Page 4 - Microsoft Word - Equilibrium State key notes

(ii) Qc = Kc, the reaction is at equilibrium.
Le – Chatelier’s Principle
There are three main factors which affect the state of equilibrium.

They are

1. concentration
2. temperature
3. pressure.
Le – Chatelier’s principle states that if a system at equilibrium is subjected to a change in
concentration. pressure or temperature. the equilibrium equilibrium change.

Effect of Change of Concentration
If at equilibrium the concentration of one of the reactants is increased. the equilibrium will
shift in the forward direction and vice-versa.

Effect of Change in Pressure
No effect of pressure on equilibria having same moles of reactants and products. e.g., N2 +
O2 ⇔ 2NO.
When there is change in the number of moles, the equilibrium will shift in the direction having
smaller number of moles when the pressure is increased and vice-versa, e.g.,

N2 + 3H2 ⇔ 2NH3 [High p. high yield of NH3]
Effect of Temperature
When process is exothermic, low temperature favours the forward reaction. When process is
endothermic. high temperature favours the formation of products.

Effect of Addition of Inert Gas
(i) Addition of inert gas at constant pressure At constant pressure. if an inert gas is added. it
will increase the volume of the system. Therefore. the equilibrium will shift in a direction in
which there is an increase in the number of moles of gases.
(ii) Addition of inert gas at constant volume If keeping volume of the system constant, an inert
gas is added. the relative molar concentration of the substance will not change. Hence. the
equilibrium position of the reaction remains unaffected.
Effect of Catalyst
The presence of catalyst does not change the position of equilibrium. It simply fastens the
attainment of equilibrium.

Le-Chatelier’s Principle Applicable to Physical Equilibrium
(i) Effect of pressure on solubility The increased pressure, will increase the solubility
of gas and vice-versa.
(ii) Effect of temperature on solubility Some substances dissolve with the absorption
of heat. Solubility of such substances will increase with increase of temperature and
vice-versa, e.g., dissolution of NH4CI, KCI, KNO3, etc. The dissolution of calcium
acetate and calcium hydroxide is exothermic, so their solubility is lowered at higher
temperature.
(iii) Effect of pressure on the melting point of ice
Ice ⇔ liquid water

The ice occupy the more volume than liquid water, so increased pressure will result
in melting of ice according to Le-Chatelier principle.