Le Chatelier principle definition
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.
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.
N2 + 3H2 ⇆ 2NH3
Hence when the pressure increases, the shift of the reaction in a direction where the sum of the stoichiometric coefficient of gaseous molecules 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. Thus the equilibrium shifted low thermodynamics enthalpy to the high enthalpy side with the absorption of specific heat.
N2 + 3H2 ⇆ 2NH3 ΔH = -22 kcal
where ΔH = ΣHproduct – ΣHreactant
- In the above reaction, the 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.
- 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.
N2 + O2 ⇆ 2NO ΔH = +44 kcal
Enthalpy of the reactants in the above reaction lower than that of the products.
- Thus with the increases in temperature forward reaction favors and production of NO increasing.
- But decreases in temperature for the above reaction backward reaction favors and production of NO 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.
- The addition of inert gas at constant volume can not affect the equilibrium. Since the concentration of the total reacting components remains unchanged.
- 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 or effects the kinetics of the reaction but 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.
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 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 fixed quantity of the 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 faction x1 = 1. When the non-volatile solute added to the solvent the mole fraction of the solvent decreased from 1 thus x1 ㄑ1.
Thus to reduce these effects, according to the Le-Chatelier principle the solvent is less vaporized. This leads to a lowering of the vapor pressure.