Barrons SAT Subject Test Chemistry, 13th Edition

forming the original reactants just as rapidly as they are being used up by the
forward reaction. Therefore, no further change in R, R′, or any of the
concentrations will occur.
If we set R′ equal to R, we have:

k 2 × [C]c × [D]d = k 1 × [A]a × [B]b

or

This process of two substances, A and B, reacting to form products C and D
and the reverse can be shown graphically to represent what happens as
equilibrium is established. The hypothetical equilibrium reaction is described by
the following general equation:

aA + bB cC + dD

At the beginning (time t 0 ), the concentrations of C and D are zero and those

of A and B are maximum. The graph below shows that over time the rate of the
forward reaction decreases as A and B are used up. Meanwhile, the rate of the
reverse reaction increases as C and D are formed. When these two reaction rates
become equal (at time t 1 ), equilibrium is established. The individual

concentrations of A, B, C, and D no longer change if conditions remain the same.
To an observer, it appears that all reaction has stopped; in fact, however, both
reactions are occurring at the same rate.