Hess's Law Energetics Worksheet

Hess's Law Energetics

Read each question carefully and use Hess's Law to determine the enthalpy change (ΔH) for the given reactions. Show all your work and calculations.

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

Given the following reactions:

C(s) + O₂(g) → CO₂(g) ΔH = -393.5 kJ

H₂(g) + ½O₂(g) → H₂O(l) ΔH = -285.8 kJ

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

2. Hess's Law states that the total enthalpy change for a chemical reaction is   of the pathway taken, as long as the initial and final conditions are the same. This means that if a reaction can be expressed as a series of steps, the ΔH for the overall reaction is the   of the ΔH values for each step.

3. Which of the following is NOT a principle used when applying Hess's Law?

4. Given the following data:

S(s) + O₂(g) → SO₂(g) ΔH = -296.8 kJ

SO₂(g) + ½O₂(g) → SO₃(g) ΔH = -98.9 kJ

Calculate ΔH for the reaction: 2S(s) + 3O₂(g) → 2SO₃(g)

5. Hess's Law is a direct application of the Law of Conservation of Energy.

6. Calculate the standard enthalpy of formation of C₂H₂(g) from the following data:

C₂H₂(g) + ⁵/₂O₂(g) → 2CO₂(g) + H₂O(l) ΔH = -1299.6 kJ

C(s) + O₂(g) → CO₂(g) ΔH = -393.5 kJ

H₂(g) + ½O₂(g) → H₂O(l) ΔH = -285.8 kJ