For which of the following reaction(s) is the enthalpy change for the reaction not equal to ΔH°_f of the product?
I. 2H(g) → H₂(g)
II. H₂(g) + O₂(g) → H₂O₂(l)
III. H₂O(l) + O(g) → H₂O₂(l)

Question

Quizplus · Accepted Answer

For reaction I, the enthalpy change for the reaction is equal to the standard enthalpy of formation of H2 minus twice the standard enthalpy of formation of H atom, which is not equal to the standard enthalpy of formation of H₂, thus the enthalpy change for the reaction is not equal to ΔH°_f of the product. For reaction II and III, the enthalpy change for the reactions are equal to the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants, which is equal to ΔH°_f of the product.

For Which of the Following Reaction(s) Is the Enthalpy Change