Dynamic Equilibrium & Le Chatelier
Chemistry: Cambridge International A Level, topic 1.7.1
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 forwards 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 forwards 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.
Test yourself on Dynamic Equilibrium & 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?