What Is an Equilibrium?

When we discussed energy diagrams, we mentioned that chemical reactions can move in the backward direction as well as the forward direction, forming reactants from the products. Such a reaction is referred to as being "reversible," and has the equation A ⇔ B.

Molecular Meanings

When the forward and reverse reactions for a reversible reaction take place at the same rate, the system is at equilibrium and the concentrations of both the products and reactants stay the same.

We learned that as the concentration of reactants increases, the rate of the reaction increases. As you might imagine, the same is true for the products—the reverse reaction will tend to speed up as the concentrations of the products increases. As a result, the concentrations of the reactants and products in a reversible reaction change in the following way over time:

  • At the beginning of a reaction, the reactants are the only chemicals present in the beaker. As a result, the forward reaction is the only one that takes place.
  • Shortly after the start of the reaction, the concentration of the reactants is decreasing because they are being consumed to make products—as a result, the forward reaction is decelerating. At the same time, the concentration of the products is small but increasing, causing the reverse reaction to accelerate.
  • At some point in the reaction, the rates of the forward and reverse reactions will become equal. When this occurs, the concentrations of the products and reactants cease changing and the system is said to be at equilibrium.

A lot of new chemists think that when a reaction has reached equilibrium, the process completely stops. Pardon my French, but non! Both the forward and reverse reactions are still taking place—they just take place at the same rate. As a result, the reaction may appear to the casual observer to have stopped even though molecules are still making the conversion between products and reactants.

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