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Name the various hydrides of group 16 elements. Arrange these hydrides in their decreasing order of (i) Volatility (ii) Bond angle (iii) Acidic characterH_2Sis less acidic thanH_2Te1, why (iv) Thermal stability and (v) Reducing character. |
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Answer» Solution :HYDRIDES of group 16 ELEMENTS. All the elements of group 16 form hydrides such as `H_2O, H_2S, H_2 Se, H_2 Te ` and `H_2 Po` . Water is a colourless, odourless liquid while hydrides of the other elements of this group are colourless, bad smelling, poisonous gases. TRENDS in characteristics : (i) Volatility. All the hydrides are volatile. The volatility increases from H20 to H S and then decreases. This is clear from their boiling points as given ahead : `H_2O "" H_2S "" H_2Te ""H_2Se` `373K ""213K ""232K ""269K` Thus, the TREND of volatility is `H_2O lt H_2S gt H_2Te gt H_2Se` The high boiling point and, therefore, low volatility of water is due to the association of `H_2O`molecules through hydrogen bonding. (ii) Bond angle. The bond angles H-X-H in the hydrides `H_2O, H_2S, H_2Se `and `H_2 Te`decrease as we move from `H_2O` to `H_2Te`as shown below: Hydride`H_2O "" H_2S "" H_2Se "" H_2Te` Bond angle`104.5^@ "" 92.1^@ "" 91^@ "" 90^@` thus `H_2O gt H_2S gt H_2Se gt H_2Te` (iii) Acidic character. The hydrides of this group are weakly acidic in character. The acidic character increases from `H_2O` to `H_2Te`as : `MH_2Te gt H_2Se gt H_2S gt H_2O` The increase of acidic character from `H_2O`to `H_2Te`can be explained on the basis of size of the central atom. As the size of the central atom increases in the order O < S < Se < Te, the distance between the central atom and hydrogen also increases. As a result of increase in bond length, the bond dissociation energy decreases and bond cleavage gets more and more easy. Therefore, the acidic strength of the hydride increases down the group. (iv) Thermal stability. The thermal stability of the hydrides decreases from `H_2O` to `H_2Te`as : `H_2O gt H_2S gt H_2Se gt H_2Te` (v) Reducing character. All the hydrides except water are reducing agents. The reducing power of these hydrides increases from `H_2O` to `H_2Te` . This is due to the decrease in thermal stability of the hydrides. `H_2Te gt H_2Se gt H_2S gt H_2O` |
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