the heat of combustion of benzene in a bomb calorimeter was found to be 3263.9 kj/mol at 25 C.calculate the heat of combustion of benzene at constant pressure?
Let us first write reaction of combustion in benzene.
C6H6 + 7½O2 (g) -----> 6CO2 (g) + 3H2O (l)
In the above reaction O2 and CO2 are the only gases.
g = no. of moles of products – no. of moles of reactants
= 6 - 7½ = - 1 ½ = -3/2
Given values U = 3263.9 kJ/mol; T = 298 K;
As we know,
H = U +ng RT R = 8.314
As this is an exothermic reaction H is negative.
H = (- 3263.9 kJ/mol) + (-3/2) (298 K)
= (- 3263.9 kJ/mol) + (-3/2) (2.477)
= (- 3263.9 kJ/mol) – 3.716 kJ/mol
H = - 3267.6 kJ/mol