CHEMICAL EQUILIBRIUM
We’ve been talking about chemical reactions as if they occur in only one
direction—from reactants to products.
aA + bB → cC + dDHowever, many reactions are reversible; as products are formed, some of them
go through the reverse reaction and reform the reactants. Reversible reactions are
written as follows:
aA + bB cC + dDWhat do the two arrows mean? They mean that the reaction is taking place in
both directions. Reactants are forming products (the forward reaction), and
products are forming reactants (the reverse reaction). Let’s talk about how this
works. When a reaction first starts, the concentration of the reactants is high, and
the concentration of the products is very low. At this point, the rate of the
forward reaction is greater than the rate of the reverse reaction. After a while, as
more of the products are formed, less of the reactants are present.
Ultimately, if pressure and temperature are maintained and the system is closed
(meaning no species are allowed to escape), the rate of the reverse reaction will
be equal to the rate of the forward reaction. In other words, reactants are being
made as rapidly as are products. When this occurs, we say that the reaction is in
equilibrium, or dynamic equilibrium. When a reaction is in dynamic
equilibrium, the forward and reverse reaction rates are equal, which also means
that the concentrations of products and reactants are constant.
But here’s what equilibrium definitely does NOT mean: It does not mean that the
concentrations of products and reactants are equal. It just means that whatever
the concentrations of products and reactants may be, they aren’t changing once
the reaction reaches equilibrium.
If we want to know about the relative concentrations of products and reactants of
a reaction at equilibrium, we must know two things: (1) the reaction’s