Chemistry

5.6 - Changing the Position of Equilibrium

8 min

Position of equilibrium

Le Chatelier's principle

Temperature and pressure

Concentration

note 1 of 2

Predicting shifts - Le Chatelier's principle

Le Chatelier's principle - When a condition is changed in a system at equilibrium, the system responds in a way that counteracts the change, shifting the position of equilibrium.

Example - ammonia synthesis:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The forward reaction is exothermic; the reverse reaction is endothermic.

Temperature

Raising the temperature shifts the equilibrium in the endothermic direction to take in energy and counteract the rise.
Lowering the temperature shifts the equilibrium in the exothermic direction to release energy and counteract the fall.
In the example: raising the temperature shifts it left (favouring N₂ and H₂); lowering it shifts it right (favouring NH₃).

N2 + 3H2 ⇌ 2NH3 drawn with molecule icons, its forward arrow labelled "exothermic" in blue and its reverse arrow "endothermic" in pink, between thermometer boxes reading "raising temperature favours" nitrogen and hydrogen and "lowering temperature favours" ammonia.

Pressure

Applies where reactants or products include gases.
Raising the pressure shifts the equilibrium towards the side with fewer gas molecules to counteract the rise.
Lowering the pressure shifts the equilibrium towards the side with more gas molecules.
In the example: there are four gas molecules on the left and two on the right, so raising the pressure shifts it right (favouring NH₃); lowering it shifts it left.

Concentration

Increasing the concentration of a substance shifts the equilibrium away from that side to use it up.
Decreasing the concentration of a substance shifts the equilibrium towards that side to replace it.
Adding more N₂ or H₂ shifts the equilibrium right, making more NH₃; removing NH₃ as it forms also shifts it right.

Catalysts

A catalyst has no effect on the position of equilibrium.
It speeds up the forward and reverse reactions by exactly the same amount, so equilibrium is reached more quickly without changing the yield.

note 2 of 2

Choosing conditions for a reversible reaction

The conditions used in industrial reactions are a compromise between yield, rate, safety, and cost.

Cost and safety of extreme conditions

Very high pressure - Needs thicker, more expensive reaction vessels and pipework to contain safely, increasing equipment and maintenance costs.
Very high temperature - Requires large amounts of energy to reach and maintain, raising running costs and fuel consumption.

Balancing rate and yield

The chosen conditions must give an acceptable rate as well as an acceptable yield.
For an exothermic reaction, a lower temperature shifts equilibrium towards the products but makes the reaction too slow. A compromise temperature is used to balance yield and rate.

key terms

TermDefinition

Le Chatelier's principleThe principle that if a condition is changed in a system at equilibrium, the position of equilibrium shifts to counteract the change.

end of notes

Key points to remember

Predicting shifts - Le Chatelier's principle

Le Chatelier's principle - When a condition is changed in a system at equilibrium, the system responds in a way that counteracts the change, shifting the position of equilibrium.

Example - ammonia synthesis:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The forward reaction is exothermic; the reverse reaction is endothermic.

Temperature

Raising the temperature shifts the equilibrium in the endothermic direction to take in energy and counteract the rise.

Lowering the temperature shifts the equilibrium in the exothermic direction to release energy and counteract the fall.

In the example: raising the temperature shifts it left (favouring N₂ and H₂); lowering it shifts it right (favouring NH₃).

Choosing conditions for a reversible reaction

The conditions used in industrial reactions are a compromise between yield, rate, safety, and cost.

Cost and safety of extreme conditions

Very high pressure - Needs thicker, more expensive reaction vessels and pipework to contain safely, increasing equipment and maintenance costs.

Very high temperature - Requires large amounts of energy to reach and maintain, raising running costs and fuel consumption.

Balancing rate and yield

The chosen conditions must give an acceptable rate as well as an acceptable yield.

For an exothermic reaction, a lower temperature shifts equilibrium towards the products but makes the reaction too slow. A compromise temperature is used to balance yield and rate.