Goal is we want to get an algebraic expression for concentration of intermediates as a function of concentration of stable species. This simplifies the rate equation Let us consider a [[Global reaction]] $\ce{2A_2 + B_2->2A_2B}$ Let us say the detail reaction steps are $\ce{A + B_2 <=>[k_f_1][k_b_1] AB + B}\tag{1}$ $\ce{B + A_2 <=>[k_f_2][k_b_2] AB + A}\tag{2}$ $\ce{AB + A_2 <=>[k_f_3][k_b_3] A_2B + A}\tag{3}$ $\ce{A + AB +M ->[k_f_4] A_2B + M}\tag{4}$ If we assume the equation 1 is in equilibrium we can say $k_{f1} C_AC_{B_2} = k_{b1} C_{AB}C_{B}$ The equilibrium constant is $k_{p1} = \frac{C_{AB}C_{B}}{C_AC_{B_2}}$ Similarly we can write for equations 2, 3 and 4. We want to express the intermediates in terms of equilibrium constants and concentration of stable species. (eg. $C_{AB}$ in terms of concentration of $A_2$, $B_2$ and $A_2B$ and $k_ps)