Rate Determining Step and Catalysts

Dick Trent                     Elk Grove High School
                               500 W. Elk Grove Blvd.
                               Elk Grove Village, IL 60007
                               708-439-4800

Objectives:

 (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.

Materials:

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

Strategy:

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.