Up to this point, most reactions you have looked at only proceed in one direction (from reactants to products) until they reach completion. There are many processes, however, which are reversible, meaning they can proceed in both the forward and reverse directions. For example:
In a closed container of water, there are water molecules evaporating from liquid into gas as given by this equation.
H2O (l) → H2O (g)
Simultaneously, there are some gaseous water molecules condensing back into liquid as given by this equation.
H2O (g) → H2O (l)
We can represent this reversible process by using a bidirectional arrow when we write out this equation.
H2O (l) ⇌ H2O (g)
What does the double arrow tell us about the processes that are occurring in the closed container of water?
The double arrow shows that both processes (evaporation and condensation) occur simultaneously.
When both the forward and reverse reactions are occurring at the same rate, we say that the reaction has reached chemical equilibrium. After this point, it may look like the reaction has stopped because the concentrations of the reactants and products do not change.
However, equilibrium in chemistry is a dynamic process, which means change is still occurring. It appears as though nothing is happening because the reactants and products are both being made and unmade at equal rates, which is why their concentrations remain constant. (It is important to note that this does not mean that the concentrations of reactants equal the concentrations of products, however.)
Question
What two characteristics define a system at equilibrium?
The forward and reverse reactions take place at equal rates.
The concentrations of reactants and products remain constant.
