Question

Karthik and Arun are studying chemical kinetics together. They came around this equation: $N_2+3H_2\to 2N{H}_3$.
Karthik concluded that the molecularity of the reaction is 4 as one molecule of $N_2$ is reacting with three molecules of $H_2$.
But Arun said that molecularity of 4 is very unlikely as the probability of 4 molecules colliding simultaneously is almost zero.
As they are both contradicting each another can you figure out who is correct and who is wrong.

• A. Karthik is correct, Arun is wrong
• B. Both are wrong
• C. Karthik is wrong Arun is correct
• D. Both are correct

Answer 1

The correct option is B Both are wrong

Arun’s conclusion is correct. Let’s see why.
For reactant molecules to change into product molecules, they must collide with each other. If molecularity is 1, it means the reactant molecules is decomposing.
Molecularities of 2, 3 are common but the probability of more than 3 molecules colliding with each other simultaneously is almost zero.

But how do these 4 molecules $(1N_2+3H_2)$ collide with each other to form ammonia?

Molecularity of a reaction is defined as the number of species colliding with each other simultaneously in an elementary reaction.
Here most people neglect 'elementary reaction' in the definition.

The reaction given, i.e.,
$N_2+3H_2\to 2N{H}_3$
does not take place in one step but in a series of steps.

Reaction that occurs in a single step are called elementary reactions, whereas reactions which occur in more than one step are called complex reactions.

So here, the reaction is a complex reaction so Karthik applied molecularity concept to a complex reaction. That’s where he went wrong.