Rate Determining Step and Catalysts

Dick Trent Elk Grove High School
500 W. Elk Grove Blvd.
Elk Grove Village, IL 60007


(Intended for high school level)
This activity will allow the student to understand the rate determining step,
the dependence of total rate on the rate determining step and the effect of a
catalyst on rate.


(for each lab group)
Table Salt Liver blood
Triple beam balance 3% Hydrogen peroxide
Calculator Manganese dioxide
Scrap paper Four test tubes and rack


This lesson is in two parts and can be used or adapted for any level student
although it was designed for high school students. The lesson attempts to
explain rate determining step, which is the slowest step of a multi-step
sequence of steps. The rate determining step determines the rate of the entire
sequence and is important to understanding chemical reactions as well as aspects
of industry, production, and time management.

Begin by telling the students to get in groups of four or five. Each member of
the group will be assigned a specific task in a sequence of steps. At the end
of the exercise the students will figure the rate of each individual step and
the rate of the total sequence and the rate determining step will be obvious.
Any sequence of steps which depend on each other and where one (in the middle of
the sequence) is considerably slower than the others can be used. Possible
steps are as follows:

(1) Tear a piece of paper and make it into a cup (fold it in quarters)
(2) Pour a sample of salt (NaCl) in the cup (about 1 gram)
(3) Precisely find the mass of salt on your balance and report the mass to
the next person
(4a) Calculate the number of moles in the sample
(4b) Calculate the molarity of the sample if it were dissolved in water to
a volume of 10 mL.

4a and 4b can be done by one person if there are four in the group or by two
people if there are five in the group or 4b can also be skipped if desired. The
data for this is the number of times each step is completed so each person must
keep track of the number of times their step is completed. If the sequence is
done correctly, the third step will be the slowest (rate determining step) and
each step after it should have the same rate as they are much faster. Possible
follow-up questions can focus on the rate of each step, the identification of
the fastest and slowest steps, the relationship between the rate determining
step and the following steps, and the rate of the total sequence.

This exercise will show the students the different aspects of rate which can be
related to rate of chemical reactions in a later lesson. For younger children,
this same lesson can be used and related to non-chemical experiences (i.e.
cooking, washing dishes, assembly lines, etc...). Discussion of this activity
should include the question "How could the rate determining step be made
faster?" The concept of a catalyst should be included in the answer and this
will lead to the next part of the activity.

The second part of this exercise involves a catalyst and the breakdown of
hydrogen peroxide. Hydrogen peroxide breaks down into water and oxygen gas.

2H2O2 ----> 2H2O + O2 In this one step reaction, the rate determining step is the only step in this reaction. The observance of bubbles in the test tube indicates the production of O2 and that the reaction is taking place. Without a catalyst, however, no
oxygen is produced because the reaction is too slow. Fresh liver blood (which
can be obtained by leaving fresh liver in a plastic bag in the refrigerator) and
manganese dioxide act as catalysts for this reaction.

A simple lab can demonstrate the usefulness of a catalyst:

Take four test tubes and put about 2 cm of hydrogen peroxide in each. In the
first, add a couple of drops of liver blood. In the second a little manganese
dioxide, some table salt in the third and nothing in the fourth. Have the
students make observations, looking for bubbles (the production of oxygen). The
fourth tube serves as a control. Tables can be made and discussion can focus on
a catalyst.

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