# Question c6aad

Aug 11, 2017

This reaction is unimolecular.

#### Explanation:

The idea here is that you're looking for the number of molecules that react in a single-step reaction.

In other words, the molecularity of the reaction tells you how many molecules are present on the reactants' side of the balanced chemical equation, i.e. how many molecules come together to, well, react.

Now, it's worth mentioning that the reaction given to you is not properly balanced because it implies that you have

overbrace("Cl"_ (2(g)))^(color(blue)("1 molecule of chlorine")) -> overbrace(1/2"Cl"_ ((g)))^(color(blue)("half an atom of chlorine"))#

This is not possible

You should actually have

${\text{Cl"_ (2(g)) stackrel(color(white)(acolor(blue)(hnu)aaa))(->) 2"Cl}}_{\left(g\right)}$

In this single-step reaction (which you'll often see referred to as an elementary reaction), $1$ molecule of chlorine gas decomposes to produce $2$ atoms of chlorine.

Since you only have $1$ molecule on the reactants' side, you can say that this reaction is unimolecular.

By comparison, a bimolecular reaction will have $2$ molecules on the reactants' side.

$\text{A"_ ((g)) + "B"_ ((g)) ->" products}$

$2 \text{A"_ ((g)) -> "products}$

Similarly, a termolecular reaction will have $3$ molecules on the reactants' side.

$\text{A"_ ((g)) + "B"_ ((g)) + "C"_ ((g)) -> "products}$

$2 \text{A"_ ((g)) + "B"_ ((g)) -> "products}$