In chemistry, a pseudo-order reaction refers to a reaction that appears to follow a first-order rate equation, even though it is not truly a first-order reaction. This situation often arises when the concentration of one of the reactants is in excess compared to the other reactants, leading to the simplification of the rate equation.

Pseudo-order reactions are commonly encountered in cases where the concentration of a reactant is intentionally kept at a high, constant level. This can be achieved by using a large excess of one reactant or by maintaining a constant concentration through a continuous supply. By doing so, the concentration of the excess reactant remains essentially constant throughout the reaction, allowing the rate equation to be simplified.

For example, consider a reaction between a reactant A and a reactant B, where the concentration of B is in excess. The reaction may have an overall reaction order greater than one, but if the concentration of B remains constant, the rate equation can be simplified to a pseudo-first-order rate equation:

rate = k[A]

In this pseudo-first-order rate equation, the reaction appears to be first order with respect to reactant A, even though the true reaction order may be different. By monitoring the change in the concentration of reactant A over time, it is possible to determine the rate constant k for the pseudo-first-order reaction.

Pseudo-order reactions are particularly useful in cases where the reaction rate is too fast to accurately measure using conventional methods. By simplifying the rate equation, the reaction can be studied over a longer period of time, allowing for more accurate determination of the rate constant.