The s-Block Elements

  • Question 221
    CBSEENCH11008407

    The alkali metals form salt-like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrieds decreases in which of the following orders?

    • CsH > RbH > KH > NaH >LiH

    • KH > NaH> LiH > CsH >RbH

    • NaH > LiH >KH> RbH> CsH

    • LiH > NaH > KH >RbH >CsH

    Solution

    D.

    LiH > NaH > KH >RbH >CsH

    Small anion forms stable compounds with small cation.
    The thermal stability of alkali metal hydrides decreases as:
    LiH> NaH > KH > RbH  >CsH
    because the size of cation increases as;
    Li+ < Na+ < K+ < Rb+ <Cs+

    Question 222
    CBSEENCH11008423

    The correct order of increasing thermal stability of K2CO3, MgCO3, CaCO3 and BeCO3 is:

    • BeCO3 < MgCO3 < K2CO3 <CaCO3

    • BeCO3 < MgCO3 < CaCO3 <K2CO3

    • MgCO3 < BeCO3 < CaCO3 < K2CO3

    • K2CO3 < MgCO3 < CaCO3 < BeCO3

    Solution

    B.

    BeCO3 < MgCO3 < CaCO3 <K2CO3

    Thermal stability of carbonates increases in a group as we move from top to bottom and decreases in a period as we move from left to right. so, the correct order of thermal stability of given carbonates is:
    BeCO3 < MgCO3 < CaCO3 < K2CO3
    Be, Mg and Ca present in second group and K present in the first group.

    Question 223
    CBSEENCH11008429

    In which of the following the hydration energy is higher than the lattice energy? 

    • BaSO4

    • MgSO4

    • RaSO4

    • SrSO4

    Solution

    B.

    MgSO4

    Hydration energy of sulphate decreases from top to bottom in an IInd group. Mg2+ is smaller than other ions of that group so Mg2+ is readily hydrated. MgSO4 has higher hydration energy than lattice energy.

    Question 224
    CBSEENCH11008446

    The correct order of the mobility of the alkali metal ions in aqueous solution is:

    • Li+>Na+ > K+ > Rb+

    • Na+>K+ > Rb+  > Li+

    • K+ > Rb+ >Na+ > Li+

    • Rb+ > K+ > Na+ > Li+

    Solution

    D.

    Rb+ > K+ > Na+ > Li+

    The correct order of the mobility of the alkali metal ions in aqueous solutions 
    Rb+ > K+ > Na+ > Lidue to following order of hydration energy of these ions Li+ > Na+ > K+ > Rb+ and due to hydration of ion, mobility decreases.

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