The Idea of Equilibrium

This lesson covers: 

  1. What reversible reactions are
  2. How reversible reactions can reach a state of dynamic equilibrium
  3. The features of dynamic equilibrium
  4. The equilibrium position

Reversible reactions

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

For example, in the reaction:

H2(g) + I2(g) ⇌ 2HI(g)

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


So reversible reactions can occur in both the forward direction:  H2(g) + I2(g) ➔ 2HI(g)

And the backward direction:  2HI(g) ➔ H2(g) + I2(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.
Graph showing the rates of forward and backward reactions over time reaching dynamic equilibrium.

So dynamic equilibrium occurs once the opposing forward and backward reaction rates are matched.

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:

Before equilibriumAt equilibriumAfter equilibrium (when conditions change)
Concentrations of reactants and productsChanging over timeRemain constantAdjust to new constant levels
Rates of forward and backward reactionsUnequalEqualUnequal initially, then re-equilibrate
Appearance of reaction mixtureVisibly changingAppears unchangingVisibly changes then new equilibrium forms

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 favors 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.