Preparation of Oxygen

1. From salt: When oxygen rich salts are heated, gets decomposed and gives oxygen.
$$\ce{2KNO3->[\Delta]2KNO2 + O2 ^ }$$
$$\ce{2KClO3->[\Delta]2KCl + 3O2 ^ }$$

2. From Potassium permanganate: When potassium permanganate is heated, it gives oxygen.
$$\ce{2KMNO4->[\Delta]K2MnO4 + MnO2 + O2 ^ }$$

3. From Potassium dichromate: On heating potassium dichromate, oxygen is obtained.
$$\ce{4K2Cr2O7->[\Delta]4K2Cr2O4 + 2Cr2O3 + 3O2 ^ }$$

4. From bleaching powder: When bleaching powder is reacted with Cobalt chloride, oxygen is obtained.
$$\ce{2CaOCl2->[\ce{CoCl2}]2CaCl2 + O2 ^ }$$

5. By heating metal oxides: Metal oxides when heated, gives oxygen.
$$\ce{2HgO->[\Delta]2Hg + O2 ^ }$$
$$\ce{2Ag2O->[\Delta]4Ag + O2 ^ }$$
$$\ce{2PbO2->[\Delta]2PbO + O2 ^ }$$
$$\ce{3MnO2->[\Delta]Mn3O4 + O2 ^ }$$
6. From acids: When concentrated sulphuric acid and concentrated nitric acid are dropped slowly in heated pumice stone, it decomposes as follows.
$$\ce{4HNO3->4NO2 + 2H2O + O2 ^ }$$
$$\ce{4H2SO4->2SO2 + 2H2O + O2 ^ }$$

Manufacture in large scale:

From air: Air mainly consists of dinitrogen and dioxygen. Both gas can be isolated from each other by fractional distillation of liquefied air. The nitrogen is distilled away first as it has lower boiling point (-196oC) and leaves the liquid oxygen (boiling point = -183oC).

From Water: Oxygen can be obtained from by water by the process of electrolysis either in the presence of acids or alkalis. Hydrogen is liberated at cathode and oxygen at anode.The following cell reaction takes place in electrolysis.

$$\ce{H2O->H^+ + OH^- }$$
At cathode: $$\ce{2H^+ + 2e^- ->2H2O + O2 ^ }$$
At anode: $$\ce{4OH^- – 4e^- ->2H2O + O2 ^ }$$

Preparation of oxygen

In the cell, iron sheet is used as cathode and nickel plated iron sheet acts as cathode. The two electrodes are separated from each other by a long rod called asbestos diaphram. This prevents the mixing of hydrogen and oxygen gas.

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