The heat of combustion at constant volume at 27°C of Benzene and acetylene are -800 k. cal and  -310 k cal. Find the heat of polymerization of acetylene to C6H6 at constant pressure

3C2H2+152O2→6CO2+3H2O;ΔU=3×(-310)=-930k.calC6H6+152O2→6CO2+3H2O;ΔU=-800k.calThe following polymerization reaction can be obtained by (1)-(2)3C2H2( g)→C6H6(l)ΔU=-930-(-800)=-130k.cal3C2H2( g)→C6H6(l);Δng=0-3=-3The relationship between heat of a reaction at constant pressure and heat of a reaction at constant volume for a chemical process is given asΔH=ΔU+ΔngRTΔH=-130-3×2×300×10-3=-131.8k.cal