Reversible reactions

Many chemical reactions are reversible - the products can react to reform the original reactants.

For example, in the reaction:

H_2(g) + I_2(g) ⇌ 2HI_(g)

The HI product can react to reform the original H₂ and I₂ reactants.

So reversible reactions can occur in both the forward direction: H_2(g) + I_2(g) ➔ 2HI_(g)

And the backward direction: 2HI_(g) ➔ H_2(g) + I_2(g)

To show that a reaction is reversible, a double arrow ⇌ is used instead of a single arrow.

Reaching dynamic equilibrium

In a closed system, a reversible reaction reaches dynamic equilibrium through the following steps:

1. The forward reaction initially proceeds rapidly as lots of reactants are present.
2. As reactants get used up, the rate of the forward reaction slows down.
3. Meanwhile, more products accumulate, speeding up the backward reaction.
4. Eventually, the rates of the forward and backward reactions become equal.
5. At this point, dynamic equilibrium is established.

So dynamic equilibrium occurs once the rates of the forward and backward reactions are equal.

Features of dynamic equilibrium

Dynamic equilibrium has several key features:

1. Equilibrium can only be achieved in a closed system - this means no substances can enter or leave.
2. The concentrations of reactants and products stay constant over time (but do not have to be the same as each other).
3. The rates of the forward and backward reactions are equal.
4. The system appears unchanging even though the forward and backward reactions are still occurring - this is why it is called a dynamic equilibrium.

Altering the conditions of the system (e.g. pressure, temperature) can disrupt the equilibrium and cause the concentrations to adjust until a new equilibrium is established.

Features of a reversible reaction:

What the position of equilibrium tells us

The position of equilibrium refers to the relative amounts of reactants and products present at equilibrium. It indicates whether the equilibrium favours the formation of products (right) or the reformation of reactants (left).

• If the equilibrium position lies to the right, the concentration of products is higher than that of reactants.
• If the equilibrium position lies to the left, the concentration of reactants is higher than that of products.