Hess's Law - Enthalpy Changes

Hess's Law: Calculating Enthalpy Changes

Read each question carefully and answer to the best of your ability. Show all your work for calculation problems.

1. Hess's Law states that the total enthalpy change for a chemical reaction is independent of the:

2. Which of the following is an application of Hess's Law?

3. Hess's Law is a direct consequence of the first law of  .

4. When a chemical equation is reversed, the sign of the enthalpy change (ΔH)  .

5. State Hess's Law in your own words.

6. Calculate the enthalpy change (ΔH) for the reaction: C(s) + 2H₂(g) → CH₄(g)

Given the following reactions:

I. C(s) + O₂(g) → CO₂(g) ΔH = -393.5 kJ/mol

II. H₂(g) + ½O₂(g) → H₂O(l) ΔH = -285.8 kJ/mol

III. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = -890.3 kJ/mol

7. Hess's Law can only be applied to reactions occurring at standard temperature and pressure.