Estimate ΔH for the reaction using bond dissociation energies from Table 7.1. Give your answer in kcal. C6H12O6 has five C-C bonds, seven C-H bonds, seven C-O bonds, and five O-H bonds.

The equation for the photosynthesis reaction in which carbon dioxide and water react to form glucose is. The hear reaction is the difference between the bond dissociation energies in the products and the bond dissociation energies of the reactants

The reactant molecules have 12 C = O, 12 H - O bonds while the product molecules have 5 C - C, 7 C - O, 5 H - O, and 6 O = O bonds. The average bond dissociation energies for the bonds involved in the reaction are 191 for C = O, 112 for H - O, 83 C - C, 99 C - H, 86 C - O, 119 O = O.

Substitute the average bond dissociation energies in the equation for and calculate as follows

= [12 (C=O) + 12 (H-O) ] - [5 (C-C) + 7 (C-H) + 7 (C-O) + 5 (H-O) + 6 (O=O) ]

= [12x191 kcal/mol + 12x112 kcal//mol] - [5x83 kcal/mol + 7x99 kcal/mol + 7x86 kcal/mol + 5x112 kcal/mol + 6x119 kcal/mol]

= 3636 kcal/mol - 2984 kcal/mol = 652 kcal/mol x 4.184 Kj/1kcal = 2.73x10^3 kJ/mol

So, enthalpy change for the reaction is 652 kcal/mol or 2.73x10^3 kJ/mol