Group Five
The Rate Determining Step
The different steps in a reaction mechanism take place at different rates. One step in a
mechanism may occur almost instantaneously, while another step occurs slowly. The rate
of the overall reaction is determined then by the rate of the slowest step. The slowest step
in a reaction mechanism is called the rate-determining step.
We can use the analogy of washing dishes to help explain how the rate-determining step
governs the whole reaction. Assume that doing dishes is a two-step process:
Step 1:
Step 2:
(washing) dirty dishes  clean wet dishes
(drying) clean wet dishes  clean dry dishes
The dirty dishes represent the reactant; the clean wet dishes represent the reaction
intermediate; and the clean dry dishes are the product. If the person washing the dishes is
very slow, then the drier will have no trouble keeping up and in fact will probably be
waiting for clean wet dishes. In this case the reaction intermediate will be short-lived and
the overall speed of doing dishes will depend on just how slowly the washer perform the
first task (Figure 14 a). But if the person washing the dishes is speedy and the drier is
slow, the clean wet dishes accumulate and the drying step is the rate-determining one
(Figure 14 b).
Figure 14
(a)
(b)
Consider how our washing dishes analogy relates to a reaction with a two-step
mechanism – the reaction of nitrogen dioxide with carbon monoxide:
Step 1:
Step 2:
NO2 + NO2  NO3 + NO
NO3 + CO  NO2 + CO2
(slow)
(fast)
In the first step, two NO2 molecules react to produce an intermediate molecule NO3. In
the second step this intermediate molecule reacts with CO very quickly to produce the
products NO2 and CO2. Increasing the concentration of CO will make Step 2 take place
faster, but will have little effect on the speed of the overall reaction because Step 1 is the
rate-determining step! However, increasing the concentration of NO2 will speed up the
overall reaction because it is a reactant in Step 1, the rate-determining step. Knowing the
mechanism of a reaction provides a basis for predicting the effect of the concentration of
a reactant on the overall rate of a reaction. If the reaction mechanism is unknown, then
experiments must be done to determine the effect.
Demo:
Materials: retort stand, 3 funnels with different bore diameters, rings to hold funnels,
beaker, graduated cylinder, coloured water to make demonstration more visible, stopwatch
Demonstration: Measure the time required for x mL of water to pass through each of the
three funnels. These values can then be converted to rates (ml/s). Next, arrange the 3
funnels in consecutively (in any order – to really convince students let them try all
possible orders to see that the result remains the same) so that water flowing out of one
will flow into the next. Then ask students to predict (and explain their predictions) what
they think the rate will be for water to flow through all three funnels. Measure the time it
takes for the water to flow through all of the funnels (start timing when water begins
flowing out of last funnel and into beaker) and convert this to a rate. Compare the rate of
water flow through each individual funnel to the rate of water flow through the series of
funnels.
Sample Questions
1. Which rate in the steps of a reaction mechanism determines the rate of a reaction?
2. An important function for managers is to determine the rate-determining steps in their
business processes. In a certain fast-food restaurant is takes 3 minutes to cook the food,
1.5 minutes to wrap the food, and 5 minutes to take the order and make change.
How would a good manager assign the work to four employees?
3. The reaction 2A + B + C  D takes place through the following mechanism:
Step 1:
Step 2:
Step 3:
A + B  AB
AB + A  A2B
A2B + C  D
(fast)
(slow)
(fast)
Predict and explain what affect on the rate of the overall reaction an increase in the
concentration of each of the following would have:
(a) substance A
(b) substance B
(c) substance C
4. Consider the following reaction mechanism:
Step 1:
Step 2:
Step 3:
Step 4:
A2  2A
2A + 2B  2AB
2AB + C2  2ABC
2ABC  2AC + 2B
(fast)
(slow)
(fast)
(fast)
(a) Write the net equation for the overall reaction.
(b) Would the speed of the overall reaction increase if additional amounts of A, B, or
C were added to the container? Explain why or why not.
5. The following potential energy diagram describes the reaction between hydrogen
bromide and oxygen gas:
4 HBr (g) + O2 (g)  2 H2O (g) + 2 Br2 (g)
(a) What is the H for the entire
reaction?
(b) Which step is the ratedetermining step for this
three step reaction?
(c) What is the Ea for the ratedetermining step?
(d) What is the Ea for the reverse direction of the rate-determining step?
(e) What is the H for the entire reaction in the reverse direction?