For which one of the following reactions is ΔH°rxn equal to the heat of formation of the product? For which one of the following reactions is H°rxn equal to the heat of formation of the product? (1/2) A. N2 (g) + O2 (g) → NO2 (g) B. P (g) + 4H (g) + Br (g) → PH4Br (l) C. 12C (g) + 11H2 (g) + 11O (g) → C6H22O11 (g) D. N2 (g) + 3H2 (g) → 2NH3 (g) E. 6C (s) + 6H (g) → C6H6 (l)

A. (1/2) N₂ (g) + O₂ (g) → NO₂ (g)

Explanation:

H°rxn = ∑n*H°f, products - ∑n*H°f, reactants (where n is the number of moles)

H°f is the enthalpy of formation at standard conditions, and it's 0 by the compounds formed by a single element at its atmospheric phase. So, to H°rxn = H°f, products, the number of moles of the product must be 1 and the H°f, reactants must be 0.

Both conditions are only satisfied in letter A.

In letter B, the reactants H and Br are not in the molecule formula, so their H°f is not 0. The mase happens in letter C for O, and in letter E for H. In letter D, 2 moles of the product are being formed.