Rucete ✏ AP Chemistry In a Nutshell
11. Thermodynamics — Practice Questions 2
This chapter introduces the principles of energy transfer, enthalpy, entropy, and Gibbs free energy, which govern the spontaneity and direction of chemical reactions.
(Multiple Choice — Click to Reveal Answer)
1. Which of the following quantities determines if a process is spontaneous at constant temperature and pressure?
(A) Enthalpy (ΔH)
(B) Entropy (ΔS)
(C) Internal energy (ΔU)
(D) Gibbs free energy (ΔG)
Answer
(D) — Spontaneity at constant temperature and pressure is determined by Gibbs free energy: ΔG < 0 means spontaneous.
2. Which change results in an increase in entropy?
(A) Freezing of water
(B) Condensation of steam
(C) Melting of ice
(D) Deposition of CO₂ gas
Answer
(C) — Melting increases disorder; entropy increases during the phase transition from solid to liquid.
3. If ΔH = +75 kJ and ΔS = +200 J/K, the reaction becomes spontaneous at temperatures:
(A) Below 100 K
(B) Above 100 K
(C) Only at 373 K
(D) Never
Answer
(B) — ΔG = ΔH – TΔS → becomes negative (spontaneous) at high T when both ΔH and ΔS are positive.
4. What is the sign of ΔH for an exothermic reaction?
(A) Positive
(B) Negative
(C) Zero
(D) Cannot be determined
Answer
(B) — Exothermic reactions release heat, meaning ΔH < 0.
5. Which of the following statements is consistent with the Second Law of Thermodynamics?
(A) The entropy of a system always decreases
(B) The entropy of the universe increases in spontaneous processes
(C) All exothermic reactions are spontaneous
(D) ΔG is always positive
Answer
(B) — Spontaneous processes result in a net increase in the entropy of the universe.
6. What is the unit of entropy (ΔS) in standard thermodynamic data tables?
(A) kJ/mol
(B) J/mol·K
(C) kJ/mol·K
(D) J/mol
Answer
(B) — Entropy is expressed in joules per mole per kelvin (J/mol·K).
7. Which of the following is always true for a spontaneous process?
(A) ΔH < 0
(B) ΔS > 0
(C) ΔG < 0
(D) TΔS > ΔH
Answer
(C) — A process is spontaneous when the Gibbs free energy change is negative (ΔG < 0).
8. The enthalpy of formation of any element in its standard state is:
(A) 0 J/mol
(B) 1 J/mol
(C) 100 J/mol
(D) –1 J/mol
Answer
(A) — By definition, ΔHf° = 0 for elements in their standard states.
9. Which of the following is not a state function?
(A) Enthalpy (H)
(B) Entropy (S)
(C) Heat (q)
(D) Internal energy (U)
Answer
(C) — Heat and work are path functions; they depend on the process, not just the state.
10. When a gas expands into a vacuum without doing work or exchanging heat, what happens to its internal energy?
(A) Increases
(B) Decreases
(C) Remains constant
(D) Becomes negative
Answer
(C) — In free expansion with no heat or work, ΔU = 0.
11. What is the value of ΔG when a reaction is at equilibrium?
(A) Positive
(B) Negative
(C) Zero
(D) Undefined
Answer
(C) — At equilibrium, ΔG = 0 because the system is in a stable state with no net change.
12. Which of the following reactions would have the greatest increase in entropy?
(A) N₂(g) → N₂(l)
(B) CO₂(s) → CO₂(g)
(C) H₂O(l) → H₂O(s)
(D) NaCl(s) → NaCl(l)
Answer
(B) — Sublimation (solid to gas) shows a large increase in entropy.
13. Which of the following is a correct expression of the First Law of Thermodynamics?
(A) ΔH = ΔS – T
(B) q = mcΔT
(C) ΔU = q + w
(D) ΔG = ΔH – TΔS
Answer
(C) — ΔU = q + w is the First Law, relating energy change to heat and work.
14. If a process is endothermic and increases entropy, what is its spontaneity dependent on?
(A) Enthalpy only
(B) Entropy only
(C) Temperature
(D) Pressure
Answer
(C) — When ΔH > 0 and ΔS > 0, spontaneity depends on temperature (ΔG = ΔH – TΔS).
15. Which of the following best describes Gibbs free energy?
(A) Maximum usable energy for work
(B) Total internal energy
(C) Total entropy
(D) Heat at constant volume
Answer
(A) — Gibbs free energy is the maximum energy available to do useful work at constant T and P.
16. What does a positive ΔH and negative ΔS imply about a reaction’s spontaneity?
(A) Spontaneous at all temperatures
(B) Nonspontaneous at all temperatures
(C) Spontaneous only at low temperatures
(D) Spontaneous only at high temperatures
Answer
(B) — ΔG = ΔH – TΔS will always be positive if ΔH > 0 and ΔS < 0.
17. For a system where ΔH = 0 and ΔS = positive, what is ΔG at any temperature?
(A) Positive
(B) Zero
(C) Negative
(D) Depends on pressure
Answer
(C) — ΔG = –TΔS; since ΔS > 0, ΔG < 0 → spontaneous at all T.
18. What happens to the entropy of the surroundings in an exothermic reaction?
(A) It increases
(B) It decreases
(C) It stays the same
(D) It becomes zero
Answer
(A) — Heat is released to the surroundings, increasing their entropy.
19. Which of the following changes represents a decrease in entropy?
(A) Evaporation
(B) Freezing
(C) Sublimation
(D) Dissolving sugar in water
Answer
(B) — Freezing creates a more ordered structure (liquid → solid), decreasing entropy.
20. Which thermodynamic variable is most directly influenced by molecular freedom of motion?
(A) Enthalpy
(B) Entropy
(C) Gibbs free energy
(D) Internal energy
Answer
(B) — Entropy measures disorder, which is tied to particle movement and arrangement.
21. A process with ΔH > 0 and ΔS < 0 will be:
(A) Always spontaneous
(B) Spontaneous only at low T
(C) Spontaneous only at high T
(D) Always nonspontaneous
Answer
(D) — With both unfavorable ΔH and ΔS, ΔG is always positive.
22. Which of the following best represents a system gaining energy from the surroundings?
(A) ΔH < 0
(B) ΔH > 0
(C) ΔS < 0
(D) ΔG < 0
Answer
(B) — A positive enthalpy change indicates the system absorbed energy (endothermic).
23. Which is the best explanation for why gas expansion is spontaneous?
(A) Enthalpy increases
(B) Work is done by the system
(C) Entropy increases
(D) Energy is conserved
Answer
(C) — Gas expansion leads to greater disorder and higher entropy.
24. In a reaction, ΔG becomes less negative as temperature increases. What is likely true?
(A) ΔH > 0 and ΔS > 0
(B) ΔH < 0 and ΔS < 0
(C) ΔH > 0 and ΔS < 0
(D) ΔH < 0 and ΔS > 0
Answer
(B) — As T increases, –TΔS becomes less favorable if ΔS < 0.
25. When ΔH = 0, under what condition is a reaction spontaneous?
(A) ΔS > 0
(B) ΔS < 0
(C) ΔS = 0
(D) ΔG > 0
Answer
(A) — ΔG = –TΔS → if ΔS > 0, then ΔG < 0 → spontaneous.
26. A reaction has ΔH = –40 kJ/mol and ΔS = –100 J/mol·K. At what temperature does this reaction change from spontaneous to nonspontaneous?
(A) 100 K
(B) 200 K
(C) 300 K
(D) 400 K
Answer
(D) — Set ΔG = 0 → ΔH = TΔS → –40,000 = T(–100) → T = 400 K.
27. The standard free energy change for a reaction is positive. What must be true about the equilibrium constant K?
(A) K > 1
(B) K = 1
(C) K < 1
(D) K = 0
Answer
(C) — ΔG° = –RT ln K. If ΔG° > 0, then K < 1, meaning reactants are favored.
28. Which of the following conditions always leads to a spontaneous process?
(A) ΔH > 0, ΔS > 0, high T
(B) ΔH < 0, ΔS > 0, any T
(C) ΔH > 0, ΔS < 0, low T
(D) ΔH < 0, ΔS < 0, high T
Answer
(B) — Negative ΔH and positive ΔS ensure ΔG < 0 for all temperatures.
29. Which of the following will always result in a nonspontaneous reaction?
(A) ΔH < 0, ΔS < 0, low T
(B) ΔH > 0, ΔS < 0
(C) ΔH < 0, ΔS > 0
(D) ΔH > 0, ΔS > 0, high T
Answer
(B) — Both unfavorable signs will always give ΔG > 0 → nonspontaneous.
30. A certain chemical reaction occurs with a decrease in both enthalpy and entropy. Which condition will make it spontaneous?
(A) High temperature
(B) Low temperature
(C) Any temperature
(D) Impossible
Answer
(B) — If both ΔH and ΔS are negative, low T makes –TΔS small enough for ΔG < 0.
31. Which thermodynamic function combines both enthalpy and entropy into one criterion for spontaneity?
(A) q
(B) w
(C) ΔG
(D) ΔU
Answer
(C) — Gibbs free energy (ΔG = ΔH – TΔS) considers both enthalpy and entropy.
32. Which of the following reactions would you expect to have a large positive entropy change?
(A) 2SO₂(g) + O₂(g) → 2SO₃(g)
(B) NH₄Cl(s) → NH₃(g) + HCl(g)
(C) H₂(g) + Cl₂(g) → 2HCl(g)
(D) N₂(g) + 3H₂(g) → 2NH₃(g)
Answer
(B) — Solid decomposing into two gases leads to a large entropy increase.
33. A student calculates ΔG° = –25 kJ/mol at 298 K. What does this imply?
(A) The reaction is not spontaneous
(B) The equilibrium favors products
(C) The system absorbs energy
(D) The entropy must be negative
Answer
(B) — A negative ΔG° means the reaction is spontaneous and favors product formation (K > 1).
34. At what temperature will a reaction with ΔH = +60 kJ/mol and ΔS = +150 J/mol·K become spontaneous?
(A) Below 200 K
(B) Above 400 K
(C) Exactly 273 K
(D) Never
Answer
(B) — T = ΔH / ΔS = 60,000 / 150 = 400 K → spontaneous above 400 K.
35. In a reaction, ΔH = –200 kJ and ΔS = –400 J/mol·K. What is the minimum temperature at which the reaction is spontaneous?
(A) Any temperature
(B) 400 K
(C) 500 K
(D) Never
Answer
(B) — Set ΔG = 0: –200,000 = T(–400) → T = 500 K; spontaneous below that.
36. Define Gibbs free energy in thermodynamic terms.
Answer
ΔG = ΔH – TΔS — Gibbs free energy determines spontaneity and combines enthalpy and entropy effects.
37. What is the significance of a negative ΔG in a chemical reaction?
Answer
The reaction is spontaneous — A negative Gibbs free energy means the process can occur without external energy input.
38. What is the entropy change when a substance transitions from solid to liquid?
Answer
Entropy increases — The molecules become more disordered during melting.
39. If a reaction is endothermic and becomes spontaneous only at high temperatures, what must be the sign of ΔS?
Answer
Positive — A positive entropy change allows ΔG to become negative at high temperatures.
40. At what value of ΔG is a reaction said to be at equilibrium?
Answer
ΔG = 0 — There is no net change; forward and reverse reactions occur at the same rate.
41. What does a large positive value of ΔS indicate about the system?
Answer
The system has increased in disorder — A large positive entropy means particles are more randomly distributed.
42. How is the spontaneity of a process affected if both ΔH and ΔS are negative?
Answer
Spontaneous at low temperatures — Only when the TΔS term is small enough for ΔG to be negative.
43. Explain what a standard enthalpy of formation means.
Answer
It is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states.
44. What does the First Law of Thermodynamics state?
Answer
Energy cannot be created or destroyed, only transferred or transformed.
45. A system absorbs 150 J of heat and does 50 J of work. What is the change in internal energy?
Answer
+100 J — ΔU = q + w = 150 J + (–50 J) = 100 J (since work done by the system is negative).
46. Why does sublimation lead to a large increase in entropy?
Answer
Because the solid directly becomes gas, increasing molecular motion and disorder.
47. What is the physical meaning of the TΔS term in the Gibbs free energy equation?
Answer
It represents the energy that is not available to do work due to disorder.
48. Why is a reaction with ΔG < 0 considered thermodynamically favorable?
Answer
It releases free energy and can occur without external input.
49. Describe the conditions under which an endothermic process can still be spontaneous.
Answer
If the entropy increases enough and temperature is high so that TΔS > ΔH.
50. If ΔG° = –40 kJ/mol for a reaction, what does that imply about the equilibrium constant K?
Answer
K > 1 — A negative ΔG° means products are favored and the equilibrium lies to the right.