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Oxygen : Oxygen, Preparation |
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Oxygen may be obtained by heating certain metallic oxides, viz., those of mercury, silver, gold, and the platinum metals. Mercuric oxide, heated in a hard glass tube, decomposes; globules of mercury collect in the cooler parts of the tube; oxygen gas is evolved, and may be collected over water: 2HgO = 2Hg + O2. Oxide of silver, precipitated from silver nitrate solution by caustic potash (in absence of carbon dioxide), gives very pure oxygen when heated: 2Ag2O = 4Ag + O2. Oxides of other metals are stable at high temperatures, but some higher oxides of metals, and hydrogen peroxide, on heating decompose into oxygen and lower oxides: 2H2O2 = 2H2O + O2 2BaO2 = 2BaO + O2 2PbO2 = 2PbO + O2 3MnO2 = Mn3O4 + O2 (at a bright red heat). Manganese dioxide evolves oxygen more readily when heated with concentrated sulphuric acid: 2MnO2 + 2H2SO4 = 2MnSO4 + 2H2O + O2. Oxygen may be obtained from water by electrolysis, or by removing the hydrogen with chlorine; the latter combines with hydrogen to form the stable hydrogen chloride, HCl, but does not unite directly with oxygen: 2H2O + 2Cl2 = 4HCl + O2. Expt. 1. A stream of chlorine is passed through water boiling in a flask, and the gas is then passed through a silica tube packed with bits of broken porcelain and heated to bright redness in a furnace. Some salts rich in oxygen: chlorates (e.g., KClO3), bromates, iodates, nitrates, dichromates (e.g., K2Cr2O7), and permanganates (e.g., KMnO4) evolve oxygen on heating.
The gas is passed through caustic soda solution in a wash-bottle to remove excess of chlorine, and hydrochloric acid, and the oxygen collected over water. In the production of oxygen by heating nitre, when potassium nitrite is left as a residue: 2KNO3 = 2KNO2 + O2, a fairly high temperature is required. Potash chlorate crystals are anhydrous; they melt at 357°, and on heating at 380° in a hard glass tube evolve oxygen: (1) 2KClO3 = 2KCl + 3O2. As the reaction proceeds, the evolution of gas slackens, and the salt becomes pastry, finally almost solid, although decomposition is not complete. At this stage the residue contains potassium chloride and potassium perchlorate, KClO4, produced by the reaction: (2) 4KClO3 = 3KClO4 + KCl. If the temperature is raised the mass fusel again (KClO4 melts at 610°), oxygen is evolved and finally solid potassium chloride (m. pt. 800°) remains: (3) KClO4 = KCl + 2O2 Reactions (1) and (2) proceed simultaneously and independently from the commencement. At high temperatures the gas contains a little chlorine owing to the reaction: (4) 4KClO3 = 2K2O + 2Cl2 + 5O2, which takes place to a slight extent. A mixture of potassium and sodium chlorates liberates oxygen at a lower temperature than potassium chlorate alone. Potassium permanganate on heating at 240° in a glass tube evolves very pure oxygen, leaving a black powdery residue of potassium manganate, K2MnO4, and manganese dioxide: 2KMnO4 = K2MnO4 + MnO2 + O2. By adding a little water to the cooled residue, a dark green solution of the manganate is formed. A solution, or paste, of bleaching powder, heated with a few drops of cobalt chloride solution, rapidly evolves oxygen (Mitscherlich, 1843), The bleaching powder, CaOCl2, is decomposed by water into calcium chloride and calcium hypochlorite: 2CaOCl2 = CaCl2 + Ca(OCl)2. Cobaltous oxide is precipitated by the free lime always contained in the bleaching powder: (a) CoCl2 + Ca(OH)2 = CoO + CaCl2 + H2O, but is at once converted by the oxidising action of the hypochlorite into a black peroxide CoO2: (b) 2CoO + Ca(OCl)2 = CoO2 + CoO2 + CaCl2, This peroxide CoO2 (or possibly Co4O7; or CoO3) then acts as a catalyst in promoting the decomposition of the calcium hypochlorite. Expt. 2. Add caustic soda solution to cobalt chloride solution: a blue precipitate, rapidly turning to a pink precipitate of hydrated CoO, is thrown down. Pour this into bleaching powder solution. The precipitate at once becomes oxidised to a black substance, CoO2, and oxygen is freely evolved on warming. Chromium trioxide and potassium dichromate evolve oxygen when heated with concentrated sulphuric acid, the red colour of these compounds changing to olive-green:K2Cr2O7 + H2SO4 = K2SO4 + 2CrO3 + H2O; 4CrO3 + 6H2SO4 = 2Cr2(SO4)3 + 6H2O + 3O2. Chromium trioxide decomposes when heated alone, although a little sublimes unchanged: 4CrO3 = 2Cr2O3 (green) + 3O2. Potassium dichromate (m.pt. 396°) when strongly heated evolves oxygen: 4K2Cr2O7 = 4K2CrO4 + 2Cr2O3 + 3O2. Potassium permanganate explodes violently when warmed with concentrated sulphuric acid, but readily yields pure oxygen if hydrogen peroxide solution is mixed with a solution of the permanganate and dilute sulphuric acid: the two highly oxidised compounds mutually decompose each other, yielding a nearly colourless solution: 2KMnO4 + 3H2SO4 + 5H2O2 = K2SO4 + 2MnSO4 + 8H2O + 5O2. Expt. 3. A solution of 5 gm. of KMnO4 in a cooled mixture of 100 c.c. of water and 50 c.c. of concentrated sulphuric acid is dropped from a tap-funnel into 100 c.c. of "20 volumes" hydrogen peroxide in a flask. The oxygen evolved is collected over water. Oxygen is evolved when water is dropped on sodium peroxide or on a mixture of barium peroxide and potassium ferricyanide:2Na2O2 + 2H2O = 4NaOH + O2. BaO2 + 2K3Fe(CN)6 = K6Ba[Fe(CN)6]2 + O2, or when barium peroxide is treated with ferric chloride solution: 6BaO2 + 4FeCl3 + 6H2O = 6BaCl2 + 4Fe(OH)3 + 3O2. |
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