The heat of combustion at constant volume at `27^(@)C` of benzene and acetylene are `-800 kcal` and `310 kcal`. Find the heat of polymerisation of acetylene to benzene at constant pressure.

Explanation

`2C_(6)H_(6) +15O_(2) rarr 12CO_(2) +6H_(2)O(l) DeltaH_(1) = -2 xx 800 …(i)`
`2C_(2)H_(2) +5O_(2) rarr 4CO_(2) +2H_(2)O(l) DeltaH_(2) =- 2 xx 310 …(ii)`
Multiply equation (ii) by `3` and then subract:
`6C_(2)H_(2) = 2C_(6)H_(6)`
`DeltaH = 3(-620) -(-1600) =- 260 kcal`
`3C_(2)H_(2)(g) =C_(6)H_(6)(l)`
`DeltaH =- (260)/(2) =- 130`
`DeltaH =0 -3 =-3`
`DeltaH =- 130000 - 3 xx 1.987 xx 300 =- 131.8 kcal`