Hess's Law - Enthalpy Changes
Explore Hess's Law and its application in calculating enthalpy changes for chemical reactions at Grade 9 level.
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Hess's Law: Calculating Enthalpy Changes
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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:
temperature of the reaction
number of steps in the reaction
pressure of the reaction
concentration of reactants
2. Which of the following is an application of Hess's Law?
Calculating the rate of a reaction
Determining the activation energy
Finding the enthalpy change for a reaction that cannot be measured directly
Measuring the spontaneity of a reaction
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.
True
False