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EFFECT OF SURFACE AREA ON THE RATE OF A HETEROGENEOUS REACTION

* PURPOSE

The objective of this demonstration is to illustrate the effect of increasing surface area upon the rate of reaction in a heterogeneous system.

* DESCRIPTION

This demonstration is appropriate for a general or first-year college-prep course. It provides a dramatic comparison of the rates of burning of similar amounts of flour when the flour is in a small pile and when it is dispersed in air.

* TIME REQUIRED

10 minutes

* MATERIALS

Chemicals:
flour
Equipment:
Bunsen burner
sheet of aluminum foil (approx. 15 *cm x 15 cm)
ringstand, ring, and clay triangle
funnel
rubber tubing
*See Modifications / Substitutions

* HAZARDS

Use caution in Part B where a large flame is produced. Goggles must be worn while doing the demonstration.

* MODIFICATIONS/SUBSTITUTIONS

  1. A candle may be substituted for the Bunsen burner, but the effect in Part B may not be as dramatic.
  2. A paper half-gallon milk carton may be substituted for the ring stand set-up. Remove the top of the carton and support the funnel in a hole cut in the top of the inverted carton. Provide a notch in the side of the carton through which the tubing can pass.

* PROCEDURE

Part A:
  1. Pour about 1/2 teaspoon of dry flour onto the aluminum foil and light it with the Bunsen burner.
  2. Note the rate of burning.
Part B:
  1. Attach a 2-foot piece of rubber tubing to the bottom of a small glass funnel and support the funnel in the ringstand set-up.
  2. While pinching the rubber tubing closed at the base of the funnel, pour about 1/2 teaspoon of flour into the funnel.
  3. Hold the burner so that the flame extends over the top of the funnel and blow sharply through the rubber tubing as you release your hold on it.
  4. Note the rate of burning.

* DISPOSAL

Unburned flour from Part A can be disposed of with solid waste.

* DISCUSSION

In Part A, it takes some time to ignite the flour. When it finally ignites, it burns very slowly. This is because the surface area of the solid exposed to the oxygen in this heterogeneous system is relatively small. In Part B, the flour ignites with a flash, producing a flame rising above the funnel because, by dispersing the flour, the surface area of the solid exposed to the oxygen is greatly increased.

* TIPS

  1. A flexible-type drinking straw might be added to the end of the rubber tubing to provide a clean surface to put in one's mouth.
  2. Discuss with students the relationship between this demonstration and the hazards associated with finely divided combustible material in grain elevators and mills, coal mines, and saw mills.

* REFERENCES

Cotton, F.A., Darlington, C.L., and Lynch, L.D., Teacher's Guide to Chemistry: An Investigative Approach, Houghton Mifflin Co., Boston, 1980. This work describes a similar demonstration.

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


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