NEET Practice Questions (MCQs) with Answers & Solutions

(a) Electronegativity as well as ionisation energy both usually decrease on moving downward a group with 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 factor, 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 increase in the atomic number of alkaline earth metals, due to large size of sulphate ion. Hence, hydration energy only governs the solubility in this case. Thus, solubility of alakaline earth metal sulphates decreases as the hydration energy decreases on moving downward the IIA group.

On the other hand, in 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 hydoxides increases with increase in the atomic number of alkaline earth metals.

Key Idea According to Fajan's rule,

 Covalent character $\propto$ 1/size of cation $\propto$size of anion

In the given options, cation is same but anions are different. Among halogens the order of size is

   F<Cl<Br<I

.^.. Order of covalent character is

     MI>MBr>MCl>MF

(d) Key Idea: 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 increase as:

     Li⁺<Na⁺<K⁺<Rb⁺<Cs⁺

(c) In I group only Li form nitride and all II group metals form nitride.

    $$\begin{gathered} Mg+N_2\to M{g}_3{N}_2\stackrel{H_{2}O\left(moist\right)}{\to }Mg{\left(OH\right)}_2+N{H}_3 \\ Ca+N_2\stackrel{H_{2}O\left(moist\right)}{\to }Ca{\left(OH\right)}_2+N{H}_3 \\ Li+N_2\to L{i}_{3}N\stackrel{H_{2}O\left(moist\right)}{\to }LiOH+N{H}_3 \\ K+N_2\to no reaction \end{gathered}$$

(a) $NaHC{O}_3$ and $KHC{O}_3$ have different crystal structure

(b) Due to strong packing of $L{i}^{+}$ and $F^{-}$ ion, lattice energy of LiF is higher than its hydration energy.

(c) Due to high hydration energy of $L{i}^{+}$ cation, the standard reduction potential of $L{i}^{+}$ is more negative among all alkali metal cations hence Li acts as strong reducing agent in water.

(b) The retardation mechanism of gypsum is: when cement is hydrated, gypsum reacts with C3A quickly to generate calcium sulfoaluminate hydrate which deposits and forms a protection film on the cement particles to hinder the hydration of C3A and delay the setting time of cement.

(c) $N{H}_3\uparrow \underset{Solvay\text{'}s process}{\overset{N{H}_{4}Cl}{\leftarrow }}\underset{\left(A\right)}{Ca{\left(OH\right)}_2\stackrel{C{O}_2}{\to }CaC{O}_3}$