AN EXPERIMENT TO SHOW THE CATALYTIC ACTION OF MANGANESE DIOXIDE ON THE DECOMPOSITION OF POTASSIUM CHLORATE AND THE MEASUREMENT OF THE PRODUCTS OF THE REACTION JOHN E. MACKENZIE. THEUNIVERSITY, EDINBURGH, SCOTLAND In various texts and manuals will be found the description of an experiment showing the catalytic action of manganese dioxide on the decomposition of potassium chlorate. For several years an extended form of the experiment has been carried out in my lectures and the following description may be of use to other teachers of chemistry. The accompanying diagram shows the apparatus, consisting of: A a hard glass tube of 25-mm. internal diameter, B a rubber stopper, C a n elbow tube of 8-mm..internal diameter and each limb 75 mm. in length, D a connecting tube of 8-mm. internal diameter and 450 mm. lenah, E a wide measurinrr of 1000 cc. capacity, F a glass trough. - cylinder . T h e powdered potassium chlorate and powdered manganese dioxide are previously dried by heating to 150' in an air oven and tubes A and C are cleaned and dried before weighing. After making the necessary weighings as shown below in the data of three experiments, the apparatus is set-up as in the diagram, except that the outer end of tube D is under the surface of the water in the trough F, but not below the cylinder E. The tube A is gradually heated until the chlorate has completely melted and air no longer escapes in bubbles through the water, thus showing the air in the apparatus to be a t a constant temperature. The opening of tube D is then brought under the cylinder and tube C is twisted through an angle of 180' so that the manganese dioxide falls down on to the melted chlorate, and the Bunsen flame is immediately removed until the first vigorous reaction has subsided. The flame is again applied to tube A until the reaction is completed as shown by no further passage of bubbles through the water into E. When this happens, the apparatus ABCD is moved away from the trough F. The volume of gas in cylinder E is measured after leveling the surfaces of water inside and out, the temperature and atmospheric pressure and the pressure of aqueous vapor noted, and the volume reduced to N.T.P. The calculation from volume of oxygen to weight of oxygen is then made. 1111
JOURNAL OF CHEMICAL EDUCATION
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JUNE. 1932
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After the tube A has cooled down, the potassium chloride and manganese dioxide are separated by adding hot distilled water-about 30 cc.-and decanting into a tared double filter paper and collecting the manganese dioxide on the paper and the potassium chloride solution in a tared porcelain basin with glass rod. The extraction with water is repeated three or four times, care being taken that all the manganese dioxide is transferred to the filter paper. The funnel and filter paper are then dried in a steam oven and the solution stirred with the rod to prevent spurting while being evaporated to dryness. The basin with the residue of potassium chloride is gently heated over a Bunsen flame and after cooling is weighed and the process repeated until constant weight is attained. DATA Expnimcnf
Weight of tube A Potassium Chlorate Weight of tube A (a) Weight of Potassium Chlorate glass rod Weight of basin glass rod Potassium Chloride Weight of basin ( 6 ) Weight of Potassium Chloride Weight of Oxygen by difference "a" - "b" Weight of tube C Manganese Dioxide Weight of tube C Weight of Manganese Dioxide Filter paper 1 = Filter paper 2 + Filter paper 1 Manganese Dioxide = Filter p w e r 2 - t Weight of Manganese Dioxide recovered Volume of Oxygen, cc. Temperature. 'C. Atmospheric pressure, mm. Pressure of Aqueous Vapor, mm. Volume of Oxygen a t N.T.P.. cc. Calculated weight of Oxygen, g.
+
+ +
+
+
+
1
2
3
K.
8.
&
35.40 35.40 35.40 37.85 37.85 37.85 2.45 2.45 2.45 61.168 23.86 41.06 25.35 42.57 62.637 1.49 1.51 1.469 0.981 0.96 0.94 14.23 14.23 14.23 14.34 14.33 14.33 0.11 0.10 0.10 0.00 0.020 0.005 0.115 0.095 0.09 0.09 0.09 0.095 700 750 750 13 14 16 747 735 732 11 12 13.5 679 672 647 0.923 0.970 0.96
These data give results in fairly close agreement with the extremely accurate data upon which the following equation is based: 2(39
2KCIO. 4 2KCI + 3 0 % + 35.5 + 48) -+2(39 + 35.5) + 3(32) g. 245 g.
149 g.
= 3 X 22,400 cc. 96 g. = 67.200 cc.
Using 1/100 of the weight of chlorate, the figures obtained were, 1.49, 1.51, and 1.47 as against 1.49 g. potassium chloride and by weight difference, 0.96,0.94, and 0.98 as against 0.96 g. oxygen; and by weight calculated from volume of gas, 0.92, 0.97, and 0.96 as against 0.96 g. oxygen. The loss in weight (0.014.02 g.) of manganese dioxide is due mainly to its not being all retained in the first filter paper.
VOL.9, NO. 0 APPARATUS FOR ILLUSTRATING ARC METHOD
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1113
First-year students perform part of this experiment as an exercise in the determination of the ratio KC1O3 : KCI, simply mixing the chlorate and manganese dioxide in an ordinary test tube before heating and omitting the collection of oxygen.
