Question

The standard heats of formation of no2 and n2o4 are 8.0 and 2.0 kcal mol^-1 respectively the heat of dimerization of no2 in kcal

Answer 1

standard heat of formation of NO2 is 8 KCal/mol
standard heat of formation of N2O4 is 2 Kcal/mol.

we have to find heat of dimerization of NO2.
we know, NO2 dimerizes in N2O4 .
e.g., NO2 + NO2 -------> N2O4
means, two mole of NO2 dimerize in N2O4 .
so, heat of dimerization = heat of prdouct - heat of reactant
= 2 - 2 × 8 = -14 Kcal

heat of dimerization is -14Kcal { here negative sign indicates that heat is evolved, e.g., reaction is exothermic}

Answer 2

N2O4 => 2 NO2

Heat of dimerisation is :

heat of formation of  no of moles of reactant - no of moles of product

 2 - 2 x 8 = -14 Kcal