Learning Objectives

  • Understand that a reversible reaction can come to a state of equilibrium
  • Explain what is meant by dynamic equilibrium


If products form reactants back as the reactants form them, this kind of reactions called reversible reactions.

Many reactions are reversible. In these reactions rate of forward reaction decreases, and the rate of reverse reaction increases by the time.

These reactions will reach a state of equilibrium when the rates of the forward reaction and reverse reaction are equal.

In general, in systems that reach equilibrium, the following two trends are in consensus in opposite directions:

  • Tendency to Maximum disorderness (towards to the side with more gas particles)
  • Tendency to Minimum energy

For example:

2NH3(g) + Heat ⇔ N2(g) + 3H2(g)

For this reaction,

  • Disorderness tendency is towards to the products and,
  • energy tendency is towards to the reactants side.

So, if the equilibrium conditions are provided, then this reaction reaches equilibrium as the rates of the forward reaction and reverse reaction are equal.

Equilibrium Conditions and Features:

  • System should be closed.
  • Temperature should be constant
  • Concentrations of both reactants and products are constants at equilibrium.
  • As equilibrium is reached, then microscopic changes (forward and reverse reactions at equal rate) continue, but macroscopic (observable) changes stop. ⇒ As a result, equilibrium is a dynamic state.

It is important to understand that even though the concentrations of reactant and product are constant at equilibrium, this in no way implies that they are equal. In fact, most commonly there will be a much higher concentration of either reactant or product in the equilibrium mixture, depending both on the reaction and on the conditions.

Which statements are always correct for a reaction at equilibrium?

I. The forward and reverse reactions both continue.

II. The rates of the forward and reverse reactions are equal.

III. The concentrations of reactants and products are equal.

I and II

Which statements is always true for a chemical reaction that has reached equilibrium at constant temperature?

A) The yield of product(s) is greater than 100%.

B) The rate of the reverse reaction is greater than that of the forward reaction.

C) The amounts of reactants and products do not change.

D) Both forward and reverse reactions have stopped.


Which statement is not true for a mixture of water vapor and water at equilibrium at constant temperature?

A) The rates of evaporation and condensation are equal.

B) The amounts of water and vapor are equal.

C) The same position of equilibrium can be reached by cooling vapor and by heating water.

D) There is no observable change in the system.


The Position of Equilibrium