The hydrocarbon acetylene, C₂H₂(g), participates in two successive hydrogenation reactions:
Reaction 1 C₂H₂(g) + H₂(g) → C₂H₄(g) ΔH₁^∘ = –175 kJ
Reaction 2 C₂H₄(g) + H₂(g) → C₂H₆(g) ΔH₂^∘ = –137 kJ
Standard molar enthalpies of formation at 298 K are provided:
C₂H₄(g) +52.3 kJ mol^{–1}
C₂H₆(g) –84.7 kJ mol^{–1}
CO₂(g) –393.5 kJ mol^{–1}
H₂O(l) –285.8 kJ mol^{–1}
H₂(g) 0.0 kJ mol^{–1}
O₂(g) 0.0 kJ mol^{–1}
No value for ΔH_f^∘[C₂H₂(g)] is tabulated.
(a) Determine the standard molar enthalpy of formation, ΔH_f^∘, of C₂H₂(g) using the information given above. Provide a clear, logically ordered calculation.
(b) Write a balanced chemical equation for the complete combustion of 1 mol of C₂H₂(g) to CO₂(g) and H₂O(l).
(c) Using the ΔH_f^∘ value obtained in part (a) together with the tabulated data, calculate the standard enthalpy change for the combustion reaction written in part (b).
(d) Ethane, C₂H₆(g), is often used as a comparison fuel. Using your calculated combustion enthalpy for C₂H₂ and the tabulated formation data, compare the energy released per mole of carbon for the combustion of C₂H₂ versus C₂H₆. Provide a brief discussion of the implications for selecting acetylene as a fuel in applications requiring high flame temperatures.