equilibrium+constants

Equilibrium Constants
An //equilibrium constant// (given the symbol K) is a mathematical ratio of the amount of products to the amount of reactants in a chemical equation at equilibrium.

It is designed to give an idea of how complete the chemical reaction is at equilibrium:
 * a large K, typically greater than 0.1, indicates that the reaction has gone to near completion and the amount(s) of products dominates the equilibrium ratio.
 * A small K, typically less than 0.1, tells us that the reaction is incomplete, and that the amount(s) of reactant(s) dominates the equilibrium ratio.

The equilibrium constant is usually followed by a sub-script letter that specifies what type of system it is referring to. Here are some examples:
 * K eq is a generic symbol that can be used in any case
 * K C refers to the use of molar concentration units in the equilibrium constant equation
 * K P is used to express an equilibrium in terms of partial pressures for gaseous systems
 * K W is defined as the auto-ionization constant for water
 * K a is used in weak acid dissociations
 * K b is used for weak base dissociations
 * K sp is used for slightly soluble (highly insoluble) solid dissociations in water (sp stands for solubility, product)

Here is how an equilibrium constant expression is written:

For a generic chemical equation where a, b, c, and d are coefficients of the balanced equation:

__//a//__ A + //__b__// B <--> //__c__// C + //__d__// D

the equation would be:



For example:

The equilibrium constant expression for the reaction:

2 SO 2 (g) + O 2 (g) <--> 2 SO 3 (g)

would look like:



Heterogeneous Equilibrium Systems
Interestingly, non-solution phases of matter - that is, liquids and solids - do not get included in these equilibrium constant expressions. These are referred to as //heterogeneous equilibria//.

For example, look at the solubility equilibrium for solid calcium hydroxide:

Ca(OH) 2 (s) <--> Ca 2+ (aq) + 2 OH - (aq)

Its equilibrium constant expression would not include solid Ca(OH) 2 :

K sp = [Ca 2+ ] x [OH - ] 2