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