Question

The combustion reaction for benzoic acid,

c6h5co2h(s) 15 2 o2(g) → 7 co2(g) 3 h2o(ℓ) has ∆h0 = −771.2 kcal/mole.

use hess’s law to calculate ∆h 0 f for benzoic acid.
1. no choice is correct.
2. −167.0 kcal/mol
3. −83.42 kcal/mol
4. −91.65 kcal/mol
5. −56.8 kcal/mol
6. −385.0 kcal/mol

Answer 1

To calculate the enthalpy of formation (∆H⁰f) for benzoic acid, one must use Hess's Law and the given enthalpy of combustion, along with the standard enthalpies of formation for the products CO2 and H2O. Proper stoichiometry must be considered to solve the equation for ∆H⁰f.

Step-by-step explanation:

The student is asking to calculate the enthalpy of formation (∆H⁰f) for benzoic acid using Hess's Law and the known enthalpy of combustion (∆H⁰comb). To begin solving this problem, we take the enthalpy change given per mole for the combustion of benzoic acid, which is −771.2 kcal/mol. Then, we need to consider the stoichiometry of the balanced combustion reaction. For one mole of benzoic acid (C6H5CO2H), 15/2 moles of oxygen are required to produce 7 moles of carbon dioxide and 3 moles of water. Using the standard enthalpies of formation for CO2 (gas) and H2O (liquid), which are −393.5 kcal/mol and −58.68 kcal/mol respectively, we can write and solve the Hess's Law equation:

∆H⁰f (benzoic acid) = [−771.2 kcal] - [7 mol CO2 (−393.5 kcal/mol) + 3 mol H2O (−58.68 kcal/mol)]

This equation will give us the enthalpy of formation of benzoic acid when solved.