In steady-state conditions, you assume that the rate of change of an intermediate species = 0
Here's an example to illustrate this:
C is an intermediate species, so we can apply steady-state conditions.
The rate equation for formation of C (change in concentration of C over time) is:
Under steady-state conditions, the change in concentration over time of intermediate species is equal to zero, so we can say that this rate equation is equal to zero .
This allows rearrangement of the equation to give rate expressions for different species in the reaction.
This is used when we want to determine a rate equation for the whole reaction that doesn't include any intermediates in the expression.
For example, if we wanted to derive an expression for the formation of product
First, you write the equation for the formation of product
we can now substitute in the expression for