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Key Idea 2
Chemical Equilibrium, Reaction Quotients and Equilibrium Constants
Concept Question: What is a chemical equilibrium and how do chemists quantitatively describe equilibria?

What do we know?

You may have learned about equilibria in previous chemistry classes. This concept is vital to building an understanding of acid-base chemistry.  Recall that in chemistry, equilibrium refers to the point in a reaction when reactants and products are being produced at the same rate.  When a state of equilibrium is reached the concentration of reactants and products will remain constant over time. This also means that the reaction is reversible; it occurs in both the forward and backward directions. At equilibrium, the forward and reverse reactions occur at the same rate.

Let us consider how the idea of chemical equilibrium applies to carbon dioxide dissolving in the ocean according to the following reaction:

CO_{2}(g) \rightleftharpoons CO_{2}(aq)

When this reaction is at equilibrium, carbon dioxide dissolves into the ocean at the same rate that aqueous carbon dioxide leaves ocean. Even when the reaction is not at equilibrium it is still occurring in both directions, but the rates of reaction are not equal. If the forward reaction occurs at a faster rate than the backward reaction, the net reaction is the formation of more products. For the above example, more carbon dioxide would dissolve into the ocean than leave. But if the backward reaction occurs at a faster rate, the net reaction is the formation of more reactants and, for the above reaction, more carbon dioxide would leave the ocean than is being dissolved.