Le-Chatelier’s principle elucidate the behaviour of a system at equilibrium, if it is subjected to changes in parameters like pressure, temperature and concentration. According to this principle system in equilibrium is subjected to a change of temperature, pressure or concentration. Hence the equilibrium will shift in a direction which tend to annul (nufly) the effect of change impressed or “if a system at equilibrium is disturbed by changing the variables such are pressure, temperature or concentration. Then the system will tend to adjust itself so as to minimize the effect of the Change as far as possible”. Thus,
- If concentration is increased equilibrium will shift in the direcrdir where concentration of that component decreases.
- If pressure is increased, equilibrium will shift in the direction of low pressure.
- If temperature is increased, equilibrium will shift in the direction of low temperature.
Effect of change in concentration
If the concentration of one of the components at equilibrium is increased, then According to this principle, the equilibrium will shift in such a way so as to decrease the concentration of that component. For example let us take synthesis of NH3 from N2 and H2.
N2 (g) + 3H2(g) ⇔ 2NH3(g)
If concentration of either N2 and H2 is increased, equilibrium will shift in forward direction where more and more N2 and H2 will change into NH3. In general increase in the concentration of reactants shift the equilibrium in forward direction, whereas increase in the concentration of products will shift the equilibrium in backward direction.
Effect of change in pressure
An increase in the pressure shifts the equilibrium in the direction where number of moles decreases.
1) The reactions for which Δn< 0 or Δn is negative, an increase in pressure will shift the equilibrium in forward direction. And a decrease in pressure will shift the equilibrium in backward direction.
Examples N2 (g) + 3H2(g) ⇔ 2NH3(g) …Δn=-2
2So2 (g)+O2 (g) ⇔ 2SO2 (g) …Δn=-1
A (g) + B(g) ⇔C (g) …Δn=-1
2) The reactions for which Δn>0 or Δn is positive an increase will shift the equilibrium in backward direction. Whereas a decrease in pressure will shift the equilibrium in forward direction. Examples.
PCl5 (g) ⇔ PCl3 (g) + Cl2 (g) …Δn=+1
N2O4 (g) ⇔ 2NO2 (g) … Δn=+1
2NOCl (g) ⇔ 2NO(g) + Cl2(g) …Δn=+1
3) The reactions for which Δn =0, there is no changes on equilibrium by increasing or decreasing the pressure. Example
N2 (g) + O2 (g) ⇔ 2NO (g) Δn=0
H2 (g) + I2 (g) ⇔ 2HI (g). Δn=0
Effect of change in temperature
In an exothermic reaction A+B ⇔ C+ heat. An increase in temperature, equilibrium will shift in backward direction since heat is absorbed in backward direction.
N2(g) + 3H2 (g) ⇔ 2NH3 (g) ΔH= -93.74 KJ
SO2 + 1/2 O2 (g) ⇔ SO3(g). ΔH= -96.25 KJ
2NO (g) + O2 (g) ⇔ 2NO2 (g) . ΔH= -113 KJ
All of the above reactions are exothermic, so equilibrium will shift in backward direction on increasing the temperature and in forward direction on decreasing the temperature.
Some examples of endothermic reactions are :
PCl5 (g) ⇔ PCl3 (g) + Cl2 (g) ΔH= +Q Kcal
N2 (g) + O2 (g) ⇔ 2NO (g). ΔH= +180.7 kj
CaCO3 (s) ⇔ CaO (g) + CO2 (g) ΔH= + 179.96 k.j
2CO2 (g) ⇔ 2CO(g) + O2 (g) ΔH = + 564 k.j
In above reactions, equilibrium will shift in forward direction when temperature is increased and in backward direction when temperature is decreased.
Effect of addition of inert gas on equilibrium
- The reaction for which for which Δn = 0, no effect of addition of inert gas since Kp does not depend upon pressure in such reactions.
- The reactions for which Δn ≠ 0 the effect of addition of inert gases is as follows :
(a) At constant volume :- If an inert gas is added without a change in volume, equilibrium constant is not affected, since partial pressure of reactants and products are not affected, through total pressure is increases.
(b) At constant pressure :- When inert gas is added at constant pressure, volume increase or number of moles increases which causes a decrease in partial pressure of reactants and products.
Thus, for the reaction for which Δn >0 , equilibrium shifts in forward direction, examples
PCl5 (g) ⇔ PCl3 (g) + Cl2 (g)
N2O4 (g) ⇔ 2NO2 (g)
The reaction for which Δn<0, equilibrium shifts in backward direction, examples
N2 (g) + 3H2(g) ⇔ 2NH3(g)
2So2 (g)+O2 (g) ⇔ 2SO3 (g)
In general it is concluded that equilibrium shifts in the direction of increased volume on addition of inert gases at constant pressure.