Le Chatelier
Chemistry: AQA A Level, topic 1.6.1
Dynamic Equilibrium
If a reversible reaction happens in a closed system, a dynamic equilibrium will eventually be reached.
Closed system
- A closed system is a system where reactants and products can neither be added nor removed.
Dynamic equilibrium
- At dynamic equilibrium, the rate of the forward reaction equals the rate of the backward reaction.
- The equilibrium is dynamic because both the forward and backward reactions are still taking place.
Le Chatelier's Principle
Le Chatelier's principle is a powerful tool for predicting the effect of physical changes on an equilibrium.
Definition
- Le Chatelier's principle is simple but powerful.
- Le Chatelier's principle states that when a change is imposed on an equilibrium, the equilibrium position will shift to oppose the change.
What changes?
- The change Le Chatelier refers to can be as a result of:
- Change in temperature.
- Change in pressure.
- Change in concentration.
Equilibrium position
- The equilibrium position describes how much of each chemical you have.
- If you have 50% reactants and 50% products, that is one position.
- If you have 75% reactants and 25% products, that is another.
Le Chatelier's principle in practice
- If you change a system by heating it, increasing the pressure, or changing the concentration of a reactant, the proportion of reactants and products will change to:
- Reduce the temperature.
- Reduce the pressure.
- Change the concentration of a reactant.
An example
- Say you have a reversible reaction, and the forward reaction is exothermic (gives out heat).
- If you cool the reaction down (e.g. by submerging it in an ice bath), the equilibrium position shifts to the right (i.e. the forward reaction increases).
- This will counteract the change and return the temperature of the system back towards its original value.
- This increases the proportion of products relative to reactants.
Industry and Yields
One of the most important places to understand equilibria is in industry.
Catalysts
- So far we've talked about heat, pressure, and concentration.
- What about catalysts?
- Catalysts have NO effect on the position of equilibrium.
- All a catalyst does is speed up the rate of the forwards and backwards reactions equally.
Industry
- Any reaction will be faster if you heat it up - so far so good!
- But exothermic reactions will have lower yields when the temperature is increased.
- Because a number of useful reversible reactions are exothermic, it is important that we find compromises between reaction rates and yields.
E.g. Ethanol production
- Ethanol is produced in an exothermic reaction between steam and ethene.
- The conditions used are 300oC and 65 atmospheres of pressure.
- Phosphoric acid is used as a catalyst.
- A lower temperature would give a better yield, but it would take longer.
- 300oC is the compromise temperature.
- A higher pressure would give a better yield, but it's very expensive to create and maintain high pressures.
- 65 atmospheres is the compromise pressure.
Test yourself on Le Chatelier
What do we call a system in which reactants and products can neither be added nor removed?
Why is chemical equilibrium described as dynamic?
Which word describes this type of chemical equilibrium?
What is Le Chatelier's Principle?
Which of the following are changes to which Le Chatelier's Principle applies?