Heat of reaction for,C6H12O6(s)+6O2(g)→6CO2(g)+6H2O(v)at constant pressure is -651 kcal at 17°C. Calculate the heat of reaction at constant volume at 17°C.

We know ΔH=ΔU+ΔnRTGiven ΔH=−651×103cal;R=2cal    T=290K;Δn=6+6−6=6∴    −651×103=ΔU+6×2×290∴    ΔU=−654480cal∴    ΔU=−654.48kcal Note: For C6H12O6(s)→6CO2(g)+6H2O(v)∆n - No. of product molecules-No. of reactant molecules (only in gaseous phase)=6+ 6-6=6