4.2.1 - Manipulating Equations and Enthalpy Changes
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Practice Questions
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What does Hess's Law state?
💡 Hint: Think of a journey taken by different routes.
What happens to the enthalpy change when we reverse a reaction?
💡 Hint: Review the direction of energy flow.
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Interactive Quizzes
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What is Hess's Law?
💡 Hint: Recall the main principle of Hess's Law.
True or False: When reversing a reaction, you must also reverse the sign of the enthalpy change.
💡 Hint: Think about how energy flows with direction.
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Challenge Problems
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Calculate the ΔH for this series of reactions involving CO2: C(s) + O2(g) → CO2(g) ΔH1 = -393.5 kJ, CO(g) + ½O2(g) → CO2(g) ΔH2 = -283.0 kJ. How does reversing reactions affect ΔH?
💡 Hint: Think carefully about how reversing a reaction changes the sign of the enthalpy.
Apply Hess's Law to find the total enthalpy change for this reaction: N2(g) + 3H2(g) → 2NH3(g). Use the given reactions: N2(g) + 2H2(g) → 2NH2(g) ΔH = -100 kJ, NH2 + H2 → NH3 ΔH = -50 kJ. What's ΔH?
💡 Hint: Manipulate the equations carefully; how do the signs play a role?
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