PV) equals the heat transferred in a constant-pressure process.
Question 2: A reaction has ΔH = +85 kJ/mol. This reaction is best described as:
- Exothermic and releases heat
- Endothermic and absorbs heat (Correct answer)
- Thermoneutral with no heat change
- Spontaneous at all temperatures
Correct answer: Endothermic and absorbs heat
A positive ΔH indicates an endothermic reaction that absorbs heat from the surroundings.
Question 3: In a bomb calorimeter experiment, the calorimeter absorbs 4.50 kJ of heat and contains 500 g of water (specific heat 4.18 J/g·°C). What temperature change occurs in the water?
- 1.07 °C
- 2.15 °C (Correct answer)
- 4.30 °C
- 9.40 °C
Correct answer: 2.15 °C
ΔT = q/(m×c) = 4500 J / (500 g × 4.18 J/g·°C) ≈ 2.15 °C.
Question 4: Hess's Law states that the total enthalpy change of a reaction is:
- Always negative for spontaneous reactions
- Independent of the pathway taken (Correct answer)
- Equal to the activation energy
- Proportional to the reaction rate
Correct answer: Independent of the pathway taken
Hess's Law states ΔH is a state function and depends only on initial and final states, not the pathway.
Question 5: Which process has the most negative (most favorable) lattice energy?
- NaF formation
- KI formation
- CsCl formation
- LiF formation (Correct answer)
Correct answer: LiF formation
LiF has the highest lattice energy because Li⁺ is the smallest cation and F⁻ is the smallest anion, maximizing ion-ion attraction.
Question 6: The standard enthalpy of formation (ΔH°f) of an element in its standard state is:
- -1 kJ/mol by convention
- 0 kJ/mol by definition (Correct answer)
- +1 kJ/mol by convention
- Determined experimentally
Correct answer: 0 kJ/mol by definition
By definition, the standard enthalpy of formation of any pure element in its standard state is zero.
Question 7: Burning 1 mol of methane (CH₄) releases 890 kJ. How much heat is released burning 8 g of CH₄? (Molar mass CH₄ = 16 g/mol)
- 111 kJ
- 222 kJ
- 445 kJ (Correct answer)
- 890 kJ
Correct answer: 445 kJ
8 g / 16 g/mol = 0.5 mol; 0.5 × 890 kJ = 445 kJ.
Chemistry Test Thermochemistry 2
Which thermodynamic quantity represents the heat exchanged at constant pressure?