Le Chatelier's Principle

Position of equilibrium

The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture.
When the position of equilibrium shifts to the left, it means the concentration of reactants increases
When the position of equilibrium shifts to the right, it means the concentration of products increases

Le Chatelier’s principle

Le Chatelier’s principle says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change
The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration

Effects of concentration on the position of equilibrium

When the concentration of a reactant increases, the rate of the forward reaction increases and the system is no longer in equilibrium
When a new equilibrium is established, there will be more product and less reactant within the reaction mixture, so the equilibrium has shifted to the right
This shift has reduced the effect of the increase by removing some of the reactant

Effects of Concentration Table

Change	How the equilibrium shifts
Increase in concentration of a reactant	Equilibrium shifts to the right to reduce the effect of an increase in the concentration of a reactant
Decrease in concentration of a reactant	Equilibrium shifts to the left to reduce the effect of a decrease in the concentration of a reactant
Increase in concentration of a product	Equilibrium shifts to the left to reduce the effect of an increase in the concentration of a product
Decrease in concentration of a product	Equilibrium shifts to the right to reduce the effect of a decrease in the concentration of a product

Effects of concentration of the value of K

If all other conditions stay the same, the equilibrium constant K is not affected by any changes in concentration of the reactants or products
For example, the decomposition of hydrogen iodide:

2HI ⇌ H₂ + I₂

The equilibrium expression is:

$K = \frac{[H_{2}] [I_{2}]}{{[H I]}^{2}} = 6.25 \times 10^{- 3}$

Adding more HI makes the ratio of [ products ] to [ reactants ] smaller
To restore equilibrium, [H₂] and [I₂] increases and [HI] decreases
Equilibrium is restored when the ratio is 6.25 x 10^-3 again

Worked example

1. Using the reaction below:

CH₃COOH (l) + C₂H₅OH (l) ⇌ CH₃COOC₂H₅(l) + H₂O (l)

Explain what happens to the position of equilibrium when:

More CH₃COOC₂H₅(l) is added
Some C₂H₅OH (l) is removed

2. Use the reaction below:

Ce⁴⁺ (aq) + Fe²⁺ (aq) ⇌ Ce³⁺(aq) + Fe³⁺ (aq)

Explain what happens to the position of equilibrium when water is added to the equilibrium mixture

Answer 1a:

The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed
The reaction moves in this direction to oppose the effect of added ethyl ethanoate, so the ethyl ethanoate decreases in concentration

Answer 1b:

The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed
The reaction moves in this direction to oppose the removal of ethanol so more ethanol (and ethanoic acid) are formed from ethyl ethanoate and water

Answer 2:

There is no effect as the water dilutes all the ions equally so there is no change in the ratio of reactants to products

Effects of pressure on the position of equilibrium

Changes in pressure only affect reactions where the reactants or products are gases
The pressure of a gas in a fixed volume increases as the number of gas molecules increases
Changes in pressure will cause the equilibrium to shift to reduce the effect of this change

Effects of Pressure Table

Change	How the equilibrium shifts
Increase in pressure	Equilibrium shifts in the direction that produces the smaller number of molecules of gas to decrease the pressure again
Decrease in pressure	Equilibrium shifts in the direction that produces the larger number of molecules of gas to increase the pressure again

Effects of pressure on the value of K

If all other conditions stay the same, the equilibrium constant K is not affected by any changes in pressure of the reactants and products

Exam Tip

If there are the same number of gas molecules on either side of the reaction, changes in pressure will not change the position of equilibrium

Worked example

Predict the effect of increasing the pressure on the following reactions:

1. N₂O₄ (g) ⇌ 2NO₂(g)

2. CaCO₃ (s) ⇌ CaO (s) + CO₂(g)

Predict the effect of decreasing the pressure on the following reaction:

3. 2NO₂ (g) ⇌ 2NO (g) + O₂(g)

Answer 1:

The equilibrium shifts to the left as there are fewer gas molecules on the left
This causes a decrease in pressure

Answer 2:

The equilibrium shifts to the left as there are no gas molecules on the left but there is CO₂ on the right
This causes a decrease in pressure

Answer 3:

The equilibrium shifts to the right as there is a greater number of gas molecules on the right
This causes an increase in pressure

Effects of temperature on the position of equilibrium

When the temperature changes, the equilibrium will respond by moving in the direction which will absorb or release energy

Effects of Temperature Table

Change	How the equilibrium shifts
Increase in temperature	Equilibrium shifts in the endothermic direction, absorbing energy to reverse the change
Decrease in temperature	Equilibrium shifts in the exothermic direction, releasing energy to reverse the change

Effects of temperature on the value of K

Changes in temperature change the equilibrium constant K
For an endothermic reaction such as:

2HI (g) ⇌ H₂ (g) + I₂(g)

K = \frac{[H_{2}] [I_{2}]}{{[HI]}^{2}}

An increase in temperature:
- [H₂] and [I₂] increases
- [HI] decreases
Because [H₂] and [I₂] are increasing and [HI] is decreasing, the equilibrium constant K increases

For an exothermic reaction such as:

2SO2 (g) + O₂ (g) ⇌ 2SO₃ (g)

K = \frac{{[{SO}_{3}]}^{2}}{{[{SO}_{2}]}^{2} [O_{2}]}

An increase in temperature:
- [SO₃] decreases
- [SO₂] and [O₂] increases
Because [SO₃] decreases and [SO₂] and [O₂] increases the equilibrium constant K decreases

Worked example

Using the reaction below:

H₂ (g) + CO₂ (g) ⇌ H₂O (g) + CO (g) ∆H = + 41.2 kJ mol^-1Predict the effect of increasing the temperature on this reaction
Using the reaction below:

Ag₂CO₃ (s) ⇌ Ag₂O (s) + CO₂(g)

Increasing the temperature increases the amount of CO₂(g) at constant pressure. Is this reaction exothermic or endothermic? Explain your answer.

Answer 1:

The reaction will absorb the excess heat and since the forward reaction is endothermic, the equilibrium will shift to the right

Answer 2:

The reaction will absorb the excess heat and since this causes a shift of the equilibrium towards the right (as more CO₂(g) is formed) this means that the reaction is endothermic (because endothermic reactions favour the products)

Effects of catalysts

A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the forward and reverse reaction equally)
Catalysts only cause a reaction to reach its equilibrium faster
Catalysts therefore have no effect on the position of the equilibrium or on the value of K

Exam Tip

When conditions of industrial processes are chosen, Le Chatelier's principle can be used to predict the conditions that would cause the equilibrium to lie towards the products, giving a high equilibrium yield
However, the kinetics of the reaction must also be considered as the rate of reaction needs to be sufficiently fast.
For example, consider a reversible reaction whose forward reaction is exothermic
- According to Le Chatelier's principle, lower temperatures would produce a higher equilibrium yield
- However, higher temperatures will give a faster rate of reaction
- A compromise temperature is used which gives a lower yield of product but is made more quickly

Heterogeneous equilibria

Le Chatelier's principle can also be applied to heterogeneous equilibria
For example, in a fizzy drink bottle, an equilibrium exists between the dissolved CO₂ and gaseous CO₂:

CO₂ (g) ⇌ CO₂ (aq)

When the bottle is opened, some CO₂ (g) escapes, the equilibrium shifts to the left to reduce the effect of this change and bubbles of CO₂ (g) are observed

Le Chatelier's Principle (SL IB Chemistry)

Revision Note