Consider the combustion reaction, 2 cap c sub 2 cap h sub 2 g plus 5 cap o sub 2 g rightarrow 4 CO sub 2 g plus 2 cap h sub 2 cap o g where cap c sub 2 cap h sub 2 , Δ⁢H_f=227.4kJ/mol , CO sub 2 , Δ⁢H_f=−393.5kJ/mol , and cap h sub 2 cap o , Δ⁢H_f=−241.82kJ/mol What is the enthalpy of the reaction? Use Δ⁢H_rxn=∑(Δ⁢H_f_1p⁢r⁢o⁢d⁢u⁢c⁢t⁢s)−∑(Δ⁢H_f_1r⁢e⁢a⁢c⁢tan⁡t⁢s)

Use ΔH_rxn = products − reactants. The products total 4(−393.5) + 2(−241.82) = −2057.64 kJ, and the reactants total 2(227.4) + 5(0) = 454 kJ. Thus ΔH_rxn = −2511.64 kJ. Choice D, negative 2,512.4 kJ, is the closest and appears to be the intended answer, though it differs slightly from the result using the stated values.

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