Problem: A gas-phase reaction was run in an apparatus designed to maintain a constant pressure.Using data from Appendix C in the textbook, determine H for the formation of one mole of the product.

We’re being asked to determine the standard enthalpy change of formation (ΔH˚_rxn) for NO. 

Recall that ΔH˚_rxn can be calculated from the enthalpy of formation (ΔH˚_f) of the reactants and products involved: 

$\boxed{\mathbf{∆}{\mathbf{H}}_{\mathbf{rxn}}^{\mathbf{\circ }}\mathbf{=}\mathbf{ }\mathbf{∆}{\mathbf{H}}_{{}_{\mathbf{f}\mathbf{ }\mathbf{products}}}^{{}^{\mathbf{\circ }}}\mathbf{ }\mathbf{-}\mathbf{ }\mathbf{∆}{\mathbf{H}}_{{}_{\mathbf{f}\mathbf{ }\mathbf{reactants}}}^{{}^{\mathbf{\circ }}}}$

The chemical equation for that reaction is:

N_2(g) + O_2(g)→ NO_(g)

This equation is not yet balanced. To balance it, we have to make sure that the number of elements on both sides is equal.

Balance N: We have 1 N₂ on the reactant side and 1 O on the product side – add a coefficient of 2 to NO:

N₂(g) + O₂(g)→ 2 NO