Chemical Equilibrium

A chemical reaction between substance A and substance B. The reaction can be written as:

If the reaction is not one-way, as indicated by the arrow, and if C and D can also react with one another to form A and B—the reverse of the process above, this process is indicated with the arrow going the other way:

This is far from an unusual situation. In fact, almost all of the chemical reactions are reversible in they proceed in either direction, as long as A, B, C and D molecules are available. After all, a chemical reaction is a process in which atoms or electrons are rearranged, reshuffled, or transferred from one molecule to another. Chemists call a two-way reaction a reversible reaction, and they write the equation with a two-way arrow:

In adverse reaction A and B molecules collide to produce C and D molecules, just as we wrote in the first equation above. As they react, the number of remaining As and Bs will be going down. But as more and more Cs and Ds are produced, they will begin to collide and reproduce some of the As and Bs. The reaction is now going in both directions-to the right and to the left.

As more and more Cs and Ds build up, they will be bumping into each other more and more often, thereby reproducing As and Bs at a faster and faster clip. Pretty soon, the C+D (leftward) reaction will be going as fast as the A+B (rightward) reaction; As and Bs will be reappearing just as fast as they are disappearing. The result is that there will be no further change in the numbers of As and Bs. The system is at equilibrium.

A and B molecules continue to collide and produce C and D molecules. Likewise, we see that Cs and Ds are still madly bumping into each other to produce As and Bs. In other words, both reactions are still vigorous. But they're going on at exactly the same rate. If somebody is giving you money and taking it away at the same rate, the amount you have isn't changing. The total amounts of A, B, C and D aren't changing at all. It looks as if the reaction is over, and for all our practical purposes it is.

When the rate of leftward reaction is the same as the rate of rightward reaction, the system stays at equilibri um—no net movement in either direction.