Problem 137 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement(s) is/are true? (a) Stability of alkali metal peroxide increases with increase in atomic number (b) Hydration energy of \(\mathrm{AgF}\) is higher than its lattice energy (c) Anhydrous \(\mathrm{MgCl}_{2}\) cannot be prepared by direct heating of \(\mathrm{MgCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (d) The milk of magnesia used as antacid is chemically \(\mathrm{MgO}+\mathrm{MgCl}_{2}\)

Step-by-Step Solution

Answer

Statements (a), (b), and (c) are true, while (d) is false.

Step 1: Understanding the stability of Alkali Metal Peroxides

Alkali metal peroxides become more stable as the atomic number of the metal increases. This is due to the larger size and more polarizable nature of heavier alkali metals which can stabilize the larger peroxide ion better than lighter alkali metals. Hence, statement (a) is true.

Step 2: Comparing Hydration Energy and Lattice Energy of Silver Fluoride

The hydration energy of \(AgF\) is indeed higher than its lattice energy. This is because \(Ag^{+}\) is small and highly charged, interacting very strongly with water molecules, making hydration energy larger. Thus, statement (b) is true.

Step 3: Preparation Issues with Anhydrous Magnesium Chloride

Anhydrous \(MgCl_2\) cannot be prepared by the direct heating of \(MgCl_2 \cdot 6H_2O\) because upon heating, the water of crystallization decomposes, forming \(MgO\) rather than removing water to make anhydrous \(MgCl_2\). Therefore, statement (c) is true.

Step 4: Composition of Milk of Magnesia

Milk of Magnesia is composed of \(Mg(OH)_2\) and not a combination of \(MgO+MgCl_2\). It is primarily used as an antacid due to the neutralizing effect of \(Mg(OH)_2\). Thus, statement (d) is false.

Key Concepts

Stability of Alkali Metal Compounds

Alkali metal compounds, including peroxides, vary in stability based on the alkali metal involved. As we move down the group in the periodic table, the atomic number of alkali metals increases. This increase results in larger and more polarizable ions, which can effectively stabilize larger anions like the peroxide ion due to the size match and electron cloud dynamics. For instance, potassium peroxide (K2O2) is more stable than lithium peroxide (Li2O2). Therefore, statement (a) from the exercise is true, highlighting that the stability of alkali metal peroxides increases with higher atomic numbers. This is due to the enhanced ability of larger alkali metal ions to stabilize the compound's ionic structure.

Hydration Energy vs Lattice Energy

The concept of hydration energy and lattice energy is crucial in understanding the dissolution and stability of ionic compounds. Hydration energy refers to the energy released when ions are solvated by water molecules. In the case of silver fluoride (AgF), the hydration energy exceeds the lattice energy, which is the energy required to separate oppositely charged ions in a compound. This disparity arises because the small silver ion (Ag+) can interact strongly with the dipole in water, releasing significant energy during dissolution. Hence, for AgF, the statement (b) that its hydration energy is higher than its lattice energy is indeed correct.

Preparation of Anhydrous Compounds

Creating anhydrous compounds often requires specific methods due to their complex behaviors on heating. A typical example is magnesium chloride (MgCl2). When hydrated magnesium chloride (MgCl2·6H2O) is heated to remove water, it doesn't simply transition to an anhydrous state. Instead, heating causes the hydration water to decompose and react, forming magnesium oxide (MgO) rather than yielding anhydrous MgCl2. This is why direct heating is ineffective for preparing anhydrous MgCl2 from its hydrated form, making statement (c) true.

Composition of Milk of Magnesia

Milk of Magnesia is a common antacid used to neutralize stomach acid. It is primarily composed of magnesium hydroxide (Mg(OH)2) and not a mixture of magnesium oxide (MgO) and magnesium chloride (MgCl2) as incorrectly suggested by statement (d). The formulation of Milk of Magnesia leverages the alkaline property of Mg(OH)2, which helps to neutralize excess gastric acid effectively. Hence, the composition provided in statement (d) is false, clarifying the true chemical makeup of this medicinal compound.