ΔH of which of the following reaction(s) may be directly taken as enthalpy of combustion of the conc — Thermodynamics and Thermochemistry Chemistry Question

💡 Solution & Explanation

By definition, the standard enthalpy of combustion is the enthalpy change when exactly 1 mole of a substance undergoes complete combustion in excess oxygen to form standard products (like $CO_2$(g) and $H_2O$(l)).<br>(a) C(s) + $O_2$(g) → $CO_2$(g): Exactly 1 mole of carbon is completely burned to $CO_2$. Thus, ΔH equals the enthalpy of combustion of carbon.<br>(b) 2$C_2H_6$(g) + 7$O_2$(g) → 4$CO_2$(g) + 6$H_2O$(l): This is the combustion of 2 moles of ethane. To represent the enthalpy of combustion, it must be per mole of $C_2H_6$. Thus, ΔH here is twice the enthalpy of combustion.<br>(c) $C_3H_8$(g) + 7/2 $O_2$(g) → 3$CO$(g) + 4$H_2O$(l): This is incomplete combustion of propane (forming $CO$ instead of $CO_2$), so it does not represent the standard enthalpy of combustion.<br>(d) 1/2 $N_2$(g) + 1/2 $O_2$(g) → $NO$(g): This is the formation of $NO$, not combustion of a fuel. (Nitrogen does not support combustion in this manner).<br>Thus, only reaction (a) directly represents the enthalpy of combustion. (Answer key on page 5.35 confirms (a)).