Thermodynamics Solutions: #14

14.* (1995 F 5)

A. What are the conditions on the sign of DH and DS so that:

  1. the reaction is always spontaneous?

    2. DH < 0, DS > 0

  2. the reaction occurs spontaneously at low temperature but not at high temperature?

    3. DH < 0, DS < 0

  3. the reaction occurs spontaneously at high temperature but not at low temperature?

    4. DH > 0, DS > 0

  4. the reaction never occurs spontaneously?

    5. DH > 0, DS < 0

    Note: on these problems, to see why these answers are correct, use the equation DG=DH - TDS with various values of DH, DS, and T.

  5. the reaction is reversible

    For the reaction to be reversible, DG = 0. This requires (b) or (c). The only way that (a) or (c) can be held at DG = 0 is for DS = DH = 0.

    B. If a reaction is nonspontaneous, how would addition of a catalyst affect yield?

    Adding a catalyst increases the rate at which equilibrium is established by lowering the activation energy of the reaction. The yield of the products cannot be improved beyond what is predicted thermodynamically by DG = DG° + RT ln K. In short, catalysts can overcome kinetic barriers, but not thermodynamic ones.