11th Class Chemistry The s-Block Elements

  • question_answer 83)   The s-block elements are characterised by their-large atomic sizes lower ionisation enthalpies, invariable +1 oxidation state and solubility of their oxosalts. In the light of these features describe the nature of their oxides, halides and oxosalts.      


      Nature of oxides The normal oxides \[({{M}_{2}}O)\] are basic in nature. They form strong alkalies (hydroxides) when dissolved in water. The basic nature increases on moving down the group.                                 \[{{M}_{2}}O+{{H}_{2}}O\to 2MOH\] Besides monoxides, sodium and higher alkali metals form peroxides and super oxides. The peroxides \[({{M}_{2}}{{O}_{2}})\] and superoxides \[(M{{O}_{2}})\]form hydroxides when hydrolysed with water. \[{{M}_{2}}{{O}_{2}}+2{{H}_{2}}O\to 2MOH+{{H}_{2}}{{O}_{2}}\] \[2M{{O}_{2}}+2{{H}_{2}}O\to 2MOH+{{H}_{2}}{{O}_{2}}+{{O}_{2}}\] Peroxides and super oxides are strong oxidising agents. Nature of halides The halides \[({{M}^{+}}{{X}^{-}})\] are crystalline and have high melting and boiling points. The fused halides are good conductors of electricity. All halides except \[LiF\]dissolve in water. Halides are colourless and on heating turn yellow, blue, etc. Halides of potassium, rubidium and caesium have a property of combining with extra halogen atoms forming polyhalides \[KI+{{I}_{2}}\to K{{I}_{3}}\] Nature of oxosalts Alkali metals readily react with oxy acids forming corresponding salts with evolution of hydrogen. Lithium salts behave some what abnormally due to polarising power and lattice energy effects. The carbonates of alkali metals are highly stable towards heat and readily soluble in water. The stability increases from Li to Cs as electropositive nature increases. Solutions are alkaline in nature due to hydrolysis \[{{M}_{2}}C{{O}_{3}}+2{{H}_{2}}O\rightleftharpoons 2MOH+{{H}_{2}}C{{O}_{3}}\] Nitrates of the type, MNO^ are known. These are colourless and soluble in water and electrovalent in nature. With the exception of\[LiN{{O}_{3}}\], the other nitrates decompose to nitrites and oxygen. \[2MN{{O}_{3}}\to 2MN{{O}_{2}}+{{O}_{2}}\] \[LiN{{O}_{3}}\]on heating gives \[N{{O}_{2}}\] and \[{{O}_{2}}\]. \[2LiN{{O}_{3}}\to L{{i}_{2}}O+2N{{O}_{2}}+\frac{1}{2}{{O}_{2}}\] Sulphate of the type \[{{M}_{2}}S{{O}_{4}}\] are known. With the exception of\[L{{i}_{2}}S{{O}_{4}}\], other sulphates are soluble in water. These are reduced by carbon oh heating. \[{{M}_{2}}S{{O}_{4}}+4C\to {{M}_{2}}S+4CO\]                 The sulphates of alkali metals form double sulphates with the sulphates of trivalent metals like Fe, Al, Cr, etc.                

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