Question
Answers

Among the statements (a)-(d), the correct ones are:
(A) Lithium has the highest hydration enthalpy among the alkali metals.
(B) Lithium chloride is insoluble in pyridine.
(C) Lithium cannot form ethynide upon its reaction with ethyne.
(D) Both lithium and magnesium react slowly with \[{{\text{H}}_{\text{2}}}{\text{O}}\] .

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Hint: - The term ‘enthalpy of hydration’ or ‘hydration enthalpy’ is used to refer to the amount of energy which is released when one mole of gaseous ions is dissolved in water to form an infinitely dilute solution. The extent of hydration depends upon the size and charge of the ions, i.e., the extent of hydration depends on the charge density of ions.
- Covalent compounds are soluble in organic solvents.
- Alkali metals with small size cannot react with ethyne.
- Elements having a diagonal relationship have similar chemical behavior.

Complete step by step answer:
The enthalpy of hydration is directly proportional to the charge density of the ion. Now, the charge density is directly proportional to the charge of the ion and inversely proportional to the radius of the ion. In other words, charge density of an ion is the ratio of the charge of the ion to its radius. So, we can write:
Charge density ${\text{ = }}\dfrac{{{\text{Charge}}}}{{{\text{Radius}}}}$
This means the ion with highest charge and smallest radius will have the highest charge density and hence the highest enthalpy of hydration. This is because when the charge density of an ion is high, the force of attraction between the ion and the polar end of the water molecule is also high. This makes the hydration energy smaller for smaller ions.
Now, the alkali metals group consists of lithium, sodium, potassium, rubidium and caesium. Among these, lithium lies at the top of the group and so it has the smallest size and thus the highest charge density. So, it will have the highest hydration enthalpy among the alkali metals. Hence, statement (a) is correct.
Lithium chloride has covalent character because of the small size of the lithium cation. Because of this covalency, lithium chloride is soluble in organic solvents like pyridine. Hence, statement (b) is wrong.
Lithium has very small size and high polarising power and hence high ionization energy due to which it is difficult to remove the outermost electron. So, lithium is the only alkali metal which does not react with ethyne to give ethynide. So, statement (c) is correct.
The density of lithium is about half that of water and so it first floats on the water surface and ultimately vanishes forming a colorless lithium hydroxide solution. Thus, it reacts very slowly with water. Magnesium has a diagonal relationship with lithium and hence it also reacts very slowly with water. So, statement (d) is also correct.

Hence the correct option is A, C and D.

Note: Because of the small size of lithium, it is very different from the rest of the alkali metals. Some more differences are:
Lithium forms a nitride but none of the other alkali metals form nitrides.
Lithium hydroxide is less basic than the other hydroxides of the group and so many of its salts are less stable.