Question
Consider the following equations.
\({\text{2Fe(s)}} + {\text{1}}\frac{1}{2}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{(s) }}\) \(\Delta {{\text{H}}^\Theta } = x\)
\({\text{CO(g)}} + \frac{1}{2}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{C}}{{\text{O}}_{\text{2}}}{\text{(g)}}\) \(\Delta {{\text{H}}^\Theta } = y\)
What is the enthalpy change of the reaction below?
\[{\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{(s)}} + {\text{3CO(g)}} \to {\text{3C}}{{\text{O}}_{\text{2}}}{\text{(g)}} + {\text{2Fe(s)}}\]
A. \(3y – x\)
B. \(3y + x\)
C. \( – 3y – x\)
D. \( – 3y + x\)
Answer/Explanation
Markscheme
A
Examiners report
This question was answered correctly by nearly 85% of the candidates which was encouraging.
Question
Hydrazine reacts with oxygen.
N2H4(l) + O2(g) → N2(g) + 2H2O(l) ΔHθ = -623 kJ
What is the standard enthalpy of formation of N2H4(l) in kJ? The standard enthalpy of formation of H2O(l) is -286 kJ.
A. -623 – 286
B. -623 + 572
C. -572 + 623
D. -286 + 623
Answer/Explanation
Markscheme
C
Examiners report
Question
What is the enthalpy of combustion of butane in kJ mol−1?
2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(l)
\[\begin{array}{*{20}{l}} {{\text{C(s)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{C}}{{\text{O}}_2}{\text{(g)}}}&{\Delta H = x{\text{ kJ}}} \\ {{{\text{H}}_2}{\text{(g)}} + \frac{{\text{1}}}{2}{{\text{O}}_2}{\text{(g)}} \to {{\text{H}}_2}{\text{O(l)}}}&{\Delta H = y{\text{ kJ}}} \\ {4{\text{C(s)}} + {\text{5}}{{\text{H}}_2}{\text{(g)}} \to {{\text{C}}_4}{{\text{H}}_{{\text{10}}}}{\text{(g)}}}&{\Delta H = z{\text{ kJ}}} \end{array}\]
A. 4x + 5y − z
B. 4x + 5y + z
C. 8x + 10y − 2z
D. 8x + 5y + 2z
Answer/Explanation
Markscheme
A
Examiners report
Question
Enthalpy changes of reaction are provided for the following reactions.
\[\begin{array}{*{20}{l}} {{\text{2C(s)}} + {\text{2}}{{\text{H}}_2}{\text{(g)}} \to {{\text{C}}_2}{{\text{H}}_4}{\text{(g)}}}&{\Delta {H^\Theta } = + {\text{52 kJ}}\,{\text{mo}}{{\text{l}}^{ – 1}}} \\ {{\text{2C(s)}} + {\text{3}}{{\text{H}}_2}{\text{(g)}} \to {{\text{C}}_2}{{\text{H}}_6}{\text{(g)}}}&{\Delta {H^\Theta } = – {\text{85 kJ}}\,{\text{mo}}{{\text{l}}^{ – 1}}} \end{array}\]
What is the enthalpy change, in \({\text{kJ}}\,{\text{mo}}{{\text{l}}^{ – 1}}\), for the reaction between ethene and hydrogen?
\[{{\text{C}}_2}{{\text{H}}_4}{\text{(g)}} + {{\text{H}}_2}{\text{(g)}} \to {{\text{C}}_2}{{\text{H}}_6}{\text{(g)}}\]
A. –137
B. –33
C. +33
D. +137
Answer/Explanation
Markscheme
A
Question
B. 2 × (−394) + (−572) − (−2602)
C. 2 × (−394) + \(\frac{1}{2}\) (−572) + \(\frac{1}{2}\) (−2602)
D. 2 × (−394) + (−572) + (−2602)
Answer/Explanation
Markscheme
A