Hess's Law Energetics Worksheet
This worksheet provides problems for students to practice applying Hess's Law to calculate enthalpy changes for chemical reactions.
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Hess's Law Energetics
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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?
If a reaction is reversed, the sign of ΔH is reversed.
If a reaction is multiplied by a factor, ΔH is multiplied by the same factor.
The enthalpy of a reaction depends on the reaction rate.
Intermediate steps in a reaction mechanism cancel out.
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.
True
False
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
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