Question

Based on lattice energy and other considerations which one of the following alkali metal chlorides is expected to have the highest melting point? (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 the Concept

The melting point of ionic compounds is affected by the lattice energy, which is the energy required to separate one mole of a solid ionic compound into gaseous ions. Lattice energy is influenced by the charge and size of the ions involved.

Considering Lattice Energy

Lattice energy generally increases with higher ionic charges and smaller ionic sizes because smaller ions can pack closely together, increasing the electrostatic attraction between them. As a result, compounds with higher lattice energies typically have higher melting points.

Linking Alkali Metals with Lattice Energy

Among alkali metals ( Li, Na, K, Rb ) paired with chloride (Cl) ions, the Li+ ion is the smallest, followed by Na+, K+, and Rb+. The smaller the size of the cation, the greater the lattice energy, hence a higher melting point.

Concluding with Highest Melting Point

Since Li+ is the smallest cation among the given options, LiCl is expected to have the highest lattice energy and, therefore, the highest melting point of the alkali metal chlorides given.

Key concepts

Melting Point

The melting point of a substance is a critical property in understanding how that substance behaves under heat. It is the temperature at which the crystalline structure of a solid breaks down to become a liquid. For ionic compounds, the melting point is significantly influenced by the lattice energy. Lattice energy is tied to the bonds holding the ionic crystals together. Strong electrostatic forces result from oppositely charged ions, and these forces must be overcome for melting to occur. This means that heightened lattice energy leads to a higher melting point. When examining different ionic compounds, smaller ions generally create greater lattice energies as they pack tightly together. Thus, compounds with small ions nearly always boast much higher melting points. Therefore, by knowing lattice energy and ion size, one can predict melting points in ionic compounds efficiently.

Ionic Compounds

Ionic compounds are formed when metal and non-metal ions bond through electrostatic attraction. They arrange themselves into an ionic lattice, a repeating three-dimensional pattern that maximizes the ionic interactions. Key characteristics of ionic compounds include:

• High melting and boiling points due to strong ionic bonds.
• Conductivity when dissolved in water, as ions are free to move and conduct electricity.
• Generally solid and crystalline at room temperature.

The properties of ionic compounds are shaped by the lattice structure and the strength of the ionic bonds. A compound with high lattice energy indicates a strong ionic interaction, which correlates with high melting points.

Alkali Metal Chlorides

Alkali metal chlorides consist of alkali metals like lithium ( Li ), sodium ( Na ), potassium ( K ), and rubidium ( Rb ) paired with chlorine ( Cl ). These compounds show fascinating trends in their chemical and physical properties, primarily influenced by their ionic sizes and lattice energies. Alkali metal chlorides share common ionic properties:

• They are typically white, crystalline solids at room temperature.
• Easily soluble in water due to the interaction between the ions and water molecules.
• High melting points related to their lattice energy, which decreases as the ionic radius increases.

As we go down the group in the periodic table, alkali metals increase in ionic size. Therefore, lithium chloride ( LiCl ) has the smallest cation and thus the highest lattice energy, leading to the highest melting point among the alkali metal chlorides.