Steady-state Approximation and Rate-determining Step: Example Problems

Try to solve these problems before watching the solutions in the screencasts.

Example Problem 1

Compare the rate expression obtained by applying the steady-state approximation to a sequence of elementary steps to the measured rate expression.

Overall reaction: \(2A \, + \, B\, \rightleftarrows \, A_2B\)

Experimental rate expression: \(r_{_{A_2B}}\, = \,[0.72(A)^2(B)]/[1\,+\,2(A)]\)

Elementary steps:

\(A \,+\, B \, \overset{k_1}{\underset{k_{-1}}\rightleftarrows} \, AB^*\)

\(AB^* \,+\, A \, \overset{k_2}{\underset{k_{-2}}\rightleftarrows} \, A_2B\)

Example Problem 2

Derive the rate expression for the Eley-Rideal surface reaction represented by the following mechanisms:

\(A(g) \,+\, S\, \rightleftarrows \, A \cdot S\)

\(A \cdot S \,+\, B(g) \, \rightleftarrows \, AB \cdot S\)

\(AB \cdot S \, \rightarrow \, AB(g)\)

where \(S\) represents an adsorption site on the catalyst surface