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Energy from Fuels · Paper 2 — Free Chemistry HL Practice Questions

1ChallengeSAQ-LCombustion reactions and energy release7 marksPaper 2~11 min

Data

ΔHf⊖(CO2(g))=−393.5 kJ mol−1\Delta H_f^\ominus(\text{CO}_2(g)) = -393.5\,\text{kJ mol}^{-1} ΔHf⊖(CO(g))=−110.5 kJ mol−1\Delta H_f^\ominus(\text{CO}(g)) = -110.5\,\text{kJ mol}^{-1} ΔHf⊖(H2O(l))=−285.8 kJ mol−1\Delta H_f^\ominus(\text{H}_2\text{O}(l)) = -285.8\,\text{kJ mol}^{-1} ΔHf⊖(CH4(g))=−74.8 kJ mol−1\Delta H_f^\ominus(\text{CH}_4(g)) = -74.8\,\text{kJ mol}^{-1}
Methane (CH4\text{CH}_4) is a common fuel used in domestic heating. The complete combustion of methane is represented by: CH4(g)+2O2(g)→CO2(g)+2H2O(l)ΔHc⊖=−890 kJ mol−1\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l) \qquad \Delta H_c^\ominus = -890\,\text{kJ mol}^{-1} In poorly ventilated gas heaters, incomplete combustion occurs, producing carbon monoxide: CH4(g)+32O2(g)→CO(g)+2H2O(l)\text{CH}_4(g) + \tfrac{3}{2}\text{O}_2(g) \rightarrow \text{CO}(g) + 2\text{H}_2\text{O}(l)
(a)
Calculate the enthalpy change, in kJ mol−1\text{kJ mol}^{-1}, for the incomplete combustion of methane using the standard enthalpies of formation below. [3 marks]
(b)
Explain, in terms of bond breaking and bond forming, why incomplete combustion of methane releases less energy per mole than complete combustion. [2 marks]
(c)
A household uses a gas heater in a poorly ventilated room. Evaluate whether the reduced energy output and the nature of the combustion products make incomplete combustion more hazardous than simply being less efficient. [2 marks]
diagram

Solutions

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