Question

Based on lattice energy and other considerations which one of the following alkali metal chlorides is expected to have the highest melting point? [2005] (a) \(\mathrm{LiCl}\) (b) \(\mathrm{NaCl}\) (c) \(\mathrm{KCl}\) (d) \(\mathrm{RbCl}\)

Short answer

LiCl is expected to have the highest melting point due to its high lattice energy.

Step-by-step solution

Understanding Lattice Energy

Lattice energy is the energy released when ions come together to form a crystalline ionic solid. It is influenced by the charges of the ions and their distances. Higher lattice energy generally contributes to higher melting points.

Factors Affecting Lattice Energy

The two main factors are the charge of the ions and the size of the ions. All given options contain similar charges (1+ and 1-), so the focus is on ion size. Smaller ions lead to higher lattice energies because they can pack more closely together.

Comparing Ionic Sizes

Looking at ionic radii, Li+ is the smallest, followed by Na+, K+, and Rb+. Smaller cation sizes mean higher lattice energy. Therefore, LiCl is expected to have the highest lattice energy among the options.

Predicting Melting Points

Higher lattice energy typically results in a higher melting point. Since LiCl has the smallest cation and hence the highest lattice energy, it should theoretically have the highest melting point.

Key concepts

Ionic Size

Ionic size plays a critical role in determining the physical properties of ionic compounds, such as lattice energy and melting point.
The size of an ion is influenced by the number of electrons and the energy levels they occupy. In a group, as you move down the periodic table, ionic size generally increases.

• Li⁺ ion is smaller than Na⁺, K⁺, and Rb⁺ ions.
• A smaller ionic size means that ions can pack more closely together in a crystal lattice.
• Smaller distances between ions lead to stronger electrostatic forces.

These stronger forces result in greater lattice energy, as ions are tightly bound in the crystal structure. Hence, smaller ions like Li⁺ contribute to higher lattice energies when compared to larger ions like Rb⁺.

Melting Point

The melting point of an ionic compound is the temperature at which it turns from solid to liquid.
It is directly related to the strength of the forces holding the ions together.

• Ionic compounds with high lattice energy tend to have high melting points, as more energy is required to break the lattice apart.
• Melting involves overcoming the electrostatic forces of attraction between oppositely charged ions.
• Smaller ions, like Li⁺, create stronger attractions, and thus higher melting points.

This means that LiCl, with its small ionic size, will have the highest melting point among alkali metal chlorides.

Ionic Compounds

Ionic compounds are substances composed of ions that are held together by ionic bonds.
These bonds are formed through the transfer of electrons from one atom to another, resulting in oppositely charged ions.

• The primary characteristics of ionic compounds are:
  • They possess high melting and boiling points.
  • They are typically solid and crystalline at room temperature.
  • They conduct electricity when dissolved in water, as ions are free to move.
• Lattice energy is a key factor in determining these properties.

In simpler terms, ionic compounds form strong, stable structures and require significant energy to break down, which is reflected in their melting and boiling points. This stability is apparent in compounds like LiCl, which has a notably high melting point due to its strong ionic interactions.