Practice Manipulating Equations and Enthalpy Changes - 4.2.1 | Chapter 4: Energetics/Thermochemistry | IB Grade 12-Chemistry
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Practice Questions

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Question 1

Easy

What does Hess's Law state?

💡 Hint: Think of a journey taken by different routes.

Question 2

Easy

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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Question 1

What is Hess's Law?

  • Energy change is always the same regardless of the path taken.
  • Energy changes depend on the path taken.

💡 Hint: Recall the main principle of Hess's Law.

Question 2

True or False: When reversing a reaction, you must also reverse the sign of the enthalpy change.

  • True
  • False

💡 Hint: Think about how energy flows with direction.

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Challenge Problems

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Question 1

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.

Question 2

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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