MCAT General Chemistry Question 200: Answer and Explanation

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Question: 200

4. If Step 1 were simply a fast reaction and not a fast equilibrium, what would be the expected rate law for the decomposition of nitramide in water?

  • A. ∆Hfus = -20 kJ/mol, DHvap = 66.7 kJ/mol
  • B. ∆Hfus = 40.7 kJ/mol, DHvap = 6.0 kJ/mol
  • C. ∆Hfus = 6.0 kJ/mol, DHvap = 40.7 kJ/mol
  • D. ∆Hfus = 12.0 kJ/mol, DHvap = 81.4 kJ/mol

Correct Answer: C

Explanation:

C Both fusion (melting) and vaporization (boiling) require energy and are endothermic, eliminating choice A. Comparing both processes, vaporization takes substantially more energy. During vaporization, intermolecular forces are essentially completely overcome, and gaseous molecules separate widely due to their increased kinetic energy. Choice B is therefore eliminated. A 36 gram sample of water is 2 moles, so the heat of sublimation of 1 mole is half of 93.4 kJ, or 46.7 kJ/mol. This eliminates choice D. Examining the fusion and vaporization of water and adding their enthalpies by Hess's law gives choice C as the correct answer:

H2O(s) → H2O(l) ∆Hfus = X (6.0 kJ/mol)
H2O(l) → H2O(g) ∆Hvap = Y (40.7 kJ/mol)
H2O(s) → H2O(g) ∆Hvap = X + Y = 46.7 kJ/mol

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