Question

Inversion of cane sugar in dilute acid is:
(A)- bimolecular reaction
(B)- pseudo-unimolecular
(C)- unimolecular reaction
(D)- trimolecular reaction

Answer 1

Hint : Simple bimolecular reactions which are non first order reactions become reactions of first order by changing the reactions conditions. Such reactions are known as pseudo-unimolecular reactions.

Complete step by step solution :
The inversion of cane sugar in presence of an acid is shown the following equation:
$\begin{aligned} & {{C}_{12}}{{H}_{22}}{{O}_{11}}+{{H}_{2}}O\xrightarrow{{{H}^{+}}}{{C}_{6}}{{H}_{12}}{{O}_{6}}+{{C}_{6}}{{H}_{12}}{{O}_{6}} \\\ & \text{Sucrose Glucose Fructose} \\\ \end{aligned}$

The reaction appears to be bimolecular but it has been found to follow first order kinetics.
Here one of the reactants i.e. water, is acting as solvent. Water is present in large excess such that its concentration remains almost the same during the reaction. Therefore, its concentration will not affect the rate constant of the reaction.
Thus, the reaction becomes independent of the concentration of water. And, the rate of the reaction now depends only on the concentration of sucrose.

 $$\text{Reaction rate }\propto \text{ }\left[ {{C}_{12}}{{H}_{22}}{{O}_{11}} \right]\text{ only}\text{.}$$   

The reaction was not truly of first order but using water as solvent in excess made the reaction a first order reaction. Thus, it is an example of pseudo-first order reaction.
For a single step reaction (or elementary reactions), molecularity and order are the same. Thus, we can say that the acid catalyzed inversion of cane sugar is a pseudo-unimolecular reaction.

So, the correct answer is “Option B”.

Note : Note that though the inversion of sugar in dilute acid becomes a first order reaction in excess of water, it is not a unimolecular reaction truly. Water molecules do not affect the reaction rate but are still present in the reaction and colliding simultaneously