Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

The halide having the highest melting point is (a) \(\mathrm{NaF}\) (b) \(\mathrm{NaCl}\) (c) \(\mathrm{NaBr}\) (d) NaI

Step-by-Step Solution

Answer

The compound with the highest melting point is NaF.

Step 1: Understanding the Problem

The problem asks us to determine which sodium halide compound has the highest melting point among the given options.

Step 2: Identify Factors Affecting Melting Point

Sodium halides are ionic compounds, and their melting points are influenced by the strength of the ionic bonds. The ionic bond strength is largely determined by the lattice energy, which depends on the charge and size of the ions.

Step 3: Comparing Ion Sizes

The compounds given are formed with halide ions: F⁻, Cl⁻, Br⁻, and I⁻. The size of these ions increases in the order F⁻ < Cl⁻ < Br⁻ < I⁻. As the size of the ion increases, the lattice energy decreases because the distance between ions increases, thus reducing the ionic bond strength.

Step 4: Determining the Compound with Highest Melting Point

Since \(\mathrm{NaF}\) has the smallest halide ion size, it will have the highest lattice energy and therefore the strongest ionic bonds. Hence, \({\mathrm{NaF}}\) is likely to have the highest melting point.

Key Concepts

Ionic Compounds

Ionic compounds are made up of positive and negative ions that are bonded together through electrostatic forces. These forces are known as ionic bonds and are usually formed between metals and non-metals. For example, in sodium fluoride (\(\mathrm{NaF}\)), sodium (\(\mathrm{Na}^+\)) is a metal, and fluoride (\(\mathrm{F}^-\)) is a non-metal. The two ions attract each other due to their opposite charges, creating a stable ionic compound.

Characteristics of ionic compounds include:

• They have high melting and boiling points due to the strong attraction between ions.
• They are typically solid at room temperature.
• They can conduct electricity when dissolved in water or melted because the ions are free to move.

These characteristics are important in determining properties such as melting point, which is directly related to the strength of the ionic bonds present in the compound.

Melting Point

The melting point of a substance is the temperature at which it transitions from a solid to a liquid. Ionic compounds generally have high melting points because substantial energy is required to overcome the strong ionic bonds holding the ions in place.

Several factors can affect the melting point of ionic compounds:

• Ionic bond strength: The stronger the ionic bonds, the higher the melting point.
• Lattice energy: A greater lattice energy implies a higher melting point.
• Size of ions: Smaller ions typically contribute to stronger ionic bonds, leading to higher melting points.

In the case of sodium halides, \(\mathrm{NaF}\) has the highest melting point among the options provided. This is due to the smallest ion size of fluoride, which results in a strong bond and high lattice energy.

Lattice Energy

Lattice energy is a measure of the strength of the ionic bonds in a crystalline compound. It is the energy required to completely separate one mole of a solid ionic compound into its gaseous ions. This measure is crucial in determining the properties of an ionic compound.

Several key factors influence lattice energy:

• Ionic charge: Higher charges result in greater lattice energies.
• Ion size: Smaller ions lead to larger lattice energies as the ions can come closer together, enhancing the electrostatic interactions.

With sodium halides, fluoride ions (\(\mathrm{F}^-\)) are the smallest among the halide ions listed, which means \(\mathrm{NaF}\) will have the highest lattice energy. This correlates to its high melting point as more energy is required to break the strong bonds between ions in the lattice structure.