For the combustion reaction carried out at constant pressure

CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics

Problem

For the combustion reaction CH4(g)+2 O2(g)→CO2(g)+2 H2O(l)CH_4(\text{g}) + 2\,O_2(\text{g}) \rightarrow CO_2(\text{g}) + 2\,H_2O(\text{l}) carried out at constant pressure, which of the following correctly describes the sign of the enthalpy change ΔH\Delta H? (a) ΔH\Delta H is positive because there are more moles of gas on the reactant side than on the product side, so work must be done on the system. (b) ΔH\Delta H is positive because energy must be supplied to break the C–H and O=O bonds in the reactants. (c) ΔH\Delta H is negative because heat is released to the surroundings during the reaction. (d) ΔH\Delta H would be negative only if the reaction were carried out at constant volume rather than constant pressure.

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