1.6.1

# Le Chatelier

Test yourself

## 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.
• 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.