reversible reaction

A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.
\[\ce{aA + bB <=> cC + dD}\], where both A, B can react to form C, D, and C, D can react to form A, B.

An example would be \[\ce{H2 + I2 <=> 2 HI}\]. The forward reaction would be \[\ce{H2 + I2 -> 2 HI}\], while the backwards reaction would be \[\ce{2 HI -> H2 + I2}\].

When hydrogen and iodine gases are mixed in a sealed container, they begin to react and form hydrogen iodide. At first, only the forward reaction occurs because no \[\ce{HI}\] is present. As the forward reaction proceeds, it begins to slow down as the concentrations of the \[\ce{H2}\] and the \[\ce{I2}\] decrease. As soon as some \[\ce{HI}\] has formed, it begins to decompose back into \[\ce{H2}\] and \[\ce{I2}\]. The rate of the reverse reaction starts out slow because the concentration of \[\ce{HI}\] is low. Gradually, the rate of the forward reaction decreases while the rate of the reverse reaction increases. Eventually the rate of combination of \[\ce{H2}\] and \[\ce{I2}\] to produce \[\ce{HI}\] becomes equal to the rate of decomposition of \[\ce{HI}\] into \[\ce{H2}\] and \[\ce{I2}\]. When the rates of the forward and reverse reactions have become equal to one another, the reaction has achieved a state of chemical equilibrium.