11th Class Chemistry The p-Block Elements-I

  • question_answer 93)   (i) What are silicones? State the uses of silicones. (ii) What are boranes? Give chemical equation for the preparation of diborane.


      (i) Silicones are organosilicon polymers containing Si?O?Si linkages. They contain repeated \[\left[ -O-\underset{\underset{\text{R}}{\mathop{\text{ }\!\!|\!\!\text{ }}}\,}{\overset{\overset{\text{R}}{\mathop{\text{ }\!\!|\!\!\text{ }}}\,}{\mathop{Si}}}\,- \right]\] units. Silicones are non-toxic, stable towards heat and are hydrophobic (water repellent) in nature. Uses: (a) Silicones are used in making waterproof cloth, paper, wool, wood, etc., by exposing them to silicone vapour. (b) Silicone oils are used as lubricants at high as well as low temperatures. (c) These are used as insulating materials for electric motors and other electrical appliances. (d) Silicone rubbers are useful as they retain their elasticity over a range of temperatures. (e) These are mixed with paints and enamels to make them resistant to the effect of high temperature, sunlight and damp. (ii) Boron forms a number of hydrides. These are called boranes. Boranes can be grouped into two series : (a) \[{{B}_{n}}{{H}_{n+4}}:{{B}_{2}}{{H}_{6}},{{B}_{5}}{{H}_{9}},{{B}_{10}}{{H}_{14}},etc.\] (b) \[{{B}_{n}}{{H}_{n+6}}:{{B}_{4}}{{H}_{10}},{{B}_{5}}{{H}_{11}},{{B}_{9}}{{H}_{15}},etc.\] The simplest boron hydride, \[B{{H}_{3}}\] is unknown. The most important hydride is diborane, \[{{B}_{2}}{{H}_{6}}\]. It is prepared by the action of \[LiAl{{H}_{4}}\] on boron trichloride in presence of ether. \[4BC{{l}_{3}}+3LiAl{{H}_{4}}\xrightarrow{{}}2{{B}_{2}}{{H}_{6}}+3LiCl+3AlC{{l}_{3}}\]

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