We know about the rate law or rate expression for a chemical reaction. The rate law is an equation that gives the relation between the rate of a reaction and its dependence on the concentration of reactants. There are numerous factors that affect the rate of a chemical reaction; some of them play a very significant role, such as temperature, concentration, catalysts, etc. We know that the concentration of reactants play a major role in reaction kinetics for any particular chemical reaction so the scientists decided to find a quantitative relationship between the reaction rate and the concentration of reactants.
Let us consider the following reaction with the rate expression as:
m M + n N → p P + q Q
Rate = k [M]x [N]y
In the rate law, x and y are experimentally determined values and can be 0, 1, 2, 3 and even a fraction. Now if we look closely, we would notice that the behavior of the reaction rate with respect to the concentration of reactant M will depend on the value of x. Hence we call x as the order of reaction with respect to reactant M.
Similarly, the order of reaction with respect to reactant N is y. The overall order of reaction is the sum of powers of the concentrations of the reactants in the rate law equation. So in the above case, the overall order of reaction is (x+y). Depending on the overall order of reaction the reactions are classified as 0 order reaction, 1st order reaction, 2nd order reaction and so on. A zero-order reaction implies that the rate of reaction is independent of the concentration of the reactants.
Molecularity of a Reaction:
We often see a single chemical equation showing the transformation of reactants into products. It has been observed that rarely a reaction gets completed in a single step. The reactions that occur in one single step are called elementary reactions. Most of the reactions occur as a combination of elementary reaction, called as complex reactions. The combination may consist of a set of consecutive reactions, a combination of consecutive and parallel reaction, side reactions etc.
So to understand the mechanisms of the reactions in better ways another property named molecularity is introduced. Molecularity is the number of reacting species such as atoms, ions or molecules taking part in an elementary reaction that must make simultaneous collisions in order to bring about the chemical reaction. The reaction is said to be unimolecular when only one reacting species is involved. If two species are making simultaneous collisions in a chemical reaction, it is called bimolecular. If the number of reacting species is three then it is said to be a trimolecular reaction. It has been observed that the probability that more than three species can collide in a reaction simultaneously is very small.
Comparison of molecularity and order of reaction:
- Reaction order is an experimentally determined value, it can be 0, 1, 2 and even a fraction but molecularity cannot be zero or a fraction.
- Reaction order is applicable for all types of reaction (elementary or complex) but molecularity is applicable only to elementary reactions.
- Order of reaction for a complex reaction is given by the slowest step, also known as the rate determining step. Molecularity of the rate determining step is same as the overall order of reaction.
So now we have the concept of order of a reaction, the overall reaction order and the molecularity of a reaction. All these terms are helpful in understanding a chemical reaction. These properties play important role in industries that manufacture commodities using various chemical reactions.