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Valency : Causes Of Variation Of Valency |
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The valency of an element may alter as a result of physical or chemical causes. Phosphorus pentachloride, PCl5, containing quinquevalent phosphorus, is decomposed by heat into chlorine and phosphorus trichloride, PCl3, containing tervalent phosphorus: PCl5 <=> PCl3 + Cl2. Mercurous oxide is decomposed by exposure to light into mercuric oxide and metallic mercury: Hg2O = Hg + HgO. Phosphine, PH3, and hydrogen chloride, HCl, do not combine at ordinary pressure, but under increased pressure they give solid phosphonium chloride containing quinquevalent phosphorus: PH3 + HCl <=> PH4Cl. By electrolysis, a solution of manganous sulphate, MnIISO4, in sulphuric acid, is converted into permanganic acid, HMnVIIO4. Chemical changes often lead to alteration of valency, according as an excess or deficit of an element or radical is present during the preparation of the compound: 2Hg (excess) + I2 = 2HgI; Hg + I2 (excess) = HgI2. In some cases only one compound is formed under all conditions from the elements, e.g., tin always forms stannic chloride with chlorine: Sn + 2Cl2 = SnCl4, even if tin is in excess, but the action of hydrochloric acid always leads to the formation of stannous chloride: Sn + 2HCl = SnCl + H2. Changes of oxidation and reduction bring about changes of valency: Sn -> SnIIO -> SnIVO2 (oxidation); Fe2IIIO3 -> FeIIO (reduction). Oxidation leads to increase, reduction to decrease, of positive valency. The change of a ferrous to a ferric salt, for example, is also called oxidation, because the valency of iron in ferric salts is higher than that in ferrous salts, and the two series of salts may, on the old dualistic theory, be regarded as derived from a higher and lower oxide of iron, respectively: Ferrous sulphate - FeIISO4 FeO, SO3 Ferric sulphate - Fe2III (SO4)3 Fe2O3,3SO3. Ferrous chloride, FeCl2, is said to be "oxidised" to ferric chloride, FeCl3, since an increase in positive valency results: neither compound contains oxygen. By passing chlorine through a green solution of potassium man-ganate, K2MnO4, it is oxidised to a purple solution of potassium permanganate, KMnO4, and the valency of manganese is raised from +6 to +7:  + Cl ->  + KCl.Removal of an electropositive atom (K) is therefore equivalent to oxidation; addition of an electronegative atom is also oxidation: FeIICl2 + Cl -> FeIIICl3. The reverse changes are equivalent to reduction. An example of oxidation and reduction occurring simultaneously is the action of ferrous sulphate on potassium permanganate: 2KMnVIIO4 + 10FeIISO4 + 8H2SO4 = K2SO4 + 2MnIISO4 + 5Fe2III(SO4) 3 + 8H2O. 10FeII becomes 10FeIII, an increase of 10 units of positive valency (oxidation). 2MnVII becomes 2MnII, a decrease of 10 units of positive valency (reduction). |
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