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THE EFFECT OF A CATALYST ON THE RATE OF A REACTION

* PURPOSE

The purpose of this demonstration is to show the effect of a catalyst on the rate of a reaction.

* DESCRIPTION

This demonstration is appropriate for a general or first-year college prep class. Students will observe that the addition of a catalyst increases the rate of production of oxygen from hydrogen peroxide.

* TIME REQUIRED

10 minutes.

* MATERIALS

Chemicals
6% hydrogen peroxide solution*
liquid dishwashing detergent
manganese dioxide*
Equipment
250-mL beaker*
chemical scoop or spoon
stirring rod or wooden splint
*See Modifications/Substitutions

* HAZARDS

Caution should be used in handling hydrogen peroxide solution and in recovering manganese dioxide from a battery; both are caustic. Wear gloves. Carry out the reaction on a tray to contain any overflow. Wear goggles throughout preparation and demonstration.

* MODIFICATIONS/SUBSTITUTIONS

  1. 6% hydrogen peroxide solution is available from a drugstore or beauty supply store as Clairoxide.
  2. Manganese dioxide may be obtained from a battery. Use the solid substance between the electrodes.
  3. Clear plastic cups may be substituted for the beakers.

* PROCEDURE

  1. Add 100 mL of the hydrogen peroxide solution to each beaker.
  2. Add a large squirt of liquid detergent to each beaker. Mix and observe; note the absence, or near absence, of bubbles being formed.
  3. Add about 1/8 to 1/4 tsp. of manganese dioxide to one beaker. Compare to the behavior of the mixture without the manganese dioxide.

* DISPOSAL

Flush down the drain using lots of water.

* DISCUSSION

A catalyst is a substance that is added to a reaction to increase the reaction rate. The catalyst allows the reaction to proceed by an alternate reaction mechanism with a lower activation energy. A catalyst is not consumed in a chemical reaction.

The reaction for the decomposition of hydrogen peroxide is:

2 H2O2(aq) ---> 2 H2O(l) + O2(g)

This reaction normally proceeds very slowly. When the catalyst, manganese dioxide, is added, the rate of the reaction increases appreciably; this can be seen by the large volume of foam that is generated as the oxygen gas bubbles through the detergent solution. Little, if any, foam forms without the catalyst. The foam is gray in color because of entrapped, black manganese dioxide.

* REFERENCE

Summerlin, L.R. and Ealy, J.L., Jr., Chemical Demonstrations - A Sourcebook for Teachers, American Chemical Society, Washington, DC, 1985, p. 71. Describes a similar demonstration using KI as the catalyst.

Submitted by Patti Ruff, Bill Vitori, Irene Walsh, Doug Wilbur and Joe Don Wilkins

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