Question

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.

Answer 1

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