**[B] = [A] _{0} [k_{1} /(k_{2} – k_{1}) (e^{-k1t} – e^{-k2t})**

By consideration of each of the terms of the equation find approximate expressions for [B] and d[B]/dt when k_{1} is very much larger than k_{2} i.e. k_{1} >> k_{2}.

When k_{1} >> k_{2} then the following approximations may be made for negative exponential functions. The condition (k_{1} >> k_{2}) means k_{1} is much larger than k_{2} and thus exp(—k_{1} t) ≈ 0 and as k_{2} is small and positive then exp(—k_{2} t) ≈ exp(0) ≈ 1 and (k_{2} — k_{1}) ≈ k_{1}.

**[B] = [A] _{0} [k_{1} /(k_{2} – k_{1}) (e^{-k1t} – e^{-k2t}) ≈ [A]_{0} [A]_{0} [k_{1} / -k_{1}] (0-1) ≈ [A]_{0 }**

As [B] ≈ [A]_{0} over part of reaction time, see below Figure, the final product concentration [C] does not start to increase appreciably until the intermediate concentration [B] has passed its maximum by which time the reactant concentration [A] in negligible in chemical reactions. Using the same approximations as above with the equation for the rate of forming the intermediate d[B]/dt we have

Over much of the reaction the rate of change of the intermediate concentration d[B]/dt to form the product C will approximately depend on the rate of the slow “Rate Determining Step” k2.