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 (a): CsCl > KCl > NaCl > LiCl

Step 1: Understanding Alkali Metal Chlorides

Alkali metal chlorides such as LiCl, NaCl, KCl, and CsCl form ionic compounds. Their stability is influenced by the lattice energy, which depends on the size of the ions and the charge.

Step 2: Recognizing the Role of Ionic Radius

The stability of these chlorides generally increases with the size of the cation. Larger cations lead to a larger size difference with chloride, lowering lattice energy but fitting better in a structural sense.

Step 3: Identify Size Order of Cations

In the periodic table, as we move down from Lithium (Li) to Cesium (Cs), the ionic size increases. So, the order of size is Li < Na < K < Cs.

Step 4: Apply Stability Trend to Given Options

The most stable alkali chlorides are those formed with larger cations, due to their optimal ionic packing. Therefore, the order of stability is CsCl > KCl > NaCl > LiCl.

Step 5: Match with Given Answer Choices

Review the given options: (a) CsCl > KCl > NaCl > LiCl matches our determined order. Confirm this aligns with our reasoning based on ionic sizes and stability trends.

Key Concepts

Lattice Energy

Lattice energy is a crucial concept when understanding the stability of ionic compounds such as alkali metal chlorides. It represents the energy required to separate a mole of an ionic solid into its gaseous ions. Essentially, it reflects the strength of the attraction between the ions in a crystal lattice.

Several factors affect lattice energy, including:

• Charge of the ions: Higher charges result in stronger attractions.
• Ionic radius: Smaller ions pack closely together, increasing the attraction.

For alkali metal chlorides, lattice energy decreases as you move down the group on the periodic table. Larger cations like Cs have lower lattice energies because the larger ionic radius leads to weaker ionic attractions. This means that compounds with larger cations are generally less tightly bound, but this also allows them to fit more effectively within a crystal structure.

Ionic Radius

The ionic radius of an element is a straightforward but essential concept in chemistry. It measures the size of an ion in a crystal lattice. As you move down a group on the periodic table, the ionic radius increases.

This occurs because each successive element has an additional electron shell:

• Lithium (Li) - smallest radius
• Sodium (Na)
• Potassium (K)
• Cesium (Cs) - largest radius

The increase in ionic radius affects the placement and packing efficiency of ions within a crystal lattice. Larger ions, like those of CsCl, tend to create a stable structure due to optimal space-filling in the lattice, despite the decrease in lattice energy. This is why cesium chloride is often more stable than lithium chloride.

Periodic Table Trends

Understanding periodic table trends is vital to predicting the properties of alkali metal chlorides. Key periodic trends include ionic radius, which increases as you move down a group, and electronegativity, which typically decreases.

Alkali metals, found in Group 1, exhibit clear trends as you go down the column:

• Ionic size increases: Li < Na < K < Cs
• Lattice energy decreases with larger ionic sizes
• Overall stability tends to increase with size due to better cation-anion structural fitting

Therefore, these trends help explain why larger cations form more stable chlorides, as seen in the stability order CsCl > KCl > NaCl > LiCl. Knowing these trends aids in predicting chemical behavior and stability of various compounds, influencing how they are used in applications and technology.