Hess's Law for Enthalpy Calculation

Steps:

• Identify the target reaction from the table's given reactions.
• Manipulate equations by reversing endothermic steps (sign flip) and multiplying as needed.
• Sum the adjusted ΔH values for reactants and products.
• Calculate net ΔH as -393.5 kJ/mol (CO₂) + (-285.8 kJ/mol × 2 for H₂O) - (+110.5 kJ/mol for C₂H₆) - (0 for O₂) = -635.9 kJ/mol.

Why A is correct:

• Hess's law states total ΔH is sum of step enthalpies, yielding -635.9 kJ/mol for the combustion reaction.

Why the others are wrong:

• B omits water formation enthalpies.
• C reverses the sign, implying endothermic instead of exothermic.
• D adds an incorrect positive value from misapplying reversals.

Final answer: A