Using average bond energies and the standard enthalpy of formation of C(g) (718.4 kJ/mol), estimate the standard enthalpy of formation of gaseous benzene, C6H6(g). Which of the following values is closest to the correct estimate?

• A. 250 kJ/mol
• B. -49 kJ/mol
• C. 500 kJ/mol
• D. 150 kJ/mol

Answer: (B)

Verified step by step guidance

1. Start by understanding the concept of standard enthalpy of formation, which is the change in enthalpy when one mole of a compound is formed from its elements in their standard states.
2. Identify the chemical reaction for the formation of benzene, C6H6(g), from its elements in their standard states: 6C(graphite) + 3H2(g) → C6H6(g).
3. Use the given standard enthalpy of formation for gaseous carbon, C(g), which is 718.4 kJ/mol. This value is needed to convert carbon from its standard state (graphite) to gaseous carbon.
4. Calculate the total energy required to break the bonds in the reactants and form the bonds in the product using average bond energies. Consider the bonds in benzene and the bonds in the reactants (C-C, C-H, H-H).
5. Apply Hess's Law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for each step. Use the bond energies and the enthalpy of formation of gaseous carbon to estimate the enthalpy of formation for benzene.