11th Class Chemistry The s-Block Elements

  • question_answer 1) In what ways lithium shows similarities to magnesium in its chemical behaviour?

    Answer:

    Lithium resembles magnesium mainly due to the similarity in sizes of their atoms (Li = 152pm, Mg = 160 pm) and ions (\[L{{i}^{+}}\] = 76 pm, \[M{{g}^{2+}}\] = 72 pm). Both have nearly same electro negativity. Some of chemical similarities are: (i) Both \[LiOH\] and \[Mg{{(OH)}_{2}}\] are weak bases. These are decomposed on heating. \[2LiOH\to L{{i}_{2}}O+{{H}_{2}}O\] \[Mg{{(OH)}_{2}}\to MgO+{{H}_{2}}O\] (ii) Both form ionic nitrides which are decomposed by water. \[6Li+{{N}_{2}}\to 2L{{i}_{3}}N\] \[3Mg+{{N}_{2}}\to M{{g}_{3}}{{N}_{2}}\] \[L{{i}_{3}}N+3{{H}_{2}}O\to 3LiOH+N{{H}_{3}}\] \[M{{g}_{3}}{{N}_{2}}+6{{H}_{2}}O\to 3Mg{{(OH)}_{2}}+2N{{H}_{3}}\] (iii) Both combine with carbon on heating. The carbides yield acetylene on hydrolysis. \[2Li+2C\to L{{i}_{2}}{{C}_{2}}\] \[Mg+2C\to Mg{{C}_{2}}\] \[L{{i}_{2}}{{C}_{2}}+2{{H}_{2}}O\to 2Li(OH)+{{C}_{2}}{{H}_{2}}\] \[Mg{{C}_{2}}+2{{H}_{2}}O\to Mg{{(OH)}_{2}}+{{C}_{2}}{{H}_{2}}\] (iv) Carbonates of Li and Mg decompose on heating. \[L{{i}_{2}}C{{O}_{3}}\to L{{i}_{2}}O+C{{O}_{2}}\] \[MgC{{O}_{3}}\to MgO+C{{O}_{2}}\] (v)\[LiF,L{{i}_{3}}P{{O}_{4}},L{{i}_{2}}{{C}_{2}}{{O}_{4}}\]and\[L{{i}_{2}}C{{O}_{3}}\]like the corresponding salts of magnesium are insoluble in water. (vi) \[LiCl\]like\[MgC{{l}_{2}}\] is deliquescent. Both arc soluble in alcohol and pyridine. Show covalent nature. (vii) The bicarbonates are not known in solid state but only known in solutions.  

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