Le Chatelier's principle states that if a stress is applied to a system at equilibrium, it will shift in a way to reduce the effect of the stress. For reactions with gaseous components, an increase in pressure favors the side with fewer moles of gas, while a decrease in pressure favors the side with more moles of gas. However, if the total moles of gas are equal on both sides of the reaction, as in the reaction H2(g) + I2(g) ⇌ 2HI(g), a change in pressure will not affect the equilibrium position.

1. Le Chatelier’s Principle
Effect of Pressure Change

2. Theory Le Chatelier’s Principle
 If you walking up an
escalator at the same speed
as it is moving down, there
is no overall change in your
position because the two
opposing motions are
balanced.
Start the Escalator

3. Theory Le Chatelier’s Principle
 Le Chatelier’s principle gives
insight into how the system
responds when equilibrium is
altered. It states that if there is a
change in any of the factors that
govern the equilibrium conditions
of a system, then the system will
undergo a change to reduce the
effect of the change.
Add N2O4
Molecule

4. Theory Le Chatelier’s Principle-Effect of Pressure
 Let us consider the synthesis
of ammonia from nitrogen
and hydrogen. Let the
system be allowed to attain
equilibrium in a cylinder
with a piston.
N2(g) + 3H2(g) ⇌ 2NH3(g)

5. Theory Le Chatelier’s Principle-Effect of Pressure
• The change in pressure has
significant effect only on
equilibrium systems with
gaseous components. When
the pressure on the system
is increased, the volume
decreases proportionately
and the system responds by
shifting the equilibrium in a
direction that has fewer
moles of gaseous molecules.
i.e. it favours the formation
of ammonia.
Rotate the handle

6. Theory Le Chatelier’s Principle-Effect of Pressure
• The change in pressure has
significant effect only on
equilibrium systems with
gaseous components. When
the pressure on the system
is increased, the volume
decreases proportionately
and the system responds by
shifting the equilibrium in a
direction that has fewer
moles of gaseous molecules.
i.e. it favours the formation
of ammonia.
Rotate increase
pressure
Rotate decrease
pressure

7. Theory Le Chatelier’s Principle-Effect of Pressure
 If we pull the piston upwards to
reduce the pressure, the volume
increases. It favours the
decomposition of ammonia
Rotate increase
pressure
Rotate decrease
pressure

8. Theory Le Chatelier’s Principle-Effect of Pressure
Add Hydrogen
Add Iodine

9. Theory Le Chatelier’s Principle-Effect of Pressure
However, when the total number of
the moles of the gaseous reactants
and the gaseous products are equal,
the change in pressure has no effect
on system at equilibrium.
Let us consider the following reaction
H2(g) + I2 (g) ⇌ 2HI(g)
2 moles of reactants ⇒ 2 moles of
product
Here, the number of moles of
reactants and products are equal. So,
the pressure has no effect on such
equilibrium with Δng = 0.
Add Hydrogen
Add Iodine
H2(g) + I2 (g) ⇌ 2HI(g)
2 moles of reactants ⇒ 2 moles of product

10. Theory Le Chatelier’s Principle-Effect of Pressure
However, when the total number of
the moles of the gaseous reactants
and the gaseous products are equal,
the change in pressure has no effect
on system at equilibrium.
Let us consider the following reaction
H2(g) + I2 (g) ⇌ 2HI(g)
2 moles of reactants ⇒ 2 moles of
product
Here, the number of moles of
reactants and products are equal. So,
the pressure has no effect on such
equilibrium with Δng = 0.
Add Hydrogen
Add Iodine
H2(g) + I2 (g) ⇌ 2HI(g)
2 moles of reactants ⇒ 2 moles of product

11. Theory Le Chatelier’s Principle-Effect of Pressure
However, when the total number of
the moles of the gaseous reactants
and the gaseous products are equal,
the change in pressure has no effect
on system at equilibrium.
Let us consider the following reaction
H2(g) + I2 (g) ⇌ 2HI(g)
2 moles of reactants ⇒ 2 moles of
product
Here, the number of moles of
reactants and products are equal. So,
the pressure has no effect on such
equilibrium with Δng = 0. H2(g) + I2 (g) ⇌ 2HI(g)
2 moles of reactants ⇒ 2 moles of product
Rotate the handle