Explanation:
We have to find the equation for which its ΔH°rxn is equal to its ΔH°f for the product.
By definition: "the ΔH°f is a measure of the energy released or consumed when one mole of a substance is created under standard conditions from its pure elements".
The key to solve this problem is in the last part of the definition. We have to find a reaction where the reactants are pure elements and the coefficient for the product is 1.
1) CO (g) is not a pure element and the coefficient for the product is 2. Incorrect.
2) This one is a bit confusing. It could be our choice but the enthalpy of formation of C (diamond) is not considered as zero, because the most stable form of C as a solid is graphite. Incorrect.
3) O₃ (g) is not a pure element. Oxygen gas is a diatomic gas. Incorrect.
4) We have two different products. Incorrect.
5) Xe (g) + 2 F₂ (g) ---> XeF₄ (g) Correct
In thjs reaction XeF₄ is produced by two pure elements, Xe and F₂. So for this reaction ΔH°rxn is equal to the ΔH°f of XeF₄.
Answer: 5) Xe (g) + 2 F₂ (g) ---> XeF₄ (g)
