Equillibrum | OCR B Chemistry

Equillibrium

There are 2 types of equilibrium:

Dynamic equilibrium = Where the rate of the forward reaction equals the rate of the backward reaction.
Position equilibrium = The point in a chemical reaction at which the concentrations of reactants and products are no longer changing.

Dynamic equilibrium:

There are 2 possible scenarios where this may happen in a chemical reaction.

Example 1:

A ⇌ B

At A, the position of equilibrium is to the right.
As time increases, the concentration of [A] decreases until it reaches equilibrium.
At B, the position of equilibrium is to the left.
As time increases, the concentration of [B] increases until it reaches equilibrium.
[A] and [B] do not change.
[A] and [B] reach equillibrium when the concentration of both [A] and [B] is constant.

Example 2:

A ⇌ B

At A, the position of equilibrium is to the left.
As time increases, the concentration of [A] decreases.
At B, the position of equilibrium is to the right.
As time increases, the concentration of [B] increases.
[A] and [B] reach equillibrium when the concentration of both [A] and [B] is constant.

Le Châtelier’s Principal:

Le Châtelier’s principle states that ‘when any change is made to the conditions of the equilibrium, the position of the equilibrium will move in the direction to counteract the change’.

There are 3 Factors that affect Equilibrium position:

Let’s go through an example question covering all conditions...

Temperature:

E.g.:

State and explain the effects on the position of the equilibrium if the temperature increases.

As we can see from the equation the forward reaction is endothermic (energy is absorbed from the surroundings). Therefore the ΔH will be positive.
If the temperature increases, the system will act to make the system cooler = counteract the change.
To counteract the change, the equilibrium will favor the forward reaction because it is endothermic. The position of the equilibrium will move to the right.

Pressure:

State and explain the effects on the position of the equilibrium if the Pressure increases.

First of all, count the number of moles on both sides of the equation. On this equation above there is 2 moles on the right and one mole on the left.
If the pressure increases, the equilibrium will move to the left to oppose the change.
This is because there are fewer moles of gas on the reactant side.

Concentration:

State and explain the effects on the position of the equilibrium if the concentration of Nitrogen oxide increases.

If the concentration of Nitrogen oxide increases, the system will act to counteract the change.
Hence, the equilibrium position will move to the right to restore dynamic equilibrium