Here is an illustration of one of the most important concepts in CRE, the rate-determining step (RDS), using what will become a very familiar example from reaction kinetics:
A --> R --> S
In these two reactions in series, with 1st order rate constants k1 and k2, the RDS is the step that "consumes" the largest portion of the total concentration gradient (or the largest fraction of the driving force for reaction). It is also the step that has the smallest rate constant. (At steady state, all steps in the sequence have the same rate, so there is a compensation between the rate constant term and the driving force term.) Here the gradient is that of the reactant A (thick curve below), cA - 0 (at t=0, cA=cA0). Note how small it is at any given time (i.e., how quickly cA decays with time) when k1>>k2, and how large it is (i.e., how slowly cA decays with time) when k1<<k2.

k1 = 30;
k2 = 1;
cAoverCA0 = Exp[-k1 t];
cRoverCA0 = (k1/(k2 - k1))*(Exp[-k1 t] - 
              Exp[-k2 t]);
cSoverCA0 = 1 - cAoverCA0 - cRoverCA0;
Plot[{cAoverCA0, cRoverCA0, cSoverCA0}, {t, 0, 0.3}, 
        PlotStyle -> {Thickness[0.0150], Thickness[0.009], 
            Thickness[0.005]}, PlotRange -> {0, 1}];

Clear[k1, k2];
k1 = 1;
k2 = 30;
cAoverCA0 = Exp[-k1 t];
cRoverCA0 = (k1/(k2 - k1))*(Exp[-k1 t] - 
              Exp[-k2 t]);
cSoverCA0 = 1 - cAoverCA0 - cRoverCA0;
Plot[{cAoverCA0, cRoverCA0, cSoverCA0}, {t, 0, 0.3}, 
        PlotStyle -> {Thickness[0.0150], Thickness[0.009], 
            Thickness[0.005]}, PlotRange -> {0, 1}];