Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following orders are correct? (i) Thermal stability \(\mathrm{BeCO}_{3}<\mathrm{MgCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{BaCO}_{3}\) (ii) Basic nature \(\mathrm{LiOH}>\mathrm{NaOH}>\mathrm{KOH}>\mathrm{RbOH}>\mathrm{CsOH}\) (iii) Solubility in water \(\mathrm{BeSO}_{4}<\mathrm{MgSO}_{4}<\mathrm{CaSO}_{4}<\mathrm{SrSO}_{4}<\mathrm{BaSO}_{4}\) (iv) Melting point \(\mathrm{NaCl}>\mathrm{KCl}>\mathrm{RbCI}>\mathrm{CsCl}>\mathrm{LiCl}\) (a) (i), (iv) (b) (i), (ii), (iv) (c) (ii), (iii) (d) (i), (ii) (iii), (iv)

Step-by-Step Solution

Answer

The correct orders are (i) and (iv), so option (a) is correct.

Step 1: Analyze Thermal Stability Order

The order of thermal stability for alkaline earth metal carbonates is based on the size of the cation. As we move down the group, the cation size increases and the lattice energy decreases, leading to increased thermal stability. Therefore, the order \( \mathrm{BeCO}_{3}<\mathrm{MgCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{BaCO}_{3} \) is correct.

Step 2: Examine Basic Nature Order

The basic nature of hydroxides increases down the group because the larger the cation, the more soluble the hydroxide is in water, which increases its basic nature. Hence, the expected order is \( \mathrm{LiOH}<\mathrm{NaOH}<\mathrm{KOH}<\mathrm{RbOH}<\mathrm{CsOH} \), which is the reverse of the order given. Thus, this order is incorrect.

Step 3: Evaluate Solubility in Water Order

The solubility of sulfates decreases down the group due to the increase in lattice energy. The given order \( \mathrm{BeSO}_{4}<\mathrm{MgSO}_{4}<\mathrm{CaSO}_{4}<\mathrm{SrSO}_{4}<\mathrm{BaSO}_{4} \) is incorrect because it should be reversed.

Step 4: Check Melting Point Order

The melting point order for alkali metal chlorides is based on the packing efficiency and size of the ions. Generally, the melting point decreases down the group because larger ions result in less efficient packing. Thus, the given order \( \mathrm{NaCl} > \mathrm{KCl} > \mathrm{RbCl} > \mathrm{CsCl} > \mathrm{LiCl} \) is generally correct, except for \( \mathrm{LiCl} \), which should be placed between \( \mathrm{NaCl} \) and \( \mathrm{KCl} \).

Key Concepts

Thermal Stability of Carbonates

The thermal stability of carbonates is heavily influenced by the size of the cation linked to the carbonate. As we look at alkaline earth metal carbonates, the cation size increases down the group: starting from beryllium carbonate (\(\mathrm{BeCO}_3\)) to barium carbonate (\(\mathrm{BaCO}_3\)). Larger cations result in lower lattice energy loss during decomposition, thereby increasing thermal stability.

Here's a simple way to remember it:

• Larger cations = Higher thermal stability
• Lattice energy decreases with increasing cation size

Imagine small boxes (cations) holding even smaller marbles (carbonate). Bigger boxes don't release the marbles easily, making them more stable under heat. That’s why \(\mathrm{BeCO}_3\) is the least stable, while \(\mathrm{BaCO}_3\) is the most stable in this sequence.

Basic Nature of Hydroxides

In alkaline hydroxides, the basic nature is directly related to the cation size. The larger the cation, the more the hydroxide can dissociate in water, enhancing its basic properties. This means that as we move down the group in the periodic table, the basicity of hydroxides increases.

Key points are:

• Bigger cations mean more dissociation in water
• More dissociation equals stronger base

The incorrect order given (\(\mathrm{LiOH} > \mathrm{NaOH} > \mathrm{KOH} > \mathrm{RbOH} > \mathrm{CsOH}\)) should be flipped to reflect the correct sequence - \(\mathrm{LiOH}\) being the least basic and \(\mathrm{CsOH}\) as the most basic.

Solubility of Sulfates

Sulfates generally become less soluble in water as you move down the group from beryllium to barium. This is due to the increase in lattice energy, which overpowers the hydration energy provided by water molecules.

Important things to remember:

• Solubility of sulfates decreases with cation size
• Lattice energy's role becomes more significant

Therefore, the given order \(\mathrm{BeSO}_4 < \mathrm{MgSO}_4 < \mathrm{CaSO}_4 < \mathrm{SrSO}_4 < \mathrm{BaSO}_4\) is incorrect, as it should actually be reversed to correctly represent increasing insolubility.

Melting Points of Chlorides

The melting points of alkali metal chlorides are dictated by the size of the ions and how efficiently they pack together in a crystal lattice. Larger ions hinder efficient packing, thus lowering the melting points.

Keep in mind:

• As ions grow bigger, melting points drop
• Packing inefficiency leads to lower melting points

Thus, while the given order \(\mathrm{NaCl} > \mathrm{KCl} > \mathrm{RbCl} > \mathrm{CsCl} > \mathrm{LiCl}\) generally makes sense, \(\mathrm{LiCl}\) is an exception due to its unique small size and packing efficiency, and should be between \(\mathrm{NaCl}\) and \(\mathrm{KCl}\).