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

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