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Le Chatelier's Principle






Le Chatelier's Principle:-
In a system under equilibrium if there is change in pressure, temperature or concentration then the equilibrium shifts in such a manner as to reduce or counteract the reflect of change.
We will examine the impact of temperature , pressure , change in concentration , addition of catalyst in the chemical reaction based on that

Effect of temperature
Equilibrium constant value depends on the temperature. Also the temperature dependence of the equilibrium constant depends on the sign of $ \Delta H$ for the reaction
a. The equilibrium constant for an exothermic reaction (negative $ \Delta H$ ) decreases as the temperature increases.
b. The equilibrium constant for an endothermic reaction (positive $ \Delta H$ ) increases as the temperature increases.
 So , On increasing temperature equilibrium shifts towards endothermic direction.
Example
$N_2 (g) + 3H_2 (g) \rightleftharpoons 2NH_3 (g)$
  $  \Delta H = -92.38 KJ/mol$
So as per Le Chatelier's Principle, on increasing the temperature , the equibrium will shift towards left and less ammonia will be formed

Effect of pressure
On increasing pressure equilibrium shifts where no.of moles are less.
$CO(g) + 3H_2 (g)  \rightleftharpoons  CH_4 (g) + H_2O(g) $
Here reactants =4 moles
Products =2 moles
So ,on increasing the pressure, the equilibrium will shifts towards less mole side. So it will move in the forward direction

Effect of change in concentration
On change in concentration equilibrium shifts to compensate the change. When the concentration of any of the reactants or products in a reaction at equilibrium is changed, the composition of the equilibrium mixture changes so as to minimize the effect of concentration changes
If we add more reactants or products , the equilibrium will shifts in such a manner so that add products or reactants are consumed
If we remove reactants or products , the equilibrium will shifts in such a manner so that remove products or reactants are replenished
Example
$H_2 (g) + I_2 (g) \rightleftharpoons 2HI(g)$
If H2 is added to the reaction mixture at equilibrium, then the equilibrium of the reaction is disturbed. In order to restore it, the reaction proceeds in a direction wherein H2 is consumed, i.e., more of H2 and I2 react to form HI and finally the equilibrium shifts in right (forward) direction

Effect of catalyst
Catalyst increases both forward & backward reaction at the same rate.
Therefore , No. effect or equilibrium




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