Question

For a reaction ${H_2} + C{l_2} \to 2HCl$ , Rate = k.
1. Write the order and the molecularity of this reaction.
2. Write the unit of k.

Answer 1

Order of a reaction is the sum of the powers to which the concentration terms are raised in rate law expression. And molecularity is the no. of molecules involved in the step leading to reaction.

Complete step by step solution:
Let us look at the definitions of the order and the molecularity of the reaction,
Order of a reaction: Order of a reaction is the sum of the powers to which the concentration (pressure) terms are raised in the rate- law expression.
Molecularity of a reaction: It is defined as the number of molecules involved in the step leading to the chemical reaction.
Now the given reaction is:
${H_2} + C{l_2} \to 2HCl$ , and Rate = k.
Since the rate of the reaction is independent of the concentration of reactants, so it is a zero order reaction.
Now molecules involved in the reaction are one molecule of each ${H_2}$ and $C{l_2}$ . So the total no. of molecules involved in the reaction is 2,

Therefore molecularity of given reaction = 2

And it is given to us that the rate of reaction is k.
So the unit of k will be the same as the rate of the reaction.

**So the unit of k will be $mol{L^{ - 1}}$ ${\sec ^{ - 1}}$.

Note: **
The order of reaction is an empirical quantity determined by experiment from the rate law of the reaction. It is the sum of the exponents in the rate law equation. Molecularity, on the other hand, is deduced from the mechanism of an elementary reaction, and is used only in context of an elementary reaction.