In an equilibrium reaction, both the forward and reverse reactions are occurring.
Equilibrium is still shown with a double arrow:
A double arrow is used to show that reactants can react to form products (the forward reaction) but products react to go back to reactants (reverse reaction). Note the substances on the left are still called “reactants” and the substances on the right are still called “products”
When the reaction is at equilibrium, the forward and reverse reactions are proceeding at the same rate. Therefore concentrations of the reactants and products no longer change with time (remain constant). However, both reactions are still occurring.
Consider the equilibrium:
Figure 13.2 in OpenStax (a three part Figure consisting of parts 13a, 13b, and 13c) and the surrounding text provide an excellent description of how this reaction reaches equilibrium and what happens when it is at equilibrium. Take a look at it and the surrounding reading.
A common misconception is that there is an equal amount of reactant and product at equilibrium. The reactant concentrations do not have to equal the product concentrations at equilibrium.
If mostly products are present at equilibrium, we say the equilibrium “lies to the right”. If mostly reactants are present at equilibrium,, we say the equilibrium “lies to the left”.
You have seen equilibrium earlier in the semester: weak electrolytes (substances that only partially dissociate and ionize in water) are one example of equilibrium. For example, the slightly soluble salt silver chloride forms an equilibrium in water:
Weak acids and weak bases also are weak electrolytes and ionize in equilibrium reactions in water. Acetic acid is an example:
The principles and calculations learned in our study of equilibrium will be necessary for the study of acids and bases next week. In weak electrolytes, only a small fraction of the compound ionizes, so the equilibrium lies to the left.
