Le Chatelier's Principle - Principle, Factors Affecting the Equilibrium State, Practice Problems, and FAQs

Suppose a man is moving (upwards) with the same speed in the opposite direction of an escalator. Let the movement of the man be the forward process and the movement of the escalator be the backward process. At the same speed, the man neither moves forward nor backwards and is said to be in equilibrium. The man will be in equilibrium irrespective of the speed-low or high, but as long as both speeds are the same.

The same concept is applicable in chemical changes also under Le Chatelier's Principle.

Table of Content

• Le Chatelier's Principle
• Application of Le Chatelier's Principle
• Practice Problems
• Frequently Asked Questions- FAQs

Le Chatelier's Principle

The principle of Le Chatelier asserts that if an outside influence disturbs the state of equilibrium, the system will change. This adjustment counteracts the disturbance and restores the equilibrium state of the system.

Application of Le Chatelier's Principle

• Change in the concentration of reactant and product
• Change in the pressure of the system
• Change in the volume of the system
• Addition of inert gas
• Change in temperature
• Addition of catalyst

1. Change in concentration of reactant and product

(i) Change in concentration of reactant

Consider an equation

Any stage in reaction except equilibrium

If we add reactant A into the mixture, reaction quotient Q becomes less than the equilibrium constant.

We know that at equilibrium . So, in order to re-establish equilibrium, the reaction quotient (Q) must rise, implying that the amount of product must rise while the amount of reactant must fall. As a result, there is a forward shift in the equilibrium.

(ii) Change in the concentration of products

The reaction quotient Q becomes more than the equilibrium constant when the concentrations of C or D increase.

To re-establish equilibrium, the reaction quotient (Q) must decrease, implying that the number of reactants must increase while the number of products must decrease. As a result, there is a backward shift in the equilibrium.

Important Point: The addition of a reactant (solid or liquid state) to the reaction has no effect on the equilibrium.

Change in the pressure of the system

Only the gaseous equilibrium is affected by a change in pressure P at a constant temperature.

From ideal gas equation:

( = total gaseous moles at equilibrium)

Case-1: When >0

[=  ( moles of gaseous products) -  ( moles of gaseous reactants)]

; = 3, = 2

Now, as the pressure increases at a constant temperature,  increases. To cancel out the effect, the reaction moves in the direction of a decrease in the number of gaseous moles (). As a result, does not affect moves backwards since there are less gaseous moles on the reactant side.

Case-2: When <0

; = 1, = 3

the reaction moves forward since there are less gaseous moles on the product side.

Case-3: When = 0

; = 2, = 2

if pressure changes then there will be no shift in equilibrium as the number of gaseous moles on reactant and product side are equal.

Change in the volume of the system

Pressure is inversely proportional to the volume at a constant temperature as per ideal equation

Case-1: When >0

; = 3, = 2

Now, as the volume increases at a constant temperature, pressure decreases and   decreases. As a result, the reaction moves forward since there are more gaseous moles on the product side.

Case-2: When <0

; = 1, = 3

As a result, the reaction moves backwards since there are more gaseous moles on the reactant side.

Case-3: When = 0

; = 2, = 2

If volume changes then there will be no shift in equilibrium as the number of gaseous moles on the reactant and product side are equal.

Addition of inert gas

The reactant and product molecules do not react with inert gas molecules(He, Ne, Ar, Kr, Xe, Rn).

The effect of inert gas addition can be studied in two conditions

• Addition of inert gas at a constant volume

Adding an inert gas at a constant volume has no effect on the molar concentrations   or partial pressure of the substances involved in the reaction (reactants and products). Hence, there will be no shift in equilibrium.

• Addition of inert gas at a constant pressure

Case-1: When >0

; = 3, = 2

When an inert gas is added to such a system, the volume increases, molar concentration decreases. To cancel out the effect, the reaction moves in the direction of an increase in the number of gaseous moles (n_g). As a result, the injection of an inert gas will cause the reaction to go forward.

Case-2: When <0

; = 1, = 3

As a result, the injection of an inert gas will cause the reaction to go backward.

Case-3: When = 0

; = 2, = 2

To such a system, when the inert gas is added, the volume increases, then there will be no shift in equilibrium as the number of gaseous moles on reactant and product side are equal.

Effect of Temperature

For endothermic reactions

;

When temperatures increase, the heat is absorbed in the system. To cancel out the effect, the reaction moves in the direction where heat gets consumed. The equilibrium moves in the forward direction to maintain the equilibrium.

For exothermic reactions

;

When temperatures increase, the heat is released from the system. To cancel out the effect, the reaction moves in the direction where heat gets consumed. The equilibrium moves in the backward direction to maintain the equilibrium.

Addition of a Catalyst

A catalyst boosts the pace of a chemical reaction by opening up a new low-energy pathway for the conversion of reactants to products. It does not affect the equilibrium since it boosts the rate of forwarding and reverses reactions that pass through the same transition state by the same amount.

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Practice Problems

Q1. What effect will adding a catalyst at a steady temperature have?

A. The equilibrium constant does not change.

B. The enthalpy change of reaction will not change.

C. Both K_f  and K_b will increase in the same proportion.

D. The composition of the equilibrium will vary.

Solution: We already know that when a catalyst is used, both forward and backward rate constants increase accordingly. As a result, there is no change in the reaction's equilibrium constant, equilibrium concentration or composition, or enthalpy change.

Hence, options (A), (B) and (C) are correct.

Q2.  When

A. an inert gas is introduced at constant pressure

B. the volume of the container is increased, the reaction should proceed in which direction?

Solution:

Adding an inert gas at constant pressure causes the equilibrium to move in the forward direction of more gaseous moles.

As the volume of the system increases, the pressure in the system decreases. As a result, the equilibrium will change in the direction of forward direction having more gaseous moles.

Q3. Take the following reaction: ; . The dissociation of  (g) will be favoured by which of the following?

A. Raising the temperature

B. Increasing the container's volume

C. Adding F₂ gas

Solution: The dissociation of (g) indicates that the equilibrium should shift backwards.

 In option A,  Because the reaction is exothermic, increasing the temperature will cause the equilibrium to shift backwards.

 In option B, As the volume of the system grows, the equilibrium will change in favour of a higher number of gaseous moles. The reactant side of the reaction contains a higher number of gaseous moles. As a result, the balance will shift in the opposite direction.

 In option C, We know that adding the reactant will cause the equilibrium to shift forward.

 As a result, alternatives A & B are correct.

Q4. When the pressure for the following equilibrium is increased:

;

1. more  is formed
2. more  is formed

Solution: As the pressure rises, the equilibrium shifts to a backward direction where the number of gaseous moles decreases.

;  = 1, = 0

As a result, more  is formed.

Frequently Asked Questions- FAQs

Question 1. How did Le Chatelier come up with his theory?

Answer. Le Chatelier conducted investigations on explosive materials and published his first scientific publications. Based on the thermocouple principle, these investigations led to advancements in measuring high temperatures.

Question 2. What does Le Chatelier's principle mean in terms of industrial applications?

Answer. Le Chatelier's Principle can be used to forecast how a change in temperature, concentration, or pressure would affect the equilibrium position in a chemical process. This is critical, especially in industrial applications where yields must be predicted and maximised with precision.

Question 3. Is the concept of Le Chatelier applicable to irreversible reactions?

Answer. The principle of Le Chatelier applies to all systems in equilibrium, whether chemical or physical. Only reversible reactions can achieve equilibrium. As a result, we are unable to apply Le Chatelier's concept to irreversible reactions.

Question 4. Is there a way around Le Chatelier's rule?

Answer. When applied to very small changes in an equilibrium system, Le Chatelier's principle is shown to have no exceptions. Finite changes, on the other hand, may break the principle.

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