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 \(\mathrm{MgCl}_{2} .6 \mathrm{H}_{2} \mathrm{O}\) (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; (d) is false.

Step 1: Evaluate Statement (a)

Statement (a) is about the stability of alkali metal peroxides. As you move down the group in the alkali metals, the size of the metal cations increases, resulting in larger ionic sizes, which can better stabilize the large peroxide ion. Therefore, the stability of alkali metal peroxides increases with an increase in the atomic number. Thus, Statement (a) is true.

Step 2: Evaluate Statement (b)

For AgF , the compound is ionic in nature, and it's common for ionic compounds to have higher lattice energies than hydration energies. However, AgF is unique due to the small size of the Ag^+ ion and the highly polarizable F^- ion, leading to a high hydration energy that exceeds its lattice energy. Thus, Statement (b) is true.

Step 3: Evaluate Statement (c)

Anhydrous MgCl_2 cannot be prepared by directly heating MgCl_2 .6H_2O because upon heating, MgCl_2 .6H_2O tends to lose HCl gas due to hydrolysis, rather than simply losing water molecules. This results in the formation of a oxychloride rather than pure anhydrous MgCl_2 . Thus, Statement (c) is true.

Step 4: Evaluate Statement (d)

Milk of magnesia is actually a suspension of Mg(OH)_2 in water. It does not consist of MgO or MgCl_2 . Thus, Statement (d) is false.

Key Concepts

Alkali Metal Peroxides

Alkali metal peroxides are chemical compounds that consist of alkali metals such as lithium, sodium, and potassium bonded with the peroxide ion (O₂²⁻). As you go down the group on the periodic table, from lithium to cesium, the atomic number increases. This also leads to an increase in ionic size. In terms of stability, larger cations stabilize the peroxide ion more effectively, meaning the stability of these peroxides increases with the atomic number.
This is because the larger cation can better accommodate the bulky peroxide ion within its ionic structure.
Some points about alkali metal peroxides:

• They can serve as oxygen carriers.
• They react with water to produce hydrogen peroxide and a hydroxide.
• Applications include use in certain batteries and as bleaching agents.

Hydration Energy

Hydration energy is the amount of energy released when ions are surrounded by water molecules during the dissolution process. It is directly related to the strength of the ion's interaction with water. In the case of silver fluoride (AgF), something interesting happens. Usually, for ionic compounds, the lattice energy is greater than the hydration energy. However, AgF is an exception.
The small size of the silver ion ( Ag^+ ) and the polarizable fluoride ion ( F^- ) lead to a particularly high hydration energy that surpasses its lattice energy. This unusual situation promotes the dissolving of AgF in water. Remember:

• Higher hydration energies make compounds more soluble in water.
• The comparison between lattice and hydration energies informs about the solubility and stability of ionic compounds.

Lattice Energy

Lattice energy is a measure of the strength of the forces between the ions in an ionic solid. It's the energy required to form a crystalline lattice of ions from gaseous ions. Among the factors influencing lattice energy: - The charge on the ions: Greater charges result in stronger attractions. - The size of the ions: Smaller ions pack more closely, increasing the electrostatic forces. For AgF , the lattice energy is lower than the hydration energy due to its specific ionic characteristics. This affects its behavior when dissolved in water, making it more easily solvable than one might typically expect. Important points:

• High lattice energy usually implies a high melting point.
• It contributes to non-volatile and non-conductive properties in solid form.
• Informs about the solubility trends in ionic compounds.

Anhydrous Compounds

Anhydrous compounds are substances that do not contain water, especially within their crystal structure. Preparing anhydrous magnesium chloride ( MgCl₂ ) directly from its hydrated form ( MgCl₂ .6H₂O ) involves complexities. Upon heating MgCl₂ .6H₂O , instead of just losing water, hydrolysis can occur, releasing HCl gas and forming a basic salt. This makes obtaining pure anhydrous MgCl₂ directly through heating difficult.
Key considerations with anhydrous compounds:

• The absence of water is crucial for certain industrial applications.
• They are typically more stable than their hydrated counterparts against certain reactions.
• Often prepared through methods other than simple heating, due to potential decomposition.

Milk of Magnesia

Milk of magnesia is a common over-the-counter remedy used primarily as an antacid and a laxative. Chemically speaking, it is a suspension of magnesium hydroxide ( Mg(OH)₂ ) in water. Contrary to some misconceptions, it does not comprise magnesium oxide ( MgO ) or magnesium chloride ( MgCl₂ ). The suspension form makes it easy to ingest and effective in neutralizing stomach acids and providing relief from indigestion.
Reasons milk of magnesia is useful:

• Neutralizes stomach acid and reduces heartburn.
• Acts as a gentle laxative by drawing water into the intestines.
• It's a safe, accessible option available in liquid or tablet form.