I would like to understand under what circumstances rate laws can leave out rate terms from certain reactions, in general, as a process of approximation. I am familiar with the basic method regarding the steady-state approximation already. Any books, websites or articles which are good or comprehensive on this would be appreciated as suggestions and/or links. An example of where I am stuck: in the following problem (see the question), why is it that when writing the differential rate laws d[I1]/dt and d[I2]/dt, the equilibrium between I1 and I2 is omitted? e.g. my original equation was d[I1]/dt = k1[A][M] -k-1[I1] -k2[I1] +k-3[I2] -k3[I1] ≈ 0 but the solutions gives d[I1]/dt = k1[A][M] -k-1[I1] -k2[I1] ≈ 0 omitting the terms concerning the equilibrium between the intermediates I1 and I2. How is this further approximation - that +k-3[I2] -k3[I1] is negligible - justified for the question given?