#ΔH^° "(reaction)" = ∑ΔH_f^° "(products)" − ∑ΔH_f^° "(reactants)"#. I'm having such a hard time understanding this equation. What exactly is happening?
1 Answer
This is a statement of Hess's Law using heats of formation.
Here's a standard Hess's Law problem
Calculate
NH₃(g) + ⁷/₄O₂(g) → NO₂(g) + ³/₂H₂O(g)
given the information
½N₂(g) + O₂(g) →NO₂(g);
H₂(g) + ½O₂(g) → H₂O(g);
½N₂(g) + ³/₂H₂(g) → NH₃(g);
You would write
½N₂(g) + O₂(g) →NO₂(g);
³/₂H₂(g) + ¾O₂(g) → ³/₂H₂O(g);
NH₃(g) →½N₂(g) + ³/₂H₂(g) →;
NH₃(g) + ⁷/₄O₂(g) → NO₂(g) + ³/₂H₂O(g);
Note that the given equations are all for the formation of the reactants from their elements. So the
The (+34 – 363) kJ term is the sum of the heats of formation of the products.
(-46 kJ) is the sum of the heats of formation of the reactants, since the heat of formation of oxygen is by definition zero.
We can write the answer as
-283 kJ = (34 – 363) kJ – (-46 + 0) kJ
Heat of reaction = sum of heats of reaction of products - sum of heats of reaction of reactants or
So