4.8 - Changing the Position of Equilibrium

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).

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.

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.

How quickly equilibrium is reached

How quickly a system reaches equilibrium depends on the reaction rate, which is separate from the position of equilibrium.

Factors that increase the rate of reaching equilibrium

• Higher temperature - Particles have more kinetic energy and collide more frequently and energetically.
• Higher pressure (for gases) or higher concentration - Particles are closer together, increasing the frequency of collisions.
• Adding a catalyst - Speeds up both the forward and reverse reactions by the same amount, reaching equilibrium faster without changing the yield.

These graphs show only how quickly equilibrium is reached. A change in temperature also moves the position of equilibrium, so a hotter run levels off at a different amount of product: more if the forward reaction is endothermic, less if it is exothermic. A change in pressure or concentration can also move the position; only a catalyst leaves the final amount of product unchanged.