Berthollet’s reasoning that reactions are reversible was an important step in
understanding chemical reactivity. When we mix together solutions of Na2CO3
and CaCl2, they react to produce NaCl and CaCO3. If we monitor the mass of
dissolved Ca2+ remaining and the mass of CaCO3 produced as a function of
time, the result will look something like the graph in Figure 6.2. At the start of
the reaction the mass of dissolved Ca2+ decreases and the mass of CaCO3 increases.
Eventually, however, the reaction reaches a point after which no further
changes occur in the amounts of these species. Such a condition is called a state
of equilibrium.
Although a system at equilibrium appears static on a macroscopic level, it is
important to remember that the forward and reverse reactions still occur. A reaction
at equilibrium exists in a “steady state,” in which the rate at which any species
forms equals the rate at which it is consumed.