Inversion of cane sugar is a pseudo first order reaction
Choose the correct reason from the statements given below.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The concentration of water does not get altered during the reaction so the reaction behaves as a first order reaction
Definition: A reaction that is truly higher-order but behaves as a first-order reaction due to one reactant being in large excess is called a pseudo first order reaction.
- In the inversion of cane sugar, sucrose is hydrolysed by water in the presence of acid:
$C_{12}H_{22}O_{11} + H_2O \overset{H^+}{\longrightarrow} \text{Glucose} + \text{Fructose}$
- Although the reaction is actually bimolecular (depends on sucrose and water), water is present in very large excess.
- Hence, the concentration of water remains practically constant during the reaction.
- As a result, the rate depends only on the concentration of sucrose, and the reaction appears to follow first-order kinetics → pseudo first order reaction.
Other options are incorrect because:
- Water is involved in the reaction. Water is a reactant in the hydrolysis of sucrose.
- Order with respect to water is not zero.The reaction is actually first order with respect to water, but it appears zero order because its concentration does not change appreciably.
- The given rate law expression is incorrect. The rate law is proportional to the concentration of sucrose (and water), not inversely proportional to water.
Therefore, the correct reason is: The concentration of water does not get altered during the reaction so the reaction behaves as a first order reaction.