Le Chatelier’s Principle
If a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will move to counteract the change
What does Le Chatelier’s principle only apply to?
Homogenous equilibria- reactions where every species is in the same physical state (eg all liquid or all gas)
Describe increasing the concentration of a reactant and its effect on equilibrium
If you increase the concentration of a reactant, the equilibrium tries to get rid of the extra reactant.
It does this by making more product.
So the equilibrium shifts to the right
Describe increasing the concentration of a product and its effect on equilibrium
If you increase the concentration of product, the equilibrium tries to remove the extra product.
This makes the reverse reaction go faster.
So the equilibrium shifts to the left
What does decreasing the concentrations of reactants or products have on equilibrium?
It has the opposite effect to increasing the concentration of reactants or products.
Describe increasing pressure and its effect on equilibria involving gases?
Increasing the pressure shifts the equilibrium to the side with fewer gas molecules.
This reduces the pressure
Describe decreasing pressure and its effect on equilibria involving gases?
Decreasing the pressure shifts the equilibrium to the side with more gas molecules.
This raises the pressure again
Describe increasing the temperature and its effect on equilibrium?
Increasing the temperature means adding heat.
Therefore, the equilibrium shifts in the endothermic (positive delta H) direction to absorb this heat
Describe decreasing the temperature and its effect on equilibrium?
What does it mean if the forward reaction is endothermic?
Decreasing the temperature means removing heat.
The equilibrium shifts in the endothermic (negative delta H) direction to produce more heat, in order to counteract the drop in temperature.
The reverse reaction will be exothermic, and vice versa
Catalysts and effect on equilibrium
Catalysts have no effect on the position of equilibrium.
They can’t increase yield, but they do mean equilibrium is reached faster
Describe temperature changes and their effect on Kc?
The value of Kc is only valid for one particular temperature.
If you change the temperature of the system, you will also change the equilibrium concentrations of the products and reactants, so Kc will change
Effect on Kc if temperature change means there’s more product at equilibrium?
Kc will rise
Effect on Kc if temperature change means there’s less product at equilibrium?
Kc will decrease
Effect of changing the concentration of a reactant or product on Kc
Will not affect the value of Kc
Catalysts and Kc
Catalysts don’t affect Kc.
They’ll speed up the reaction in both directions by the same amount, so they just help the system to reach equilibrium faster
Two conditions for dynamic equilibrium