Demonstration 12
Hydrogen peroxide
The oscillating reaction demonstrated in the previous experiment was catalysed by manganese(II) ions. The present experiment illustrates the effect of a catalyst on the speed of chemical reaction in a more visual way. Solutions of hydrogen peroxide are unstable and the H2O2 present in them slowly decomposes forming oxygen gas and water,
H2O2==> H2O + 1/2 O2 (12.1)
The reaction is slow at room temperature; however if you open a fresh bottle of a hydrogen peroxide solution and dip a glowing splint into the mouth of the bottle - the presence of oxygen will be revealed [1]. Upon heating, the hydrogen peroxide decomposes vigorously. The decomposition reaction can be accelerated dramatically even at room temperature by employing a catalyst. A great variety of substances will catalyse the decomposition of hydrogen peroxide, e.g., manganese dioxide, blood, dust, and even rough surfaces [2,3]. In our demonstration we use potassium iodide. To make the reaction more visible and visual we also add some washing-up liquid to the hydrogen peroxide before adding the catalyst.
Preparation. No specific preparation is needed. The experiment can be performed in almost any vessel but we found that a gas jar (ca. 1-1.5 in dia.) works best.
Demonstration. Place about 30 ml of 30% (100 vol.) hydrogen peroxide into the gas jar. The add a couple of squirts of any washing-up liquid (using a small pipette). Place the jar on a sheet of bench coat (or in a large trough) and drop some solid potassium iodide (ca. 1 g) into the liquid using a spatula. The solution begins vigorously to decompose and the froth of escaping oxygen can be seen rising in the gas jar. As the reaction progresses it also speeds up and soon the froth is pouring out of the jar. The reaction is exothermic and steam coming out the mixture is clearly visible. The colour of the froth is light brown because some potassium iodide is oxidised during the reaction with the formation of iodine.
Coloured foams can be produced easily by using food coloring.
The catalytic decomposition of hydrogen peroxide can be demonstrated in many ways, see, for example, cited references.
Safety. 30% (100 vols.) hydrogen peroxide is corrosive and a strong oxidising agent, contact with skin and eyes must be avoided.
References.
1. G. Fowles, Lecture Experiments in Chemistry, 3-rd ed., London, G.Bell & Sons, Ltd., 1947, p. 287.
2. M.A. Ivanova and M.A. Kononova, Chemical Lecture Experiment, 2-nd ed., Moscow, Vyschaya Shkola, 1984, p. 23 (in Russian).
3. Tested Demonstrations in Chemistry, ed. G.L. Gilbert, et al., Denison University, Granville, OH, 1994, vol. 1, p. I-39.