Pseudo Order Reaction:

A pseudo order reaction, also known as a pseudo first-order reaction, is a type of reaction that appears to follow first-order kinetics even though it may actually be a higher-order reaction. This behavior can occur when the concentration of one reactant is much larger than the concentration of the other reactants, leading to the approximation that the reaction rate depends only on the concentration of the limiting reactant.

Explanation:

In a typical chemical reaction, the rate of the reaction is determined by the concentrations of the reactants. For example, a second-order reaction rate is proportional to the product of the concentrations of two reactants, while a first-order reaction rate is proportional to the concentration of a single reactant.

However, in certain cases, the concentration of one reactant is significantly larger than the concentration of the other reactants. This can happen, for instance, when one reactant is present in excess or when it is continuously supplied to the reaction mixture. In such situations, the reaction rate becomes dependent only on the concentration of the reactant that is present in limited quantities.

Example:

Let's consider the reaction between a reactant A and a reactant B, where A is present in excess and B is the limiting reactant. Initially, the reaction may appear to be second-order as the rate is proportional to both the concentrations of A and B. However, as the concentration of A remains relatively constant, the rate equation can be simplified to a pseudo first-order rate equation:

rate = k[B]

In this pseudo order reaction, the rate of the reaction is directly proportional to the concentration of B, making it appear as a first-order reaction. The reaction rate constant k is determined experimentally and can be used to calculate the rate of the reaction under different conditions.

Applications:

Pseudo order reactions are commonly encountered in various fields of chemistry, particularly in chemical kinetics studies. They are often used to simplify reaction mechanisms and determine rate constants. By manipulating the concentration of one reactant to be in excess, researchers can effectively study the kinetics of a reaction as if it were a first-order reaction.

These reactions are also useful in determining the order of reactions when the true order is difficult to determine experimentally. Pseudo order reactions allow for the simplification of complex reaction mechanisms and provide a more straightforward approach to studying reaction rates.

Conclusion:

A pseudo order reaction is a reaction that appears to follow first-order kinetics due to the excess concentration of one reactant. By simplifying the rate equation to depend only on the concentration of the limiting reactant, the reaction can be treated as a pseudo first-order reaction. This simplification allows for easier determination of reaction rates and rate constants, making it a valuable tool in chemical kinetics studies.