Le Chatelier’s principle

Le Chatelier principle dynamic equilibrium

Le-Chatelier principle predicts quantitatively the effect on the system at equilibrium when some of the variables such as temperature, pressure, and concentration of the equilibrium of a chemical reaction.

If a system at equilibrium point subjected to change, the system will react in such a way so as to oppose or reduce the change.

If this possible, the system tends to balance or counteract the effects of any imposed stress.

How does pressure affect the rate of a chemical reaction?

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 + 3 H₂ ⇆ 2 NH₃

When the pressure increases, the shift of the reaction in a direction where the sum of the stoichiometric number 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 with the increase of temperature, the equilibrium will shift in the endothermic direction. Equilibrium shifted to the high enthalpy side.

If the reaction proceeds from low enthalpy side to high enthalpy side heat absorbed and for this reason, this direction known as endothermic direction.

N₂ + 3 H₂ ⇆ 2 NH₃ ΔH = -22 kcal
where ΔH = ΣHproduct – ΣHreactant

Thus the enthalpy of the reactants in the above reaction higher than that of the products. Thus with the increases in temperature backward reaction favors and thus the equilibrium shifted to the higher enthalpy side and the production of ammonia decreased.

With the decrease of temperature, the equilibrium will shift in the exothermic direction that is shifted to the low enthalpy side.

If the reaction proceeds from high enthalpy side to low enthalpy side heat released and for this reason, this direction is known as exothermic direction.

Thus with the decreases in temperature forward reaction favors and thus the equilibrium shifted to the low enthalpy side and the production of ammonia is increased.

N₂ + O₂ ⇆ 2 NO 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 thus the equilibrium shifted to the lower enthalpy side and the production of NO- is increasing.

Decreases in temperature for the above reaction backward reaction favors and the equilibrium shifted to the high enthalpy side and the production of NO decreased.

Effect of inert gas on reaction rate

Addition of inert gas(He, Ne, Ar, etc) at constant temperature by two way

Constant volume

The addition of inert gas at constant volume can not affect the equilibrium since the concentration of the total reacting components remain unchanged.

Constant pressure

When inert gas is added to the system at equilibrium at constant pressure the volume of the reacting system increased and thereby total concentration decreased.

According to the Le-Chatelier principle, the system will move in the direction in which no of moles is increases and thereby the concentration of the system is also increased.

Le Chatelier’s principle
Le Chatelier’s principle

Question
What happens to the NH₃ yield when pressure increased?

Answer
According to the Le-Chatelier principle yield of NH₃ increased if pressure is increasing.

Question
N₂ + 3 H₂ ⇆ 2 NH₃ ΔH = -22 kcal equilibrium shifted to forward direction when

  1. the concentration of NH3 increases
  2. pressure is decreasing
  3. the concentration of N2 and H2 decrease
  4. pressure increases and temperature decreases

Answer
Pressure increases and temperature decreases

Effect of catalyst on chemical equilibrium

Catalyst can speed up the reaction it does not affect the chemical equilibrium of the reaction. A reversible reaction, the equilibrium state is one in which the forward and reverse reaction rates are equal.

The presence of a catalyst, speeds up both the forward and backward reaction, thereby allowing the system to reach equilibrium faster.

This is very important that the addition of a catalyst has no effect on the final equilibrium position of the reaction. Thus we can not increases the production of the product.

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.

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

Answer
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.

To reduce the effect according to the Le-Chatelier principle the solvent is less vaporized and the mole fraction of the solvent in a solution is thus improved. Thus there occurring a lowering of vapor pressure.

Le Chatelier’s principle facts

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

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