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Le Chatelier–Braun Principle

Le Chatelier’s principle states that if an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset.

Change in number of moles of reactant

For any equilibrium, when more reactant is added to (or some product is removed from) an equilibrium mixture, the net reaction moves in the forward direction (as Q < K) to establish a new equilibrium state.

Change in number of moles of product

For any equilibrium, when product is added to (or some reactant is removed from) an equilibrium mixture, the net reaction moves in the reverse (backward) direction (as Q > K) to establish a new equilibrium state.

Change in volume

For any equilibrium, on increasing the volume of the container, the net reaction shifts in the direction of more moles of the gases, while on decreasing the volume of the vessel, the reaction goes in the direction of fewer moles of the gases.

Addition of an inert gas

For any equilibrium, when an inert gas is added at constant volume, the equilibrium remains unaffected whether the equilibrium reaction is equal to zero or non-zero. Thus, for equilibrium having Δn = 0, when an inert gas is added at constant pressure, the equilibrium remains unaffected (since V does not appear in the expression of KP), while for equilibrium having Δn ≠ 0, the addition of an inert gas at constant pressure causes reaction to move in the direction of more moles of the gases.

Addition of a catalyst

The presence of a catalyst does not alter the equilibrium constant nor does it shift the position of an equilibrium system.

Change in temperature

In general, with the increase in temperature, the net reaction moves in that direction where the heat is absorbed and the effect of increasing temperature is nullified.

Change in more than one parameter

When two or more parameters are simultaneously changed for any equilibrium, find the changed value of Q and K and compare them. If Q = K, there will be no effect on the reaction. If Q > K, the net reaction moves in the backward direction. If Q < K, the net reaction moves in the forward direction.




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