IB Chemistry Sub-topic R2.3 Notes

Properties and Types of Equilibrium 

In Topic R1.1 you learned about reaction kinetics and in Topic R2.2 you learned about reaction rate. As mentioned in those topics, once a reaction is complete, it reaches a state of equilibrium.

Equilibrium is defined as the position of a reaction in which the concentration of both the products and reactants remains constant. The position of equilibrium varies for every reaction, as in some it lies closer to the reactants and in others closer to the products.

There are a few important properties of equilibrium:

1. It can only be obtained in a closed system. This means the reactants and products can't exit into the surroundings.
2. There are no overall changes in concentration.
3. The concentrations of reactants and products are not necessarily the same.

Before discussing the types of equilibrium, you need to review the types of reactions:

1. Non-reversible reactions - reactions that only occur forwards to convert reactants to products.
2. Reversible reactions - reactions that occur forwards to convert reactants to products and reverse to convert products to reactants.

In these reactions, two types of equilibrium can occur:

1. Static equilibrium - in this, the rate of forwards reaction reaches zero.
2. Dynamic equilibrium - in this, the rate of the forwards reaction equals the rate of the reverse reaction.

An additional types of equilibrium is phase equilibrium. This is an equilibrium between phase changes. For example:

Br_2(l) ⇌ Br_2(g)

In dynamic and phase equilibria, despite forwards and reverse reactions, the overall concentrations of reactants and products remain the same.

Equilibrium Constant 

You are expected to be able to quantify the position of equilibrium, which is done by the equilibrium constant K_c. In the reaction:

aW + bX ⇌ cY + dZ

The formula for this is:

$K_{c} = \frac{[Y]^{c}[Z]^{d}}{[W]^{a}[X]^{b}}$

As you can see, this places the concentration of products as the numerator and concentration of reactants as the denominator.

Thus:

1. Equilibrium lies close to the products - this means the container is mostly products and so K_c > 1.
2. Equilibrium lies close to the reactants - this means the container is mostly reactants and so K_c < 1.