Animated Solution for Chemistry - s and p-Block Elements: Among the statements (A)-(D), the correct ones are (A) lithium has the highest hydration enthalpy among 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 ${\text{H}}_2\text{O}$.

When we study the s-block elements, the first member of each group always demands special attention. Lithium, sitting at the very top of the alkali metals, is a classic rebel. Because of its exceptionally small atomic and ionic size, it exhibits properties that are quite different from its heavier siblings like Sodium and Potassium. This question is a perfect test of how well you understand these anomalous properties.

Let's break down the four statements provided and uncover the chemistry behind each one.

The Power of Being Small

The first statement claims that lithium has the highest hydration enthalpy among alkali metals. To verify this, we need to recall what hydration enthalpy depends on. When an ion is dropped into water, the polar water molecules surround it. The energy released in this process is the hydration enthalpy.

This energy is directly proportional to the charge density of the ion, which is the ratio of its charge to its radius. Since all alkali metal ions have a $+1$ charge, the deciding factor is the ionic radius. The $L{i}^{+}$ ion is the smallest of the bunch. Therefore, it has the highest charge density, allowing it to attract water molecules most intensely.

This results in the maximum release of energy, meaning Statement (A) is absolutely correct.

Fajans' Rules and Solubility

Next, we look at the solubility of Lithium Chloride ($LiCl$) in pyridine. Pyridine is an organic, non-polar solvent. A common trap is to assume that since $LiCl$ is formed from a metal and a non-metal, it must be purely ionic and therefore insoluble in organic solvents.

However, we must apply Fajans' Rules. The tiny $L{i}^{+}$ ion has a very high polarizing power. When it approaches the larger $C{l}^{-}$ ion, it distorts the chloride's electron cloud, pulling electron density into the space between the nuclei. This introduces a significant covalent character into the $LiCl$ bond.

Because "like dissolves like," covalent compounds tend to dissolve in organic solvents. Thus, $LiCl$ is actually soluble in pyridine. Statement (B) claims it is insoluble, making it incorrect.

The Ethyne Exception

Statement (C) discusses the reaction of lithium with ethyne ($HC\equiv CH$). Terminal alkynes like ethyne have weakly acidic hydrogen atoms. Heavier alkali metals like Sodium react readily with ethyne to form ethynides (like $N{a}_2{C}_2$) and release hydrogen gas.

Lithium, however, is an exception. Due to its high ionization energy and small size, it does not react with ethyne to form ethynide under normal conditions. It prefers to react directly with carbon at high temperatures to form lithium carbide ($L{i}_2{C}_2$). Therefore, Statement (C) is correct.

The Diagonal Bond with Magnesium

Finally, Statement (D) compares the reactivity of Lithium and Magnesium with water. If you drop a piece of Sodium or Potassium into water, you get a vigorous, often explosive reaction. But Lithium? It reacts quite gently and slowly.

This brings us to the concept of the diagonal relationship. Because Lithium and Magnesium have a similar charge-to-size ratio (ionic potential), they share many chemical properties. Just like Lithium, Magnesium also reacts very slowly with cold water (in fact, Magnesium prefers hot water or steam).

Since both metals react slowly with water, Statement (D) is correct.

After a careful analysis, we have determined that Statements (A), (C), and (D) are correct, while Statement (B) is incorrect. Matching this with our options, the correct choice is (c).

Mastering the exceptions and diagonal relationships in the s-block is a surefire way to secure easy marks in JEE Chemistry!