Le Chatelier principle of equilibrium

Le Chatelier principle definition

Chemistry Topics
4 min readAug 13, 2019

Le-Chatelier principle quantitatively predicts the effect on the system at equilibrium when some of the variables such as temperature, pressure, and concentration are changed. Thus if the equilibrium point of a system subjected to change, the system will react in such a way so as to oppose or reduce the change if this is possible.

Le Chatelier principle of equilibrium

Effects of equilibrium on pressure

According to the Le-Chatelier principle with the increase of pressure, the reaction will shift in a direction where the no of moles reduced thus the system will try to lower the pressure. Hence when the pressure increases, the shift of the reaction in a direction where the sum of the stoichiometric coefficient of gas molecule lowered thus lowering of pressure. In other words, an increase in pressure shifts the equilibrium to the low volume side of the reaction whereas a decrease of pressure shifts it to the high volume side.

Effect of temperature on equilibrium

According to the Le-Chatelier principle when the temperature increases, the equilibrium will shift in the endothermic direction. Mathematical calculation of equilibrium change with temperature given from Van’t Hoff equation. Thus the equilibrium shifted low enthalpy or energy to the high enthalpy or energy side with the absorption of the specific heat.

  1. In the above reaction, the thermodynamics enthalpy of the reactant is greater than the product. Thus with the increases in temperature backward reaction favors where the equilibrium shifted to the higher enthalpy side and the production of ammonia decreased.
  2. But with the decrease of temperature, the equilibrium will shift in the exothermic direction and shift to the low enthalpy side. Thus with the decreases in temperature forward reaction favors and the production of ammonia increased.

Enthalpy of the reactants in the above reaction lower than that of the products.

  1. Thus with the increases in temperature forward reaction favors and production of nitrogen oxide increasing.
  2. But decreases in temperature for the above reaction backward reaction favors and production of nitrogen oxide decreases.

Effect of inert gas on reaction rate

The addition of inert gas (He, Ne, Ar, etc) is done in two ways as constant temperature and constant volume.

  1. The addition of inert gas at constant volume can not affect the equilibrium. Since the concentration of the total reacting components remains unchanged.
  2. But when inert gas is added to the system at constant pressure the volume of the reacting system increased and thereby total concentration decreased. Thus according to the Le-Chatelier principle, the system will move in the direction in which no of moles is increases.

Le Chatelier’s principle catalyst

Catalyst can speed up the reaction or response in the kinetics of the reaction it does not affect the chemical equilibrium of the reaction. A reversible reaction where the forward and reverse reaction rates are equal. But the presence of a catalyst, speeds up both the forward and backward reaction, thereby allowing the system to reach equilibrium faster or increases the entropy of the system.

Thus catalysts can be lowering the transition state and the reaction proceeds faster rate. It can be lowering the energy of the transition state(rate-limiting step). Thus catalysts reduce the required free energy of activation to allow the reaction proceeds faster rate and reach equilibrium more rapidly.

Le Chatelier’s principle facts

The le-Chatelier principle provides the reacting system with some special features.

  • For example, if the volume of the nonreactive system decreased by a specific amount the pressure rises correspondingly. Thus the equilibrium shifted to the low volume sides (if ΔV ≠ 0). So the pressure increases become less than in the non-reactive system. This shift in the equilibrium position makes the reactive system higher compressibility than the non-reactive one.
  • Similarly, if the specific heat supplied to the non-reacting system temperature of corresponding increases. Thus heat supplied does not increase the temperature so much. Since the equilibrium shift to the higher enthalpy side and the temperature is less increased.
    This shift of equilibrium makes the heat much higher than the non-reactive system. Since the reacting system chosen as a heat storage medium.

What happens to the vapor pressure of a liquid, when a nonvolatile solute dissolved in it?

The pure solution, the mole fraction x 1 = 1. When the non-volatile solute added to the solvent the mole fraction of the solvent decreased from 1 thus x 1 ㄑ1. Thus to reduce these effects, according to the Le-Chatelier principle the solvent is less vaporized. In learning chemistry, this leads to a lowering of the vapor pressure.

Originally published at https://www.priyamstudycentre.com on August 13, 2019.



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