Q.12. Enthalpies of formation of COg, CO2g, N2Og and N2O4g 

are –110 kJ mol-1, – 393 kJ mol-1, 81 kJ mol-1 and 9.7 kJ mol-1 respectively. Find the value of rH for the reaction:

N2O4g+3COgN2Og+3CO2g

NEETprep Answer:
Formation of CO2 from carbon and dioxygen gas can be represented as:
Cs+O2gCO2g                             fH=-393.5 kJ mol-1
(1 mole = 44 g)
Heat released on formation of 44 g CO2 = –393.5 kJ mol-1
Heat released on formation of 35.2 g CO2
=-393.5 kJ mol-144 g×35.2 g
=-314.8 kJ mol-1