Question
Define and explain the term molecularity of a reaction. Give examples.
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Define the molecularity of a chemical reaction.
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Define the molecularity of a chemical reaction.
✓
Answer
Molecularity : The molecularity of an elementary reaction is defined as the number of molecules (or atoms or ions) which take part in a chemical reaction.
Explanation :
The rate law expression for this reaction is, Rate $= k \left[ N _2 O _5\right]$. Hence it is unimolecular and first order.
Other unimolecular reactions are,
$\begin{aligned}
& O _{3( g )} \rightarrow O _{2( g )}+ O _{( g )} \\
& C _2 H _5 I _{( g )} \rightarrow C _2 H _{2( g )}+ HI _{( g )}
\end{aligned}$
(B) Bimolecular reaction In this two molecules take part in the reaction,
$\begin{aligned}
& \text { e.g., } 2 HI _{( g )} \rightarrow H _{2( g )}+ I _{2( g )} \\
& O _{3( g )}+ O _{( g )} \rightarrow 2 O _{2( g )} \\
& 2 NO _{2( g )} \rightarrow 2 NO _{( g )}+ O _{2( g )}
\end{aligned}$
(c) Trimolecular reaction: In this three molecules take part in the reaction.
$\text { e.g., } 2 NO _{( g )}+ O _{2( g )} \rightarrow 2 NO _{2( g )}$
The higher molecularity is rare since the prob ability of simultaneous collisions between more molecules is very low.
- The molecularity of a reaction is always integral.
- It cannot be determined experimentally.
- The minimum value of the molecularity is one.
- It cannot have fractional or zero values.
- The reactions are classified according to the molecularity as follows :
The rate law expression for this reaction is, Rate $= k \left[ N _2 O _5\right]$. Hence it is unimolecular and first order.
Other unimolecular reactions are,
$\begin{aligned}
& O _{3( g )} \rightarrow O _{2( g )}+ O _{( g )} \\
& C _2 H _5 I _{( g )} \rightarrow C _2 H _{2( g )}+ HI _{( g )}
\end{aligned}$
(B) Bimolecular reaction In this two molecules take part in the reaction,
$\begin{aligned}
& \text { e.g., } 2 HI _{( g )} \rightarrow H _{2( g )}+ I _{2( g )} \\
& O _{3( g )}+ O _{( g )} \rightarrow 2 O _{2( g )} \\
& 2 NO _{2( g )} \rightarrow 2 NO _{( g )}+ O _{2( g )}
\end{aligned}$
(c) Trimolecular reaction: In this three molecules take part in the reaction.
$\text { e.g., } 2 NO _{( g )}+ O _{2( g )} \rightarrow 2 NO _{2( g )}$
The higher molecularity is rare since the prob ability of simultaneous collisions between more molecules is very low.
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