Hess's Law - Enthalpy Calculations

Hess's Law Worksheet

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

1. Calculate the enthalpy change (ΔH) for the reaction: C(s) + 2H₂(g) → CH₄(g), given the following thermochemical equations:

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

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

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

2. Determine the ΔH for the reaction: N₂(g) + 2O₂(g) → 2NO₂(g), using the following reactions:

a) N₂(g) + O₂(g) → 2NO(g) ΔH = +180.5 kJ/mol

b) 2NO(g) + O₂(g) → 2NO₂(g) ΔH = -113.1 kJ/mol

3. Hess's Law states that the total enthalpy change for a chemical reaction is  , regardless of the number of steps or the pathway taken.

4. If a reaction is reversed, the sign of its ΔH value must be  .

5. Hess's Law is a direct consequence of the conservation of energy.

6. When multiplying a reaction by a coefficient, the ΔH value remains unchanged.

7. Given the following data:

2C(s) + O₂(g) → 2CO(g) ΔH = -221.0 kJ/mol

2C(s) + O₂(g) → 2CO₂(g) ΔH = -787.0 kJ/mol

Calculate ΔH for the reaction: 2CO(g) + O₂(g) → 2CO₂(g)