When dissolved in water, cobalt chloride produces a reddish-pink solution. The introduction of additional chloride ions causes a shift from a fully hydrated cobalt cation ([Co(H2O)6]2+(aq)) to a new complex ([CoCl4]2–(aq)). This is accompanied by a change in color, from reddish-pink to blue. The green solution shown in the figure above is a third type of cobalt complex. The equilibrium between the two complexes is sensitive not only to the concentration of the chloride ion, but also to the temperature. Increasing the temperature causes a shift from the pink to the blue species, and lowering the temperature reverses this color change.
So far, we have generally assumed that the reactants in a chemical reaction are irreversibly transformed into products. However, many chemical reactions are reversible, meaning that it is also possible for the reaction to go "backwards". When both the forward and reverse reactions can proceed at a reasonable rate, a state of equilibrium is eventually reached, in which both reactions are occurring at the same rate and no net change is made to the amounts of reactants or products. In this chapter, we will look at the factors that govern chemical equilibria.