Le Chatelier's Principle (AQA A-Level Chemistry): Revision Notes
1.8.2 Le Chatelier's Principle
Le Chatelier's principle helps predict how a change in conditions affects the position of equilibrium in a reversible reaction.
It states that "if a system at equilibrium experiences a change in concentration, pressure, or temperature, the equilibrium will shift in a direction that opposes this change."
Effects of Changing Conditions
Change in Pressure
- Increasing pressure shifts the equilibrium towards the side with fewer gas molecules to reduce the pressure.
- Decreasing pressure shifts the equilibrium towards the side with more gas molecules to increase the pressure.
Example: For the reaction:
Increasing pressure shifts the equilibrium to the right (fewer moles of gas on the product side), increasing the yield of
Decreasing pressure shifts the equilibrium to the left (more moles of gas on the reactant side), increasing the yield of and
Change in Temperature
- If the forward reaction is exothermic, an increase in temperature shifts the equilibrium towards the reverse (endothermic) reaction, reducing the yield of products.
- A decrease in temperature favours the exothermic reaction, shifting the equilibrium towards the products.
Example: For the reaction
where
Increasing temperature shifts the equilibrium to the left (endothermic direction).
Decreasing temperature shifts the equilibrium to the right (exothermic direction), increasing the yield of
Change in Concentration
- Increasing the concentration of reactants shifts the equilibrium to the right, increasing product formation.
- Increasing the concentration of products shifts the equilibrium to the left, increasing reactant formation.
Example: For the reaction
Adding more shifts the equilibrium to the right, producing more
Adding more shifts the equilibrium to the left, increasing the concentrations of and
Use of a Catalyst
A catalyst increases the rate of both the forwards and reverse reactions equally. This means that equilibrium is reached more quickly, but it does not affect the position of equilibrium or the concentrations of reactants and products.
Industrial Applications: Compromise Conditions
In industrial processes, achieving both high yield and fast reaction rates is often necessary. Compromise conditions are used to balance the demands of maximising yield and maintaining a practical reaction rate.
Example: Ethanol Production (Hydration of Ethene) The production of ethanol from ethene is a reversible reaction:
Catalyst: Phosphoric acid speeds up the reaction but does not affect the equilibrium position.
Pressure: High pressure shifts the equilibrium to the right (fewer gas molecules), increasing ethanol yield. However, very high pressures increase costs and require stronger equipment.
Temperature: A lower temperature favours the exothermic forwards reaction, increasing yield. But, too low a temperature slows the reaction, so a compromise temperature of 300°C is used to balance yield and rate.
Excess Reactants: Using excess ethene or steam shifts the equilibrium to the right, increasing ethanol yield.
Practical Example for Students
You can observe the effect of concentration and temperature changes on equilibrium by performing a test-tube experiment with the following system:
Adding concentrated HCl shifts the equilibrium to the right, changing the solution colour from blue to green.