Welcome to Chem Zipper.com......: EQUILIBRIUM CONSTANT Kp ,Kc AND Kx (LAW OF MASS ACTION)

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Saturday, October 6, 2018


Law of mass action is applicable for only reversible chemical reactions and it is an imperial law.
The law state that “ At a fixed temperature the rate of a chemical reaction is directly proportional to the product of reactive mass of reactants raised to the their respective Stoichiometric cofficients ” The law of mass action is by Guldberg and Waage.
(1) Equilibrium Constants in term of concentration ( KC):
At the constant temperature, let us consider the following reversible reaction
According to law of mass action - The rate of forward reaction
The rate of reverse reaction-
Where Kf and Kb  is the rate constant of the forward reaction and backward reaction respectively
We know at equilibrium, the two rates of forward as well as backward are equal. ie
Rate of reaction = Rate of forward reaction – Rate of backward reaction = 0
Unit of Kc= (Conc)ng
(2) Equilibrium Constants in term of Pressure( Kp):
Consider the general chemical  reaction taking place at constant temperature.
From law of mass action- rate forward reaction is directly proportional  to product of active mass of reactants and rate backward reaction is directly proportional  to product of active mass of products.

For an ideal gas PV=nRT


P= Pressure in atm

V=Volume in liters

n=Number of gaseous moles

R=Gas constant  

  =  0.0821 L atm/mol/K or 1/12 L atm /mole/K

T=Temperature in kelvin

                                      = total number of moles of gaseous products -total number of moles of gaseous reactants
EXAMPLE (1): At 700 K, the equilibrium constant Kp, for the reaction
is 1.8 × 10kPa What is the numerical value of Kc for this reaction at the same temperature
EXAMPLE (2) : Methanol (CH3OH) is manufactured industrially by the reaction
 CO(g) + 2H2(g) CH3OH(g) The equilibrium constant (Kc) for the reaction is 10.5 at 220°C. What is the value of Kp at this temperature ?
SOLUTION: Give Data ,Kc = 10.5 ,T= 220oC = (220 + 273)K = 493 K

EXAMPLE(3): For the reaction N2(g) + 3H2(g) 2NH3(g) The Kp is 4.3 × 10-4 at 375°C. Calculate Kc for the reaction. ?

EXAMPLE(4):   The value of Kc for the reaction is 0.50 at 400º C.
What will be the value of Kp at 400ºC when concentration are expressed in mole litre.1 and pressure in atmosphere ?


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