Which expression represents the ∆h for a chemical reaction in terms of the potential energy, pe, of its products and reactants?
a) pe of products + pe of reactants
b) pe of products – pe of reactants
c) pe of reactants × pe of products
d) pe of products ÷ pe of reactants
e) pe of reactants – pe of products

The answer is the option B) PE of products - PE of reactants.

This is, the change of energy, in the form of heat, ΔH, for a chemical reaction is the potential energy of products less the potential energy of reactants: ΔH = PE products - PE reactants.

Note that this leads to:

1) when the potential energy increases, PE of products - PE of reactants > 0 => ΔH > 0, =>the reaction is absorbing energy and it is endothermic;

2) when potential energy decreases PE of products - PE of reactants < 0 => ΔH < 0, the reaction is releasing energy and it is exothermic.

Those are terms very used in chemistry and thermodynamics.