Chapter 19 Outline

Reading Notes for Chapter 19

  1. Spontaneity and Entropy. This section introduces the basic idea of entropy (S) and the entropy change (S) for a reaction. The basic idea is that [delta] S relates to the "randomness or disorder" of a system. It is favorable for the entropy of a system to increase. If a reaction causes the entropy of a system to increase, this will make the reaction more favorable. The degree of randomness depends upon the temperature. As the temperature increases, the randomness increases.

  2. Calculation of Entropy Changes. The entropy change for a reaction is calculated in several different ways.
    1. One is from the standard entropy (S), found in a table. The calculations are very similar to calculating [delta] H for a reaction, using [delta] H of formation. Remember:
      1. Products - Reactants
      2. Include the moles of each species from the balanced equation
      3. The tabulated values for S are absolute entropy.
      4. S=0, for a perfect crystal at 0 K.
    2. IF a system is at equilibrium (boiling point and freezing point are good examples), [delta]S can also be calculated from [delta]H for the reaction. You can think of these two factors as being in balance when a "system" is at equilibrium. then [delta]H = T [delta]S.

  3. Free Energy and Spontaneity. The next major concept in the chapter is Gibbs free energy ([delta]G). This is a measure of how "favorable" a reaction is. It takes both [delta] H and [delta] S into account.
    1. [delta] G = [delta] H - T [delta]S. THIS EQUATION IS IMPORTANT. KNOW IT!!!
    2. If the free energy of a system decreases ([delta]G is negative), the reaction is favorable, or spontaneous.
    3. If the free energy of a system increases ([delta]G is positive), the reaction is not favorable and is not spontaneous.
    4. If the free energy of a system does not change ([delta]G is 0), the system is at equilibrium.
    5. [delta]Go of formation is often tabulated. It is used to calculate [delta]Go for a reaction (just like with [delta]H and [delta]S).
    6. NOTE: the superscript o (as in [delta]Go, or [delta]Ho, or So. indicates the value for a standard system at a specific temperature (usually 25 C).

  4. Free Energy and Equilibrium. Because [delta]G is a measure of how favorable a reaction is, it also relates to the equilibrium constant.
    1. A reaction with a negative [delta]G, is very favorable, so it has a large K.
    2. A reaction with a positive [delta]G is not favorable, so it has a small K.
    3. A reaction with [delta]G = 0 is at equilibrium.
    4. There are several different [delta]G's. It is important to distinguish between them.
      1. [delta]Go (a delta G, with a superscript o), is the free energy change for a reaction, with everything in the standard states (gases at 1 bar, and solutions at 1 M concentration), and at a specific temperature (usually 25 C)
      2. [delta]G (just delta G). This is the free energy change for a reaction that is not at the standard state.
    5. The [delta]G's are related as follows. KNOW THIS EQUATION and understand how to use it:

      6. [delta]G =[delta]Go + RT ln Q

      Where Q is the same Q we used for calculating equilibrium (K is the special case for Q when at equilibrium.)

  5. Spontaneity and Speed of Reactions. This section simply points out that there is not any direct relationship between [delta]G and the speed of a reaction (kinetics).

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