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What is Goldsehmidt thermite process ?Explain the following terms : (a) Zone refining (Fractional crystallization) (b) Electrorefining (c) van arkel method |
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Answer» Solution :Certain oxides are not satisfactorily reduced by carbon. In such cases aluminium is used as a reducing agent. A mixture of metallic oxide and aluminium powder is ignited in a closed crucible by means of lighted magnesium ribbon. Metals like CHROMIUM, manganese etc., are extracted by thermite process as illustrated below. `{:(Cr_(2)O_(3) + 2AI rarr Al_(2)O_(3) + 2Cr),(3Mn_(3)O_(4) rarr 4Al_(2)O_(3) + 9Mn):}` The reaction is HIGHLY exothermic and the heat produced is sufficient to melt the metal. (a) Zone refining (Fractional crystallization) : This method is employed for preparing extremely pure metals. This method is based upon the principle that when a molten solution of the impure metal is allowed to cool, the pure metal crystallizes out while the impurities remain in the melt. (b) Electrorefining : In this method, the impure metal is converted into a block which FORMS the anode while cathode is a rod or PLATE of pure metal. These electrodes are suspended in an electrolyte which is the solution of a soluble salt of the metal usually a double salt of the metal. When electric current is passed, metal ions from the electrolyte are deposited at the cathode in the form of pure metal while an equivalent amount of metal dissolves form the anode and goes into the electrolyte solution as metal ion. The soluble impurities present in the crude metal anode GO into the solution while the insoluble impurities settle down below the anode as anode mud. (c) van Arkel method : In this method, the metal is converted into its volatile unstable compound such as iodide leaving behind the impurities. The unstable compound, thus formed is decomposed to get the pure metal. `Ti(s)+2I_(2)(s) + underset(540 K)rArr Til_(4)(g) underset(Ti(s))overset(1700 K)rarr Ti + 2I_(2)(g)` |
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