For the combustion reaction

CHEMISTRY · Subjective · A Level · Chemistry · Energetics · Energetics

Problem

For the combustion reaction CH4(g)+2 O2(g)→CO2(g)+2 H2O(g)\mathrm{CH_4(g)} + 2\,\mathrm{O_2(g)} \rightarrow \mathrm{CO_2(g)} + 2\,\mathrm{H_2O(g)}, a student used the standard enthalpies of formation ΔHf∘(CH4(g))=−74.6 kJ mol−1\Delta H_f^\circ(\mathrm{CH_4(g)}) = -74.6\,\text{kJ mol}^{-1}, ΔHf∘(CO2(g))=−393.5 kJ mol−1\Delta H_f^\circ(\mathrm{CO_2(g)}) = -393.5\,\text{kJ mol}^{-1}, ΔHf∘(H2O(l))=−285.8 kJ mol−1\Delta H_f^\circ(\mathrm{H_2O(l)}) = -285.8\,\text{kJ mol}^{-1}, ΔHf∘(H2O(g))=−241.8 kJ mol−1\Delta H_f^\circ(\mathrm{H_2O(g)}) = -241.8\,\text{kJ mol}^{-1}. By mistakenly substituting the liquid-water value instead of the gaseous value, the student calculated ΔH=−890 kJ\Delta H = -890\,\text{kJ}. Determine the correct ΔH\Delta H (in kJ) for the combustion reaction and state the numerical error (in kJ) introduced by using the wrong phase value.

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