Читать книгу Geochemistry - William M. White - Страница 128

Often we encounter a reaction for which we have no value of the equilibrium constant. In many cases, however, we can derive an equilibrium constant by considering the reaction of interest to be the algebraic sum of several reactions for which we do have equilibrium constant values. For example, the concentration of carbonate ion is often much lower than that of the bicarbonate ion. In such cases, it is more convenient to write the reaction for the dissolution of calcite as:

(3.87)

Given the following equilibrium constants, what is the equilibrium constant expression for the above reaction?

Answer: Reaction 3.87 can be written as the algebraic sum of three reactions:

The initial inclination might be to think that if we can sum the reactions, the equilibrium constant of the resulting reaction is the sum of the equilibrium constants of the components. However, this is not the case. Whereas we sum the reactions, we take the product of the equilibrium constants. Thus, our new equilibrium constant is:

For several reasons (chief among them that equilibrium constants can be very large or very small numbers), it is often more convenient to work with the log of the equilibrium constant. A commonly used notation is pK. pK is the negative logarithm (base 10) of the corresponding equilibrium constant (note this notation is analogous to that used for pH). The pK's sum and our equilibrium constant expression is: