Reversible Reactions and Equilibrium

In some reactions no products is formed to allow the reaction to go to completion, that is, the reactants and products can still interact in both directions. This can be shown as follows:

A+B <---> C+D

A real chemical example would be:

H2 + I2 <---> 2HI

In which the rate of the forward reaction (hydrogen and iodine combining to form hydrogen iodide) is equal to the reverse reaction (hydrogen iodide decomposes to form hydrogen and iodine)

The double arrow indicates that C and D can react to form A and B, while A and B react to form C and D.

The reaction is said to have reached equilibrium when the forward reaction rate is equal to the reverse reaction rate. Notice that this is a dynamic equilibrium, not a static one, although in appearance the reaction seems to have stopped. An example of an equilibrium is a crystal of copper sulfate in a saturated solution of copper sulfate. Although to the observer the crystal seems to remain unchanged, there is actually an equal exchange of crystal material with the copper sulfate in solution. As some solute comes out of solution, an equal amount is going into solution.