The difference between heats of reaction at constant pressure and constant volume for the reaction,
`2C_6H_6(l) + 15O_2(g) to 12CO_2(g) + 6H_2O (l)`
at `25^@C` in kJ is

Explanation

Heat of reaction at constant pressure (`DeltaH`) and heat of reaction at constant volume (`DeltaU`) are related as
`Delta H = Delta U + Deltan RT`
or `DeltaH - DeltaU = Deltan RT`
For the given reaction
`2C_6H_6(l) + 15O_2(g) to 12CO_2(g) + 6H_2O(l)`
`Deltan = Sigman_("gas. product") - Sigamn_("gas. reactions")`
` = 12 - 15`
`= -3`
Therefore, for the given reaction
`Delta H - DeltaU = Deltan RT`
`= (-3) xx8.314 xx 10^(-3) xx 298`
`( :. R = 8.314 xx10^(-3) kJ K^(-1) mol^(-1))`
`=-7.43 kJ`