How does the A Hreaction relate to the A He of molecules involved in a reaction?

A. A Hreaction = A Hf, reactants

B. AHreaction = A Hf, products - AHf, reactants

C. A Hreaction = AHf, products

D. A Hreaction = A Ht, products + AHf, reactants

B. ΔHreaction = ΔH°f reactants - ΔH°f products

Explanation:

Using Hess's law, it is possible to sum ΔH of several related reactions to find ΔH of a particular reaction.

Having in mind Hess's law, ΔH°f is defined as the change in enthalpy during the formation of 1 mole of substance from its constituent elements (That is, pure elements, mono or diatomics, that have a ΔH° = 0).

For example, in ΔH°f of H₂O, the equation is:

H₂ (g) + 1/2O₂ (g) → H₂O (g)

The constituent elements with ΔH°f = 0 are H₂ (g) and O₂ (g).

Now, using Hess's law, you can sum the ΔH°f of substance in a reaction as, for example:

NaOH + HCl → H₂O + NaCl. ΔHr

The ΔH°f for each substance in the reaction is:

NaOH: Na + 1/2H₂ + 1/2O₂ → NaOH (1)

HCl: 1/2H₂ + 1/2Cl₂ → HCl (2)

H₂O: H₂ + 1/2O₂ → H₂O (3)

NaCl: Na + 1/2Cl₂ → NaCl (4)

The algebraic sum of (3) + (4) is - (ΔH°f reactants):

H₂ + 1/2O₂ + Na + 1/2Cl₂ → NaCl + H₂O ΔH°f reactants

This reaction - { (1) + (2) } ΔH°f products

NaOH + HCl → H₂O + NaCl.

ΔHr = ΔH°f reactants - ΔH°f products

In the example, we obtain this relationship that can be expanded for all reactions. Thus, right answer is:

B. ΔHreaction = ΔH°f reactants - ΔH°f products