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react with dihydrogen at about 673K (lithium at, are ionic solids with high melting points., , PACtiVity i, ‘ A, +? vwards dihydrogen: The alkali metals, orm hydrides. All the alkali metal hydrides, + H> > 2MH (M = Li,Na,K.Rb,Cs), ity towards halogens: The, X., 2M+X)— 2MX (M = Li,Na,.K.Rb.Cs, , alkali metals readily react vigorously with halogens to form, , and X= F,ClI,Br,1), , , , i) Solutions in liquid ; ia: T i, = tions in liquid ammonia: 1 he alkali metals dissolve in li, solutions which are cor, , electron which absorbs, , — quid ammonia giving deep blue, ducting in nature. The blue colour of the solution is due to the ammoniated, energy in the visible region of light and thus imparts blue colour to the solution., M+ (x + y)NH3 —[MO(NHs); J* + [e(NH3))], © Uses:, , = ithi 5 is us 5 4 , ’, , Lithium metal is used to make useful alloys, for example with lead to make ‘white metal ., bearings for motor engines, with aluminium to make aircraft parts, and with magnesium to make |, armour plates. ;, It is used in thermonuclear reactions. Lithium is also used to make electrochemical cells. ;, , Sodium is used to make a Na/Pb alloy. These compounds were earlier used as anti-knock, additives to petrol,, , , , Liquid sodium metal is used as a coolant in fast breeder nuclear reactors., , Potassium has a vital role in biological systems. Potassium chloride is used as a fertilizer., Potassium hydroxide is used in the manufacture of soft soap., , It is also used as an excellent absorbent of carbon dioxide. ;, Caesium is used in devising photoelectric cells., , ANOMALOUS PROPERTIES OF LITHIUM ;, The anomalous behaviour of lithium is due to the:, , (i) exceptionally small size of its atom and ion, and, , (11) high polarising power (i.e., charge/radius ratio)., , As a result, there is increased covalent character of lithium compounds which is responsible for their, solubility in organic solvents. Further, lithium shows diagonal relationship to magnesium., , , , Difference between Lithium and other Alkali Metals:, , , , (i) Lithium is much harder. Its m.p. and b.p. are higher than the other alkali metals., (ii) Lithium is least reactive but the strongest reducing agent canis all the alkali metals. On combustion, in air it forms mainly monoxide, LizO and the nitride, Li3N unlike other alkali metals., (iii) LiCl is deliquescent and crystallises as a hydrate, LiC].2H2O whereas other alkali metal chlorides, do not form hydrates., (iv) Lithium hydrogencarbonate is not obtained in the solid form while all other elements form solid, hydrogencarbonates., (v) Lithium unlike other alkali metals forms no ethynide on reaction with ethyne., (vi) Lithium nitrate when heated gives lithium oxide, LizO, whereas other alkali metal nitrates, decompose to give the corresponding nitrite., (vii) LiF and Li20 are comparatively much less soluble in water than the corresponding compounds of, other alkali metals., , Difference between Lithium and other Alkali Metals:, , RIE