2840
Question
Among the statements (a)-(d), the correct ones are :
(a) Lithium has the highest hydration enthapy among the alkali metals.
(b) Lithium chloride is insoluble in pyridine.
(c) Lithium cannot form enthynide upon its reaction with ethyne.
(d) Both lithium and magnesium react slowely with H2O.
Among the statements (a)-(d), the correct ones are :
(a) Lithium has the highest hydration enthapy among the alkali metals.
(b) Lithium chloride is insoluble in pyridine.
(c) Lithium cannot form enthynide upon its reaction with ethyne.
(d) Both lithium and magnesium react slowely with H2O.
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Solution
The correct option is D (a), (c) and (d) only
The enthalpy of hydration is directly proportional to the change density of the ion. Now, the charge density is directly proportional to the charge of the ion 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. Do, we write:
charge density =ChargeRadius
This means the ion with highest charge and smallest radius will have the highest charge density and hence the highest enthlpy 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 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 colourless 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.
The enthalpy of hydration is directly proportional to the change density of the ion. Now, the charge density is directly proportional to the charge of the ion 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. Do, we write:
charge density =ChargeRadius
This means the ion with highest charge and smallest radius will have the highest charge density and hence the highest enthlpy 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 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 colourless 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.
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