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Give reasons for the following: a. Anhydrous calcium sulphate (anhydrite) cannot be used as plaster of Paris. B. Limewater turns milky on passing CO_(2) through it, but milkiness disappears on passing excess of CO_(2). C. The reacton between marble and dil H_(2)SO_(4) is not used to prepare CO_(2). d. In the manufacture of Mg by carbon reduction of MgO, the product is cooled in the stream of an inert gas. e. magnesium metal burns in air to give a white ash. when this ash is treated with water, the odour of ammonia can be detected. |
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Answer» Solution :(a). Anhydrous calcium sulphate in presence of water does not set like plaster of paris. However, it directly changes to gypsum. `CaSO_(4)+2H_(2)Orarrunderset("Gypsum")(CaSO_(4).2H_(2)O` (b). Limewater, `CA(OH)_(2)`. Reacts with `CO_(2)` to form white ppt. of `CaCO_(3)`, which gives milkiness to the solution, But on excess passage of `CO_(2)` through limewater, `CaCO_(3)`. formed initially further reacts with `CO_(2)` to form calcium bicarbonate, `Ca(HCO_(3))_(2)`. which is soluble and hence the milkiness disappears. `underset("Limewater") +CO_(2) rarr underset(("Milkiness")darr+H_(2)O` `CaCO_(3)+H_(2)O+underset(("excess"))(CO_(2))rarrunderset(("Soluble"))(Ca(HCO_(3))_(2))` (C ). Marble, `CaCO_(3)`, reacts with dil `H_(2)SO_(4)` to form `CaSO_(4)`. This `CaSO_(4)` forms a layer on the surface of marble, preventing any further reaction with dil `H_(2)SO_(4)`. Hence, EVOLUTION of `CO_(2)` ceases after sometime and hence marble cannot be used as a source of `CO_(2)`. `CaCO_(2)+H_(2)SO_(4)rarrCaSO_(4)darr+H_(2)O+CO_(2)uarr` (d).  Reducing of `MgO` by `C` to give `Mg` metal and `CO` is a reversible process. In order to prevent the backward reaction or to make the reaction proceed in FORWARD direction, the temperature of the products is reduced in a stream of an inert gas. (e). Magnesium metal on burning in air reacts with `O_(2)` and `N_(2)` to give `2Mg+O_(2)rarrunderset("Magnesium oxide")(MgO)` `3Mg+N_(2)rarrunderset("Magnesium nitride")(Mg_(3)N_(2))` White ash produced is a mixture of `MgO` and `Mg_(3)N_(2)`.On treatment with water, `Mg_(3)N_(2)` is hydrolysed, ammonia is produced. `Mg_(3)N_(2)+6H_(2)Orarr3Mg(OH)_(2)+2NH_(3)uarr` |
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