TOPIC: KINETICS

DEMO 004

CATALYSIS: HOMOGENEOUS AND HETEROGENEOUS

Hydrogen peroxide solution is inert with respect to disproportionation into water and oxygen gas until a catlayst is added to it. Iodide ion produces a slow evolution of oxygen bubbles. The reaction is more rapidly catalyzed by a solid (Ag2O). Bubbles are vigorously evolved at the surface of the catalyst, but at the end of the reaction, the solid catalyst is unchanged.

MATERIALS

3% hydrogen peroxide
0.1 M potassium iodide solution
1 M sodium hydroxide solution
silver oxide
5 50-mL beakers or a Corning 6-well Cell Well™
small spatula
small stirring rod
overhead projector
transparency made from last page of this form

PRESENTATION

Place the containers on the transparency. Add enough hydrogen peroxide to each to give a depth of 4-5 mm. Do nothing more to the first container. This will be the comparison standard for H2O2. To the second and third containers, add 1 drop of NaOH. The second shows that the reaction is not the result of adding base. To the third container, add 5 drops of KI solution and stir. Bubbles of oxygen will become visible in 2-4 minutes. Using the spatula, drop a few grains of silver oxide into the fourth container. It may be necessary to use the spatula to force the grains of solid to sink to the bottom. A vigorous evolution of gas will begin immediately and will continue for some time, at which time the grains of silver oxide will be visible (adjust the focus of the projector) if they were large enough to see initially. If possible, retrieve the grains and drop them into the fifth container to show that the material is still catalytically active.

DISCUSSION

A catalyst is a substance that changes (usually increases) the rate of a reaction without itself undergoing any permanent chemical alteration. Catalysts are generally divided into two types, those that are in the same phase as the reactants (homogeneous catalysts) and those that belong to a different phase (heterogeneous catalysts, usually solids).

The first of the two reactions in this demonstration is an example of homogeneous catalysis. Hydrogen peroxide reacts vigorously as an oxidizing or reducing agent with a number of chemicals. But the disproportion reaction below, although it has a strong driving force, cannot seem to figure out how to proceed at room temperature if the solution is free of impurities.

2H2O2 2H2O + O2,

Iodide ion is a fairly effective catalyst for the reaction. The mechanism is

H2O2 + I- OI- + H2O

H2O2 + OI- O2 + H2O + I-

both steps of which are faster than the rate of the direct reaction. The initial appearance of oxygen bubbles is slow because oxygen is fairly soluble in water, and its solubility has to be exceeded before bubbles will form.

The second demonstration, where the overall reaction is the same, is an example of heterogeneous catalysis. The addition of Ag2O provides a surface upon which a crucial step in the catalyzed reaction can take place. The reaction appears to be taking place much more rapidly than in the heterogeneous case, but it must be appreciated that the reaction is taking place at a few isolated centers and is more obvious there. A number of substances can catalyze the disproportion of hydrogen peroxide, and great care must be taken to remove impurities from the commercial 60% solution of H2O2 to give it a reasonable shelf life.

HAZARDS

Hydrogen peroxide 3 - 6% is not usually harmful, although eye protection is advisable. CHECK THE LABEL! Hydrogen peroxide available in research laboratories is often 30%, which is very reactive and takes special handling techniques, including rubber gloves, eye protection, and immediate access to running water. Skin is immediately damaged by 30% H2O2.

Silver nitrate is toxic and is reduced to silver metal by proteins, and so requires eye protection.

1 M sodium hydroxide requires the usual precautions.

REFERENCES

Lee R. Summerlin and James L. Ealy, Jr., "Chemical Demonstrations: A Sourcebook For Teachers", American Chemical Society, Washington, D. C., 1985, p. 71.

Doris Kolb, "Introduction to Overhead Projector Demonstrations", Journal of Chemical Education, 1987, 64, 348.

NOTES

If Ag2O is not available, it may be prepared by dissolving 0.5 g of silver nitrate in 20 mL of water then adding 5 mL of 1 M NaOH solution. Filter out the dark precipitate, wash it with water, and allow it to dry. MnO2 may be substituted for Ag2O, but if it is very finely divided, the reaction is harder to see.

This demonstration is commonly done without the addition of base. However in that case, an amber color develops in the reaction as the result of the oxidation of I_ to I2. The perceptive student might object that the catalyst was not supposed to undergo a permanent chemical change. Addition of base suppresses the coloration, presumably by suppressing the reaction

HOI + I- I2 + OH-

DEMO-004.DOC

 

 

 

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