Varft Hoff isotherm

When K = Q, r = rr and there is equilibrium. When K > Q r > rT and the net reaction proceeds from left to right (i.e., in the forward direction). When K< Q r reverse direction. The term K/Q will be encountered again later in this chapter, in the context of the Varft Hoff isotherm. 

The importance of the Varft Hoff isotherm lies in the fact that it gives the direction of a chemical reaction. If under a given set of conditions AG is positive, then the reaction cannot take place under these conditions. It may, however, occur if the conditions (activities) are suitably altered to make AG negative. 

It can be seen from the Varft Hoff isotherm that the sign of AG depends on the relative values of K and Q. The latter must be smaller than the former for the reaction to be spontaneous. This will be so because AG would then be negative. If the conditions are such that Q > K, the reaction will not be possible. There are, however, two ways by which this situation can be altered. First, the activities of the products may be decreased and/or those of the reactants increased, in order that the value of Q may be made smaller than that of K. Second, the temperature may be changed in such a manner as to increase K sufficiently so that the condition Q < JC is satisfied and AG becomes negative. When this happens, the reaction will occur. 

The Van t Hoff isotherm establishes the relationship between the standard free energy change and the equilibrium constant. It is of interest to know how the equilibrium constant of a reaction varies with temperature. The Varft Hoff isochore allows one to calculate the effect of temperature on the equilibrium constant. It can be readily obtained by combining the Gibbs-Helmholtz equation with the Varft Hoffisotherm. The relationship that is obtained is... 

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