To calculate the enthalpy of the reaction, N2 (g) + 3H2 (g) → 2NH3 (g), you need to subtract which heats of formation?

N2 (g) and H2 (g)

NH3 (g)

N2 (g), H2 (g), and NH3(g)

none of them

Question

To calculate the enthalpy of the reaction, N2 (g) + 3H2 (g) → 2NH3 (g), you need to subtract which heats of formation?
		
N2 (g) and H2 (g)
		
NH3 (g)
		
N2 (g), H2 (g), and NH3(g)
		
none of them

aaronq · Answer

When calculating the change in enthalpy of the reaction $\sf (ΔH_{rxn})$, you subtract the enthalpy of formation of the reactants from the products.

  $\sf  ΔH_{rxn}=H_f(products)−H_f(reactants)$