Rucete ✏ AP Chemistry In a Nutshell
11. Thermodynamics — Practice Questions 3
This chapter introduces the fundamental principles of thermodynamics, including energy changes, entropy, enthalpy, and Gibbs free energy, which determine whether reactions are thermodynamically favorable.
(Multiple Choice — Click to Reveal Answer)
1. Which of the following processes results in a decrease in entropy?
(A) Evaporation of water
(B) Sublimation of dry ice
(C) Freezing of liquid water
(D) Dissolving salt in water
Answer
(C) — Freezing decreases molecular motion and randomness, lowering entropy.
2. What does a negative value of ΔH° indicate for a chemical reaction?
(A) The reaction absorbs heat
(B) The reaction is at equilibrium
(C) The reaction releases heat
(D) The reaction has high activation energy
Answer
(C) — A negative ΔH° means the reaction is exothermic.
3. Which quantity is a measure of disorder in a system?
(A) Enthalpy
(B) Entropy
(C) Free energy
(D) Internal energy
Answer
(B) — Entropy (S) quantifies the disorder or randomness in a system.
4. What condition must be met for a process to be spontaneous at all temperatures?
(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
(C) — A negative enthalpy and positive entropy ensure ΔG is always negative.
5. What is the sign of ΔG when a reaction is nonspontaneous under standard conditions?
(A) Positive
(B) Negative
(C) Zero
(D) Undefined
Answer
(A) — Nonspontaneous reactions have positive Gibbs free energy.
6. Which of the following is a state function?
(A) Heat
(B) Work
(C) Enthalpy
(D) Pathway
Answer
(C) — Enthalpy is a state function, independent of the path taken.
7. What does it mean if ΔS is positive for a process?
(A) The system is releasing heat
(B) The system is becoming more ordered
(C) The disorder of the system is increasing
(D) The reaction is exothermic
Answer
(C) — A positive entropy change means increased disorder.
8. Which law of thermodynamics states that energy cannot be created or destroyed?
(A) Zeroth law
(B) First law
(C) Second law
(D) Third law
Answer
(B) — The First Law is the law of conservation of energy.
9. What happens to the surroundings in an exothermic reaction?
(A) The surroundings absorb heat
(B) The surroundings release heat
(C) The surroundings remain unchanged
(D) Entropy of surroundings decreases
Answer
(A) — In an exothermic process, heat is transferred to the surroundings.
10. What is the sign of ΔG for a reaction at equilibrium?
(A) Positive
(B) Negative
(C) Zero
(D) Cannot be determined
Answer
(C) — At equilibrium, Gibbs free energy is zero.
11. Which of the following reactions would show the greatest increase in entropy?
(A) H₂(g) + Cl₂(g) → 2HCl(g)
(B) C(s) + O₂(g) → CO₂(g)
(C) NH₄NO₃(s) → N₂O(g) + 2H₂O(g)
(D) 2SO₂(g) + O₂(g) → 2SO₃(g)
Answer
(C) — A solid turning into gases has the greatest entropy increase.
12. Which factor most directly influences the spontaneity of a chemical reaction?
(A) Activation energy
(B) Reaction rate
(C) ΔG (Gibbs free energy)
(D) Catalyst presence
Answer
(C) — ΔG determines thermodynamic favorability (spontaneity).
13. What is the standard entropy of a pure crystalline substance at 0 K?
(A) 0 J/mol·K
(B) 100 J/mol·K
(C) 1 J/mol·K
(D) Undefined
Answer
(A) — By the third law of thermodynamics, entropy is zero at 0 K for a perfect crystal.
14. When is a process with ΔH > 0 and ΔS < 0 spontaneous?
(A) Always
(B) Never
(C) Only at low temperatures
(D) Only at high temperatures
Answer
(B) — Both signs are unfavorable, so ΔG is always positive.
15. In thermodynamics, what does q represent?
(A) Work
(B) Heat
(C) Enthalpy
(D) Entropy
Answer
(B) — q represents heat transferred in or out of a system.
16. Which is an example of a spontaneous process?
(A) Water freezing at 50°C
(B) Water evaporating at 25°C
(C) Ice melting at –10°C
(D) Iron rusting in air
Answer
(D) — Rusting is a natural, spontaneous oxidation process.
17. The Second Law of Thermodynamics states:
(A) Energy is conserved
(B) Entropy always decreases
(C) Heat flows from cold to hot
(D) Entropy of the universe increases in a spontaneous process
Answer
(D) — The Second Law deals with entropy increasing in spontaneous processes.
18. When ΔH is negative and ΔS is negative, under what condition is the reaction spontaneous?
(A) High T
(B) Low T
(C) Any T
(D) Never
Answer
(B) — At low temperatures, –TΔS is less significant, allowing ΔG to be negative.
19. Which of the following does not affect entropy change in a system?
(A) Change in temperature
(B) Phase change
(C) Type of reaction mechanism
(D) Change in number of gas particles
Answer
(C) — Mechanism doesn't affect thermodynamic entropy; it’s related to states.
20. What does a positive value of ΔS° suggest about a reaction?
(A) The system is becoming more ordered
(B) The system is releasing heat
(C) The reaction is nonspontaneous
(D) The system is becoming more disordered
Answer
(D) — A positive ΔS° indicates increased disorder or randomness.
21. Which has the highest standard entropy?
(A) H₂O(l)
(B) H₂O(g)
(C) H₂O(s)
(D) H₂O(aq)
Answer
(B) — The gaseous state has the most disorder, thus highest entropy.
22. Which of the following increases the internal energy of a system?
(A) Heat loss and work done by system
(B) Heat gain and work done on system
(C) Heat loss and work done on system
(D) Heat gain and work done by system
Answer
(B) — Energy increases with heat input and compression (work done on).
23. A reaction has ΔH = 0 and ΔS > 0. What can be said about ΔG at any T?
(A) Always negative
(B) Always positive
(C) Depends on temperature
(D) Zero
Answer
(A) — ΔG = –TΔS. If ΔS > 0, then ΔG is always negative, so the process is spontaneous.
24. What does ΔH represent in a thermochemical equation?
(A) Energy stored in chemical bonds
(B) Change in entropy
(C) Heat absorbed or released
(D) Work done by the system
Answer
(C) — Enthalpy change (ΔH) represents heat at constant pressure.
25. Which combination of signs for ΔH and ΔS would never result in a spontaneous reaction?
(A) +ΔH, +ΔS
(B) +ΔH, –ΔS
(C) –ΔH, +ΔS
(D) –ΔH, –ΔS
Answer
(B) — Both signs are unfavorable, leading to positive ΔG regardless of temperature.
26. A reaction has ΔH = +75 kJ/mol and ΔS = +250 J/mol·K. At what temperature does it become spontaneous?
(A) Below 298 K
(B) Above 300 K
(C) Below 150 K
(D) Above 600 K
Answer
(B) — Use T = ΔH / ΔS = 75000 / 250 = 300 K. The reaction is spontaneous above this temperature.
27. Which of the following correctly describes a system with increasing entropy and decreasing enthalpy?
(A) ΔH > 0, ΔS < 0
(B) ΔH < 0, ΔS < 0
(C) ΔH < 0, ΔS > 0
(D) ΔH > 0, ΔS > 0
Answer
(C) — Negative enthalpy and positive entropy make the process highly spontaneous.
28. Which of the following reactions best exemplifies a decrease in entropy?
(A) 2SO₂(g) + O₂(g) → 2SO₃(g)
(B) H₂O(l) → H₂O(g)
(C) CaCO₃(s) → CaO(s) + CO₂(g)
(D) NH₄NO₃(s) → N₂O(g) + 2H₂O(g)
Answer
(A) — Three gas molecules becoming two reduces disorder, so entropy decreases.
29. If a process is spontaneous and exothermic, what can be inferred about the entropy change?
(A) ΔS must be positive
(B) ΔS must be negative
(C) ΔS could be positive or negative
(D) ΔS must be zero
Answer
(C) — As long as ΔG is negative, spontaneity can occur with either sign of ΔS depending on temperature.
30. A process has ΔG = 0. Which of the following statements is true?
(A) The process is spontaneous
(B) The process is nonspontaneous
(C) The system is at equilibrium
(D) The entropy is zero
Answer
(C) — ΔG = 0 indicates the system is at equilibrium.
31. What does the term "thermodynamically favorable" imply about a reaction?
(A) It occurs rapidly
(B) It requires a catalyst
(C) It has a negative ΔG
(D) It is exothermic
Answer
(C) — Thermodynamic favorability refers to a negative Gibbs free energy.
32. What is the primary difference between ΔH and q in thermodynamics?
(A) ΔH includes work done
(B) q is only defined for gases
(C) ΔH is heat at constant pressure
(D) q is a state function
Answer
(C) — ΔH represents heat transferred under constant pressure conditions.
33. Which of the following sets of conditions guarantees a nonspontaneous process?
(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
(C) — A positive enthalpy and negative entropy always lead to a positive ΔG.
34. For which of the following reactions is ΔS most likely negative?
(A) H₂O(l) → H₂O(g)
(B) N₂(g) + 3H₂(g) → 2NH₃(g)
(C) NaCl(s) → Na⁺(aq) + Cl⁻(aq)
(D) C(s) + O₂(g) → CO₂(g)
Answer
(B) — Four gas molecules form two, indicating a decrease in entropy.
35. If a spontaneous reaction becomes nonspontaneous when temperature increases, what does that imply about ΔS?
(A) ΔS is positive
(B) ΔS is negative
(C) ΔS is zero
(D) ΔS depends on pressure
Answer
(B) — A negative entropy change makes the TΔS term more positive at high T, making ΔG positive.
36. Explain why endothermic reactions can still be spontaneous under certain conditions.
Answer
When entropy increases significantly and temperature is high — A large positive ΔS and high temperature can make ΔG negative despite a positive ΔH.
37. Describe the significance of ΔG = 0 for a chemical process.
Answer
The reaction is at equilibrium — There is no net change in the forward or reverse direction.
38. A process has ΔH = –100 kJ/mol and ΔS = –250 J/mol·K. At what maximum temperature will this process remain spontaneous?
Answer
400 K — Set ΔG = 0 and solve: 0 = ΔH – TΔS → T = 100000 / 250 = 400 K.
39. In thermodynamics, what is the relationship between entropy and the number of microstates?
Answer
Entropy increases with more microstates — More microstates means higher disorder and thus greater entropy.
40. Give a real-life example of a spontaneous endothermic process.
Answer
Dissolution of ammonium nitrate in water — The process absorbs heat but occurs spontaneously due to a large entropy increase.
41. What is the formula for calculating standard Gibbs free energy change using the equilibrium constant?
Answer
ΔG° = –RT ln K — This relates free energy to the equilibrium position.
42. Explain why entropy increases when ice melts to form water.
Answer
Particles move more freely — Liquid water has more molecular motion than ice, increasing randomness.
43. State the Second Law of Thermodynamics in terms of entropy and the universe.
Answer
The total entropy of the universe always increases in a spontaneous process.
44. A chemical reaction has ΔG = –50 kJ/mol. What does this indicate about the spontaneity and equilibrium constant?
Answer
Spontaneous, and K > 1 — A negative ΔG implies the reaction favors product formation.
45. What is the meaning of a state function in thermodynamics?
Answer
A property that depends only on the initial and final states, not the path — Enthalpy, entropy, and internal energy are state functions.
46. Why does boiling water at 100°C require energy even though the temperature stays constant?
Answer
Energy is used to break intermolecular forces, increasing entropy — This is latent heat, not reflected by temperature rise.
47. Define enthalpy of formation.
Answer
ΔH°f is the change in enthalpy when 1 mol of a compound forms from elements in their standard states.
48. What conditions favor a reaction with ΔH > 0 and ΔS > 0?
Answer
High temperature — At high T, the positive entropy term can outweigh the endothermic enthalpy.
49. A gas expands into a vacuum. What is the sign of ΔS and why?
Answer
ΔS is positive — Expansion increases disorder and the number of accessible microstates.
50. What is the physical meaning of negative Gibbs free energy in terms of energy availability?
Answer
Free energy is available to do work — A negative ΔG means energy is released and the system can perform useful work.