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
Kc=Kf/Kb
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
For an ideal gas PV=nRT
Where
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 × 103 kPa What is the numerical value of Kc for this
reaction at the same temperature
SOLUTION:
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. ?
SOLUTION:
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 ?
SOLUTION:
EXAMPLE(5):
EXAMPLE(6):
SOLUTION:
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