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.
Questions
\(N_{2}O_{4(g)}\rightleftharpoons 2NO_{2(g)}\) \(K_{p}=3.0 at 70^{\circ C}\)
colorless brown
A mixture of \(NO_{2(g)}\) and \(N_{2}O_{4(g)}\) is placed in a glass tube and allowed to reach equilibrium at \(70^{\circ C}\), as represented above. Which of the following statements about \(\bigtriangleup H^{\circ }\)° for the reaction is correct?
(A) \(\bigtriangleup H^{\circ }\) < 0 because energy is released when the N-N bond breaks.
(B) \(\bigtriangleup H^{\circ }\) < 0 because energy is required to break the N-N bond.
(C) \(\bigtriangleup H^{\circ }\) > 0 because energy is released when the N-N bond breaks.
(D) \(\bigtriangleup H^{\circ }\) > 0 because energy is required to break the N-N bond.
▶️Answer/Explanation
Ans: D
The given reaction is:
\[\mathrm{N}_2 \mathrm{O}_4(g) \rightleftarrows 2 \mathrm{NO}_2(g)\]
The enthalpy change (\(\Delta H^\circ\)) for this reaction can be determined based on whether the reaction is endothermic or exothermic.
In this reaction, \(\mathrm{N}_2 \mathrm{O}_4\) decomposes into \(2 \mathrm{NO}_2\), which means the \(\mathrm{N}-\mathrm{N}\) bond in \(\mathrm{N}_2 \mathrm{O}_4\) is broken. Breaking bonds requires energy input, making the reaction endothermic.
So, the correct statement about \(\Delta H^\circ\) for the reaction is:
(D) \(\Delta H^\circ > 0\) because energy is required to break the \(\mathrm{N}-\mathrm{N}\) bond.
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.
Question
\(2Al(s)+Fe_2O_3(s)→2Fe(s)+Al_2O_3(s)\) ΔH°\(=−850kJ/mol_{rxn}\)
The chemical equation shown above represents the thermite reaction. What is the approximate amount of heat released when 108g of Al(s) reacts with excess \(Fe_2O_3(s)\) ?
A 210kJ
B 430kJ
C 850kJ
D 1700kJ
▶️Answer/Explanation
Ans:D
108g of Al(s) is equal to 4.00mol of Al(s); thus, based on the stoichiometry of the reaction, the amount of heat released is equal to 2×\(850kJ/mol_{rxn}\), or 1700kJ.
Question
\(2CO(g)+C(g)→C_3O_2(g)\) ΔH°\(=127.3kJ/mol_{rxn}\)
The equation shown above represents an endothermic reaction between CO(g) and C(g). What is the amount of heat absorbed when 1.00mol of CO(g) reacts with an excess of C(g)?
A 31.8kJ
B 63.7kJ
C 127.3kJ
D 254.6kJ
▶️Answer/Explanation
Ans:B
Based on the stoichiometry of the reaction, 127.3kJ of heat is absorbed for every 2mol of CO(g) that reacts; thus, if 1.00mol of CO(g) reacts, then 63.7kJ is absorbed.
Question
\(N_{2}O_{4}\rightleftharpoons 2NO_{2}(g)\)
A mixture of\( NO_{2}\)(g) and \(N_{2}O_{4}\)(g) is placed in a glass tube and allowed to reach equilibrium at 70°C, as represented above. \( K_{p}=3.0~at~70^{\circ}C\)
Which of the following statements about \(\Delta H°\) for the reaction is correct?
(A) \(\Delta H° <0\) because energy is released when the N-N bond breaks.
(B) \(\Delta H <0 \)because energy is required to break the N-N bond.
(C) \(\Delta H >0\) because energy is released when the N-N bond breaks.
(D)\( \Delta H°> 0 \)because energy is required to break the N-N bond.
▶️Answer/Explanation
Ans:A
Question
In which of the following processes will ∆S° be negative?
(A) \(C_{2}H_{5}OH(l) \rightarrow C_{2}H_{5}OH(g)\)
(B) \(NaCl(s) \rightarrow NaCl(l)\)
(C)\( CO_2(s) \rightarrow CO_2(g)\)
(D)\( Cl_2(g) \rightarrow Cl_2\)(l)
▶️Answer/Explanation
Ans:D