CH353 - Physical Chemistry I
Spring 2013, Unique 52575

Lecture Summary, 27 March 2013

Chemical Equilibrium Continued:  Figuring out the equilibrium composition of our reaction mixture is only one part of the problem.  The second, and more interesting problem is how to influence the reaction to move one way or another down the reaction coordinate, presumably to increase the quantity of either reactants or products.  Today we talked about three effects on z(eq). 

  Pressure:  Kp is a constant that is not dependent on total pressure of the system.  If the total pressure of the system changes, then the extent of reaction, z(eq) must change to compensate, not Kp.  By separating Kp into expressions for mole fraction of each species (which we called Kx) and Ptot, we can quantify how much z(eq) must change in order to keep Kp constant.

  Temperature:  Kp is a function of temperature, and so as temperature changes the value of the equilibrium constant changes, and thus z(eq) changes.  Using the Gibbs-Helmholtz equation (which we derived), we derived an expression for change in Kp caused by changes in temperature through delta(Hrxn).  By doing a careful accounting of signs, we find that for an endothermic reaction, Kp increases as Kp increases, as expected.

  Composition:  The extent of reaction will also be influenced by changing the composition of the reaction mixture.  Again, we can use Le Chatlier to predict what will happen if a reactant or product is added to the reaction solution.  In order to quantify this, we introduced a new constant, Q, which is defined in the same way to Kp at all points on the reaction coordinate except at equilibrium.  By comparing Q to Kp, we can determine which direction the reaction will proceed spontaneously, and how far it will go before achieving equilibrium.