The s-Block Elements

  • Question 217
    CBSEENCH11008343

    Match (I) with list (II) for the  compositions of substances and select the correct answer using the code given below the lists.


     

    List I (substance)

     

    List II Composition

    A.

    Plaster of Paris

    1.

    CaSO4.2H2O

    B.

    Epsomite

    2.

    CaSO4. ½ H2O

    C.

    Kieserite

    3.

    MgSO4. 7H2O

    D.

    Gypsum

    4.

    5.

    MgSO4.H2O

    CaSO4

    • A

      B

      C

      D

      3

      4

      1

      2

    • A

      B

      C

      D

      2

      3

      4

      1

    • A

      B

      C

      D

      1

      2

      3

      5

    • A

      B

      C

      D

      4

      3 2

      1

    Solution

    B.

    A

    B

    C

    D

    2

    3

    4

    1

    Plaster of paris = CaSO4.1/2 H2O
    Epsomite =MgSO4.7H2O
    Gypsum = CaSO4.2H2O
    Kieserite = MgSO4.H2O

    Question 218
    CBSEENCH11008360

    Property of the alkaline earth metals that increases with their atomic number

    • the solubility of their hydroxides in water

    • the solubility of their sulphates in water

    • ionisation energy 

    • electronegativity

    Solution

    A.

    the solubility of their hydroxides in water

    Electronegativity, as well as ionisation energy both, usually decrease on moving downward a group with an increase in atomic number. The hydroxides and sulphates of alkaline earth metals are ionic solids and the solubility of ionic solids is governed by two factors viz, lattice energy and hydration energy. For solubility hydration, energy > lattice energy.
    Hydration energy varies inversely with size, ie, decreases with increase in size. However, lattice energy in case of sulphates, remains almost same with an increase in the atomic number of alkaline earth metals, due to the large size of sulphates ion. Hence, hydration energy only governs the solubility of alkaline earth metal sulphates decrease as the hydration energy decreases on moving downward the II A group.

    On the other hand, in the case of hydroxides, the lattice energies are different because of medium size of hydroxide ions and decreases on moving from Be to Ba. This tends to increase the solubility and to overcome the counter-effect produced by the decrease in hydration energy. Hence, the solubility of alkaline earth metal hydroxides increases with increase in the atomic number of alkaline earth metals.
    Question 219
    CBSEENCH11008363

    Which of the following alkaline earth metal sulphates has hydration enthalpy higher than the lattice enthalpy? 

    • CaSO4

    • BeSO4

    • BaSO4

    • SrSO4

    Solution

    B.

    BeSO4

    Hydration energy varies inversely with the size and in sulphates of alkaline earth metals lattice energy remains almost constant.
    The order of size of alkaline earth metal is
    Be2+ > Ca2+ > Sr2+ > Ba2+
    Hence, BaSO4 has the hydration enthalpy higher than the lattice enthalpy.

    Question 220
    CBSEENCH11008383

    In the case of alkali metals, the covalent character decreases in the order

    • MCl> MI>MBr>MF

    • MF> MCl> MBr>MI

    • MF > MCl> MI>MBr

    • MI>MBr>MCl>MF

    Solution

    D.

    MI>MBr>MCl>MF

    According to Fajan's rule
    Covalent space character space proportional to fraction numerator 1 over denominator Size space of space cation end fraction proportional to space size space of space anion
    In the given options, the cation is same but anions are different. Among halogens the order of size is 
    F < Cl< Br< I
    therefore, Order of covalent character is 
    MI> MBr>MCl>MF

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