8.3    Equilibrium Constants

Equilibrium constants, ftc  and ftp

• Equilibrium constant is the value of the reaction quotient when the  reaction has reached equilibrium. Alternatively, it can also be defined as the measure of the extent in which reactants are converted into  products.
• At any given temperature, every reaction has a specific value of equilibrium constant which is independent of changes in concentration, pressure, use of catalyst or the amount of reactants you start with
• Equilibrium constant expressed in concentration is called ftc while expressed in partial pressure is called ftp.
• For any reaction: aA + bB ⇌ cC + dD
• For reaction involving gases, equilibrium constant can be expressed in terms of partial aA(g) + bB(g) ⇌ cC(g) + dD(g) …………..(1)

Mole fraction and partial pressures

• From equation (1), the mole fraction of A is given by:
• The partial pressure of a gas A in a mixture of gases is the pressure that gas A will exert, if gas A alone was to occupy the same volume at the same temperature. It is given by:

More about ftc and ftp

• Note:
  1. Concentration of liquids and solids are omitted in the expression of ftp and ftc.
  2. State symbols are not essential in writing the expressions

• The magnitude of ftp and ftc gives the measure of the extent of equilibrium, it does not give any information about the rate of reaction

• Examples on writing an expression for ftp and ftc.

Factors affecting the value of ftp and ftc

• In short, the equilibrium constants are constants at constant temperature. Only temperature can affect the value of Kc and Kp.
• For endothermic reaction, the value of ftc or ftp increases with increasing temperature.
• For exothermic reaction, the value of ftc or ftp decreases with increasing temperature.

[ Note: Statement (g) is omitted in this section and is discussed in Chapter 13 ]