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Oxygen : Oxygen, Preparation In The Laboratory



The evolution of oxygen from potassium chlorate is greatly accelerated if manganese dioxide is mixed with the salt. Only the chlorate is decomposed.

Expt. 4. Fuse a little potassium chlorate in a test-tube, and keep the temperature below the point at which oxygen is evolved. Now add a little powdered manganese dioxide: a rapid evolution of oxygen occurs.
A mixture of 5 gm. of manganese dioxide with 25 gm. of potassium chlorate (oxygen mixture) evolves oxygen freely when heated in a glass tube at a temperature below the melting point of the chlorate. The heating must be carefully regulated, as the decomposition of potassium chlorate, unlike that of mercuric oxide, evolves heat and under certain conditions may become explosive.

The manganese dioxide undergoes no permanent chemical change in the reaction: it may be recovered by dissolving out the potassium chloride from the residue with water.

Expt. 5. Mix 25 gm. of potassium chlorate with 5 gm. of powdered manganese dioxide in a mortar. Place the mixture in a wide: test tub and tap the tube so as to leave a free passage for the gas. Fit the tube in a horizontal position with a good cork and a wide (¼ in.) glass delivery tube to a Woulfe's bottle containing caustic soda solution,
Preparation of oxygen from potassium chlorate and manganese dioxide

Fig: Preparation of oxygen from potassium chlorate and manganese dioxide


The caustic soda removes any chlorine from the gas. Heat the mixture gently with a slightly luminous flame, beginning at the end near the cork and moving towards the closed end as the reaction proceeds. If the evolution of gas becomes violent, withdraw the flame till it slackens. The gas may be collected in jars over water, or in a metal Pepys' gas-holder, as shown. The latter stands in a trough of water, and the delivery tube is inserted into the lower opening. When the gas has been collected, this opening is closed by a screw stopper. The funnel tube, A, and gas-holder are filled with water, before the collection of the gas. When the gas is no longer evolved, the test-tube is taken off to prevent liquid being drawn back into the tube and cracking it. Jars may be filled in the upper trough of the gas-holder over the short-tube, B, by opening the taps on A and B.
Warning! Manganese dioxide adulterated with powdered coal explodes violently on heating with chlorate. More than one death has been caused in this way, and a little of the mixture should always be heated in an open test-tube before beginning the experiment, in order to be sure that no deflagration occurs.
Other oxides, such as ferric oxide and cupric oxide, act similarly to manganese dioxide: they are also left chemically unchanged after the reaction. This action, discovered by Dobereiner in 1820, is an example of numerous cases in which a substance accelerates a chemical reaction without itself undergoing any permanent chemical change, Such substances were called catalysts by Berzelius (1835).




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