Preparation of Oxygen - Form Oxides

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• "Generally oxides, do not decompose when heated."

However, oxides of mercury and silver decompose completely, to yield the respective metal and oxygen.

• The higher oxides of lead, namely lead dioxide and trilead tetroxide (red lead), when heated, decompose to yield the lower oxide and oxygen.

• Hydrogen peroxide is also a higher oxide of hydrogen. It is a colourless liquid. When boiled, it decomposes to yield water and oxygen.

Hydrogen peroxide is taken in a round-bottomed flask and heated carefully. The oxygen evolved is collected by the downward displacement of water (Fig.13.3).

If a small quantity of manganese dioxide is added to hydrogen peroxide, the latter decomposes to form water and oxygen, at ordinary temperature. Manganese dioxide is taken in a round-bottomed flask. Hydrogen peroxide is taken in a dropping funnel. A small quantity of hydrogen peroxide is added into the flask by opening the tap of the funnel (Fig.13.4).

The reaction is immediate and one can observe bubbles of oxygen gas liberated in the flask. As more and more hydrogen peroxide is added more oxygen is formed. The oxygen produced is collected in an inverted glass cylinder by the downward displacement of water in a trough.

Manganese dioxide acts as a catalyst in this reaction.

Remember :

A catalyst is a substance that accelerates the rate of a chemical reaction, without itself undergoing any permanent chemical change. The process is known as catalysis.

Conversely, a substance that decelerates the rate of a chemical reaction, without itself undergoing any permanent chemical change is called an inhibitor.

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• Water is also an oxide of hydrogen. Oxygen can be obtained from water by the process known as "Electrolysis". When a direct current of electricity is passed through water, acidified with a small quantity of sulphuric acid, it decomposes to give hydrogen and oxygen gas.

The electrolysis cell consists of a large vessel carrying two electrodes. The electrodes used here are made of platinum. Hydrogen is liberated at the cathode

(-), and oxygen is liberated at the anode (+). The gases are collected in two separate cylinders inverted on the electrodes. The volume of oxygen formed is half the volume of hydrogen (Fig.13.5).