Unsteady convective diffusion problems involving chemical reaction in a rectangular are numerically examined. The effect of various factors affecting the removal of the contaminant in the cavity has been analyzed systematically. The vorticity and streamfunction are used for numerical computations. For low Reynolds number cases, the rate of removal of the contaminant increases as the Grashof number becomes larger. For high Reynolds number, the secondary flow hinders the rate of removal of the contaminant as the Grashof number becomes larger.