AP Chemistry 6.6 Introduction to Enthalpy of Reaction - MCQs - Exam Style Questions
▶️ Answer/Explanation
1. Analyze the Reaction:
The forward reaction is \(N_{2}O_{4}(g) \rightarrow 2~NO_{2}(g)\). This reaction involves breaking the single N-N bond that holds the \(N_2O_4\) molecule together.
2. Analyze the Energetics of Bond Breaking:
Breaking any chemical bond is an endothermic process. It always requires an input of energy from the surroundings to overcome the forces holding the atoms together.
3. Relate to \(\Delta H^{\circ}\):
An endothermic process, which absorbs energy, has a positive enthalpy change. Therefore, \(\Delta H^{\circ} > 0\). This eliminates options (A) and (B).
4. Evaluate Remaining Options:
- (C) \(\Delta H^{\circ} > 0\) because energy is released. This is incorrect. Energy is required, not released.
- (D) \(\Delta H^{\circ} > 0\) because energy is required to break the N-N bond. This is correct.
✅ Answer: (D)
Question
\(CH_{4}\)(g) + Cl(g) → \(CH_{3}\)(g) + HCl(g) ΔH° = −14 \(kJ/mol_{rxn}\)
\(NH_{3}\)(g) + Cl(g) → NH2(g) + HCl(g) ΔH° = −36 \(kJ/mol_{rxn}\)
\(H_2O\)(g) + Cl(g) → OH(g) + HCl(g) ΔH° = + 40 \(kJ/mol_{rxn}\)
Based on the data above, what can be concluded regarding the strength of the C −H, N−H, and O−H bonds in the molecules shown?
(A) The C −H bond is the strongest.
(B) The N −H bond is the strongest.
(C) The O −H bond is the strongest.
(D) Nothing can be concluded without knowing the strength of the H −Cl bond.
▶️Answer/Explanation
Ans:C
From the data, we observe that:
When \(CH_4\) reacts with Cl, it releases energy, indicating that the C−H bond is weaker than the Cl−H bond in HCl.
When \(NH_3\) reacts with Cl, it also releases energy, indicating that the N−H bond is weaker than the Cl−H bond in HCl.
When \(H_2O\) reacts with Cl, it absorbs energy, indicating that the O−H bond is stronger than the Cl−H bond in HCl.
Therefore, based on the provided data:
(C) The O−H bond is the strongest.
The correct answer is (C) The O−H bond is the strongest.
Question
\(Na_2S_2O_3(aq)+4NaOCl(aq)+2NaOH(aq)→2Na_2SO_4(aq)+4NaCl(aq)+H_2O(l)\)
The enthalpy change for the reaction represented by the chemical equation shown above is ΔH°\(=−1236kJ/mol_{rxn}\) . When 2.00 mol of NaOCl(aq) reacts completely with an excess of \(Na_2S_2O_3\) (aq) and of \(NaOH(aq)\) , which of the following enthalpy changes occurs?
A 618kJ of heat is absorbed.
B 2470kJ of heat is absorbed.
C 618kJ of heat is released.
D 2470kJ of heat is released.
▶️Answer/Explanation
Ans:C
The reaction is exothermic, so heat is released. Because NaOCl(aq) is the limiting reagent and only 2.00mol of NaOCl(aq) is used, the enthalpy change is equal to 12×\(−1236kJ/mol_{rxn}\); thus, 618kJ of heat is released.