What Are the Units for a Zeroth Order Reaction?

Rate of a Reaction

• The rate of a reaction is usually dependent upon the concentration of the reactants, surface area of the reactants, catalyst activity and temperature and pressure of the reaction environment. According to the collision theory, increase in the concentration of the reactants normally increases the rate of the reaction. Temperature is also considered to be directly proportional to the rate of the reaction. Surface area of the reactants, when increased, increases the reaction rate. A catalyst can also increase the rate of the reaction up to many times.

Zeroth Order Reaction

• In a zeroth order reaction, the rate of the reaction is independent of the concentration of the reactants. Thus, any increase or decrease in the concentration of the reactants does not change the rate of the reaction. The rate is expressed as a constant, generally k: rate = k. Zeroth order reactions are only possible for a short time frame due to decreasing concentration of the reactants and decrease in the catalyst's ability to increase the rate of the reaction.

Unit of Zeroth Order Reaction

• A zeroth order reaction has the unit of rate coefficient, k. The unit is concentration per unit time. The units of concentration and time can be differently chosen. Normally, the unit is molarity/second, expressed as M/Ls. This unit chosen for concentration can be in mass, like grams, in molar like molarity or molality, in numbers or in volume such as liters or cubic centimeter. Similarly, the units for time can be seconds, minutes or hours.

Examples of Zeroth Order Reaction

• Some examples of zeroth order reaction are reversed Haber's process, photochemical reaction between hydrogen and chlorine to form hydrogen chloride and the decomposition of nitrous oxide in the presence of a platinum catalyst. The disintegration of hydrogen iodide into hydrogen and iodine is also a zeroth order reaction. Its rate does not depend on the concentration of hydrogen iodide but is constant. In a zeroth order reaction, the concentration of the reactants at any given time does not change although the quantities of the reactants decrease while those of the products increase.