Question

Write two differences between order of a reaction and molecularity.

Answer 1

The order of a reaction is an experimental quantity. It can only be derived from performing an experiment. The molecularity of a reaction is a theoretical quantity. It can only be derived by looking at the reaction mechanism.

Complete step by step answer:
The order is derived from the rate law equation. The rate law equation if we can recall can be represented in the following way:
$aA + bB \to products$
$rate = K{\left[ A \right]^m}{\left[ B \right]^n}$
Where $\left[ A \right]$ and $\left[ B \right]$ are the concentrations.

ORDER	MOLECULARITY
The terms $m$ and $n$ in the rate law equation are known as the order with respect to $A$ and $B$ respectively. The sum of both these variables $m$ and $n$ gives the order of the reaction.	It is derived from the slowest step in the entire reaction process. The slowest step is also known as the rate determining step. This means that the molecularity can be found by just adding the number of molecules that are taking part in the slowest step of the reaction.
It is important to remember that these variables can take any value for example, it can be a whole number, an integer or a fraction and even zero	This is a theoretical quantity and can only take values that are whole numbers or integers. It can never be zero or a fraction.
The overall reaction is required in order to obtain the order of the reaction.	Molecularity on the other hand, is basically the total reacting number of species of a reaction.

Note: For an order of the reaction, it may or may not be equal to the stoichiometric coefficients mentioned in the reaction. Molecularity however, is always equal to the stoichiometric coefficient of the slowest step.
The order of a bimolecular reaction can also be one as in the case of a pseudo unimolecular reaction where one of the reactants is taken in excess. This means that the order of the reactant in excess is taken to be zero and the order of the entire reaction is one.