Principles of Steady State Approximation

The steady state approximation is a method used in chemical kinetics to simplify the rate equations for complex reactions.
It’s based on the assumption that the concentration of an intermediate species remains constant throughout the reaction. This is called a steady state condition.
The steady state is achieved when the rate of production of the intermediate equals the rate of its consumption.

Steady State in Reaction Mechanisms

For a reaction mechanism with an intermediate substance, the steady state is not the same as equilibrium. Equilibrium involves reactant and product concentrations, while the steady state focuses on the intermediate.
The principle of the steady state approximation assumes that over the course of the reaction, the intermediate concentration does not change noticeably, or it reaches a steady state quickly and remains constant.
A steady state condition does not imply that the quantities of the reactants, products, or intermediates are equal.

The steady state approximation is advantageous when dealing with multiple step reactions. It helps in simplifying the rate equations and making calculations more manageable.
This approximation also allows us to predict the rate law for a reaction mechanism without needing to know the rate constants for each individual step.
However, it’s important to remember that being an approximation, the steady state assumption may not always hold true, especially when dealing with reactions with low concentrations of intermediates or at very early stages of the reaction.

Consider a simple reaction A → B → C. Here, B is the intermediate. According to the steady state assumption, the rate of production of B is equal to the rate of its consumption.
This principle can be represented by the equation: rate of B produced = rate of B consumed.
Using the steady-state approximation, we can derive the rate equation for this reaction, thereby determining the overall kinetics and reaction mechanism.