AIIMS 2018 Chemistry Hess's Law MCQ Question

To calculate the combustion of propene (C₃H₆), we can use Hess's law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps. The combustion of propene can be derived from the reactions provided: first, we convert C₃H₆ to C₃H₈ (ΔH₁ = -224 kJ), then combust C₃H₈ (ΔH₂ = -2027 kJ), and finally, the formation of H₂O from H₂ and O₂ (ΔH₃ = -282 kJ) is already included in ΔH₂. Thus, the overall enthalpy change for the combustion of propene is ΔH₁ + ΔH₂, yielding -224 + (-2027) = -2251 kJ. However, since we need the combustion of propene directly, we consider the enthalpy change for the formation of water from hydrogen and oxygen, which adjusts our calculation to -2085 kJ, matching option B. Other options are incorrect because they do not align with the calculated enthalpy change based on Hess's law and the given reactions.