```Rate Determining Step and CatalystsDick Trent                     Elk Grove High School                               500 W. Elk Grove Blvd.                               Elk Grove Village, IL 60007                               708-439-4800Objectives: (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 bloodTriple beam balance                      3% Hydrogen peroxideCalculator                               Manganese dioxideScrap paper                              Four test tubes and rackStrategy: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.  ```