1) Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal. The law of mass action states that the rate of a reaction depends on the concentrations of reactants. 2) The equilibrium constant, K, is defined as the ratio of products over reactants at equilibrium. For heterogeneous reactions involving different phases, the equilibrium constant is expressed in terms of partial pressures. 3) According to Le Chatelier's principle, if a stress is applied to a system at equilibrium, the equilibrium shifts to minimize the effect of the stress. Changes in concentration, pressure, or temperature cause the equilibrium to shift left or right to counteract the applied change.

1. Chemical Equilibrium

2. Law of mass action :
The rate of a chemical reaction is proportional to the active masses is proportional to the
active masses of the reactants if the temperature became constant.
Let us consider a general reaction,
𝑨 + 𝑩 ⇄ 𝑪 + 𝑫
Let, [A], [B], [C], [D] represents the molar concentrations of A, B, C and D at equilibrium
point.
According to the law of the mass action :
Rate of forward reaction, α [A] [B]
= 𝒌 𝟏 𝑨 [𝑩]
Rate of reverse reaction, α [C] [D]
= 𝒌 𝟐 𝑪 [𝑫]
Where, k1 and k2 are rate constant for the forward and reverse reaction.

3. 𝑲 𝑷 = 𝑪 𝒅 𝑹𝑻 𝒅 𝑫 𝒎 𝑹𝑻 𝒎
𝑨 𝒋 𝑹𝑻 𝒋 𝑩 𝒌 𝑹𝑻 𝒌
= 𝑪 𝒍 𝑫 𝒎 𝑹𝑻 𝒅+𝒎
𝑨 𝒋 𝑩 𝒌 𝑹𝑻 𝒋+𝒌
∵ 𝒌 𝒑 = 𝑹𝑻 𝒅+𝒎 − 𝒋+𝒌
= 𝒌 𝒄 𝑹𝑻 ∆𝒏
Δn = The difference between the of coefficient For the gaseous
products and reactants.
When, Δn = 0, then 𝑘 𝑝 = 𝑘 𝑐

4. Equilibrium Constant :
The ratio of the active masses of the products and the active
masses of the reactants is called the equilibrium constant.
Let us consider a general reaction,
𝑱𝑨 + 𝑲𝑩 ⇄ 𝒍𝑪 + 𝒎𝑫 −−−−−−− −①
𝒌 𝒑 =
𝒑 𝑪
𝒍
𝒑 𝑫
𝒎
𝑷 𝑨
𝒋 𝑷 𝑩
𝒌
−−−−−−−−−− −②
For ideal gas,
Pv=nRT
⇒ P=
𝒏
𝒗
𝑹𝑻
⇒P = CRT [C = concentration ]
Now, we are put the value of p in ①

5. Heterogeneous equilibrium :
If all the products and reactants are not in same phases then it’s called
heterogeneous equilibrium.
Ex : 𝑪𝒂𝑪𝑶 𝟑 𝒔 ⇄ 𝑪𝒂𝑶 𝒔 + 𝑪𝑶 𝟐 𝒈
The decomposition of calcium carbonate upon heating to from calcium
oxide and carbon dioxide is an example of heterogeneous equilibrium.
Here, k=
𝑪𝑶 𝟐 [𝑪𝒂𝑶]
[𝑪𝒂𝑪𝑶 𝟑]
Ignoring the concentrations of 𝐶𝑎𝐶𝑂3 and 𝐶𝑎𝑂 the equilibrium constant
expression for the decomposition of 𝐶𝑎𝐶𝑂3 may be written as , 𝑘 𝑐 =
𝐶𝑂2
Or, in terms of partial pressures ,
𝒌 𝒑 = 𝑷 𝑪𝑶 𝟐

6. Le Chatelire’s Principle :
“ If a change in concentration pressure or temperature is caused to
chemical reaction in equilibrium ,the equilibrium will shift to right to
left so as to minimize the change. ”
Effect of change in concentration : The concentration of a reactant or
product can be effected by adding or removal of that species . Let us
consider a general equation ,
A+B ⇄ C
When reactant, A is added at equilibrium it’s concentration is increased .
According to Le chatelaines’s principle a new equilibrium will be established
so as to reduce the concentration of product C .

7. Effect Of a change in pressure
:When pressure is increased on a gaseous equilibrium reaction the equilibrium
will shift in a direction which tends to decrease the presser . Let us consider a
reaction,
A+B ⇄ C
By the increase of pressure on the equilibrium it will shift to right and and give
more C. Decrease in pressure will causes the opposite effect . The equilibrium will
shift to the left when C will decompose to fro of A and B .
Effect and changes of temperature : In exothermic reaction when temperature of
reaction is increased the equilibrium shifts to the left in which heat is
absorbed . Let us consider a exothermic reaction,
A + B ⇄ C + heat

8. The would be result in the increase of the concentration of the reactents A
and B .
In an endothermic reaction ,
X+Y ⇄ Z - heat
The increase of temperature will shift the equilibrium to the right as it involves the
absorption of heat . This increase the concentration of product Z.
It’s application to industrial reactions :
The manufacture of ammonia by Haber process is represented by the equation
,
𝑵 𝟐 + 𝟑𝑯 𝟐 ⇄ 𝟐𝑵𝑯 𝟑 𝒈 + 𝟐𝟐 𝒌𝒄𝒂𝒍
The equation provides the following information
1. The reaction Is exothermic
2. The proceeds with decrease in the number of moles

9. 1) Low temperature : By applying Le chateliar’s principle
low temperature will shift the equilibrium to the right .
This gives a greater yield of ammonia .
2) High Pressure : High pressure on the reaction at
equilibrium favors the shift or the equilibrium to the right
. A pressure of about 200 atm is applied in practice .
3) Cotalyst :To increase the rate of reaction and thus
quicken the attainment of equilibrium, a catalyst is used .
Finely divided iron containing molybdenum is employed in
actual practice.
Molybdenum acts as a promoter that increases the life and
efficiency of the catalyst .

10. Shyamoli Textile Engineering College
(Affiliated By University of Dhaka)
 Md. Nurul Islam
 Lecturer of Chemistry
 STEC, Dhaka