1. Some reactions that are predicted by their sign of ∆Go to be spontaneous at room temperature do not proceed at a measurable rate at room temperature.

    1. Account for this apparent contradiction.

    2. A suitable catalyst increases the rate of such a reaction. What effect does the catalyst have on ∆Go for the reaction? Explain.

(SOURCE: CEEB, 1999b, p. 47)

Suggested Modification

The panel suggests that Section II, Part B, Question 6 of the AP exam, which deals with thermodynamic principles and KMT concepts, could be directed to higher levels of thinking in part (a) by relating a simple system, such as an ice cube melting, to enthalpy, entropy, and free energy. The student could be asked to explain the process using the above terms and appropriate equations. Part (b) could be similar but related to something common, such as the possible oxidation of sucrose, which has a large negative free energy. The student could discuss why the sugar does not spontaneously combust on the kitchen table since the free energy is favorable. Included in the explanation would be descriptions of the differences between thermodynamic and kinetic stability.

AP CHEMISTRY 1999, SECTION II, PART B, QUESTION 7

Original Question

Answer the following questions, which refer to the 100 mL samples of aqueous solutions at 25ºC in the stoppered flasks shown above (four partially full flasks are shown, each containing an equal volume of 0.10 M solutions of NaF, MgCl2, C2H5OH, and CH3COOH, respectively).

  1. Which solution has the lowest electrical conductivity? Explain.

  2. Which solution has the lowest freezing point? Explain.

  3. Above which solution is the pressure of water vapor greatest? Explain.

  4. Which solution has the highest pH? Explain.

(SOURCE: CEEB, 1999b, p. 48)

(NOTE: On the exam, students were asked to answer either this question or the next question concerning principles of chemical bonding and molecular structure, but not both.)

Suggested Modification

The panel suggest the following modified question:

Modification 1. Do not have all solutions be 0.10 M. Instead use 0.010 M NaF, 0.050 M MgCl2, 0.10 M C2H5OH, and 0.20 M CH3COOH. This change requires a student to think more



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