Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

The stability of the following alkali metal chlorides follows the order (a) \(\mathrm{CsCl}>\mathrm{KCl}>\mathrm{NaCl}>\mathrm{LiCl}\) (b) \(\mathrm{NaCl}>\mathrm{KCl}>\mathrm{LiCl}>\mathrm{CsCl}\) (c) \(\mathrm{KCl}>\mathrm{CsCl}>\mathrm{NaCl}>\mathrm{LiCl}\) (d) \(\mathrm{LiCl}>\mathrm{KCl}>\mathrm{NaCl}>\mathrm{CsCl}\)

Step-by-Step Solution

Answer

Option (b) is the correct answer: NaCl > KCl > LiCl > CsCl.

Step 1: Understanding Ionic Radii and Stability

The stability of ionic compounds like alkali metal chlorides is influenced by the lattice energy, which depends on the sizes of the ions involved. Generally, smaller ions form stronger ionic bonds, leading to greater stability.

Step 2: Analyzing Sodium to Cesium Chlorides

In alkali metal chlorides, as the size of the cation increases (from Li to Cs), the lattice energy decreases because larger ions lead to weaker bonds. This trend usually means that smaller cations like Na and Li form more stable chlorides compared to larger cations like Cs.

Step 3: Reviewing the Options

Given the information on ionic radii and lattice energy, we anticipate that the order from most stable to least stable should generally involve the smaller ions showing more stability than the larger ions. Hence, inspect the options provided.

Step 4: Correct Answer Verification

Option (b) `NaCl > KCl > LiCl > CsCl` suggests NaCl as most stable, which aligns more with expected trends as NaCl involves smaller, effectively charged ions compared to KCl and CsCl. LiCl, while having a small cation, is less stable due to its high lattice energy not compensating for its lower coordination number.

Key Concepts

Ionic Radii

In chemistry, ionic radii refer to the sizes of ions in a crystalline lattice.
Ions are atoms that have lost or gained electrons, resulting in a positive or negative charge.
For alkali metals, as you move down the group from lithium to cesium, the ionic radii increase.

• Lithium ions (abla\( \text{Li}^+ \)) have the smallest ionic radii among alkali metals.
• Cesium ions (abla\( \text{Cs}^+ \)) have the largest.

The size of the ionic radii affects how closely ions can pack together in a crystal lattice.
Smaller ion size generally allows for closer packing and stronger interactions between ions.

Lattice Energy

Lattice energy is the energy released when ions come together to form a solid ionic compound from gaseous ions.
This energy is a crucial factor that determines the overall stability of ionic compounds.
The smaller the ions and the higher their charges, the higher the lattice energy.

• For example, lithium chloride (abla\( \text{LiCl} \)) has a high lattice energy due to the small size of lithium ions.

Higher lattice energies indicate stronger bonds between ions, contributing to greater stability of the compound.
Thus, compounds with high lattice energies usually have high melting points and are less soluble in water.

Alkali Metals

Alkali metals include abla\( ext{Li}, ext{Na}, ext{K}, ext{Rb}, ext{Cs} \).
These metals are highly reactive and are found in Group 1 of the periodic table.
They form cations by losing one electron, which gives them a stable electronic configuration.

• abla\( \text{Li}^+ \), abla\( \text{Na}^+ \), abla\( \text{K}^+ \), abla\( \text{Rb}^+ \), abla\( \text{Cs}^+ \) are all common alkali metal cations.

Due to their larger atomic size down the group, alkali metals from top to bottom tend to form less stable compounds with non-metals.
This is because they have larger radii, resulting in weaker bond formation.

Chlorides

Chlorides are compounds where chlorine combines with other elements.
In the context of alkali metals, chlorides like abla\( \text{NaCl} \), abla\( \text{KCl} \), abla\( \text{LiCl} \), and abla\( \text{CsCl} \) form when alkali metal ions bond with chloride ions.

• The chloride ion (abla\( \text{Cl}^- \)) is relatively large compared to alkali metal cations.

The formation of these compounds strongly relies on the balance between ionic sizes and charges.
A stable chloride will typically have a well-matched size and charge between its constituent ions, but may still vary in stability depending on specific lattice interactions.

Cation Size

Cation size plays a key role in determining the properties of ionic compounds.
In alkali metal chlorides, the cation size increases as we move from lithium to cesium.

• This increase in size leads to changes in lattice energy and bonding strength.
• Smaller cations like abla\( \text{Na}^+ \) may lead to more stable ionic compounds due to stronger ionic bonds.

Therefore, while larger cations may form weaker ionic compounds, there are exceptions where small cation size can lead to less stability if not energetically favorable, as seen with abla\( \text{LiCl} \).