Question

The standard enthalpy of formation, ΔHf0, for N2O4 is the enthalpy change for which reaction?

a. 2NO(g) + O2(g) → N2O4(g)

b. 2NO2(g) → N2O4(g)

c. 2N(g) + 4O(g) → N2O4(g)

d. N2(g) + 2O2(g) → N2O4(g)

e. N2O(g) + O3(g) → N2O4(g)

Answer 1

Enthalpy of formation of a compound is the enthalpy of the reaction in which one mole of the required compound is formed from the reactants having enthalpy of formation 0.

In thermodynamics , all elements in their standard states (Oxygen gas (O2) , Nitrogen gas(N2), graphite form of solid carbon etc ) have a standard enthalpy of formation zero , as there is no change involved in their formation , or we can say it takes no energy to form a naturally-occuring compound.

Now,  ΔHfo, for N2O4 is the enthalpy change for reaction involving only O2 and N2 in the reactant side .
Thus d is the correct answer. (N2(g) + O2(g) → N2O4(g)).