Question

Why are the standard molar enthalpies for benzoic acid and biphenyl so different? Explain this in terms of bonds formed and broken in the individual combustion reactions. Identify two specific functional groups (think organic chemistry) that contribute to the enthalpic differences between these two compounds.

Answer 1

benzoic acid combustion equation

C₇H₆O₂ + 15/2 O₂ --> 7CO₂ + 3 H₂O

biphenyl combustion reaction is

(C₆H₅)2 + (29/2)O₂ ----> 12CO₂ + 5H₂O

from the combustion equation it is evident that in biphenyl reaction higher number of carbon atoms break to form carbon dioxide per mole. Benzoic acid releases 7 mole CO2 and Biphenyl release 12 mol CO2 per mole fuel.

So Biphenyl will give higher value of combustion than benzoic acid . which is also evident from literature because H_combustion of benzoic acid and biphenyl are -3226.6 KJ/mol and -6248.2 KJ/mol respectively.