Chemical equilibrium:How to find equilibrium constant 'Keq'


A chemical system can be thought of as being either:

1.At equilibrium

2.Not at equilibrium

A system which is not at equilibrium will move spontaneously to a position of being at equilibrium.

What is Keq : The "K" in Keq stands for "Constant". The "eq" means that the reaction is at equilibrium. Very roughly, Keq tells you the ratio of Products/Reactants for a given reaction at equilibrium at a certain temperature.

K eq = [Products] / [Reactants]

Consider the reaction

2HI(g )  < ---------------> H2(g) + I2(g)

that if you take the [H2], the [I2] and the [HI] in an equilibrium mixture of these at 423 °C,the expression 

[H 2 ] [I 2 ] /  2 [HI]        =      0.0183

 this is Keq expression             this is value of Keq

Writing Keq Expressions:

Consider the general reaction:

aA  + bB  <------> cC  + dD

where, A,B,C,D denote the reactants and prodcuts and a,b,c,d are coefficients in the balanced chemical equation.The equilibrium-constant expression for a reaction is an expression obtained by multiplying the concentrations of products,dividing by th concentration of reactants and raising each concentration term to a power equal to the coefficent in the chemical equation.

The equilibrium constant Kc is the value obtained for the equilibrium-constant expression when equilibrium concentrations are substituted. 

Keq = [C]c [D]d / [A]a [B] b

Here you denote the molar concentration of a substance by writing its formula is square brackets.The subscript eq on the equilibrium contant means that it is defined in terms of molar concentrations.

The Keq Expressions for Solids and Liquids

For solids: When we write the Keq expression for a reaction with solids, we simply leave out the solids.

the Keq expression for the equation:

CaCO3(s) <---------> CaO(s) + CO2(g)

is simply: Keq = [CO2]

The same argument that was used for solids can also be used for liquids. Thus, we can expand the last statement:

"When we write the Keq expression for a reaction with solids or liquids, we simply leave out the solids and the liquids".

Gases and aqueous solutions do undergo changes in concentration so they are always

included in the Keq expression.