Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

The magnitude of enthalpy for formation of alkali metal halides decreases in the order (a) fluoride \(>\) chloride \(>\) iodide \(>\) bromide (b) iodide \(>\) bromide \(>\) chloride \(>\) fluoride (c) fluoride \(>\) chloride \(>\) bromide \(>\) iodide (d) bromide \(>\) iodide \(>\) fluoride \(>\) chloride

Step-by-Step Solution

Answer

Option (c) fluoride > chloride > bromide > iodide.

Step 1: Understand the Concept

The enthalpy of formation is influenced by factors like lattice energy and ion size. For alkali metal halides, smaller halide ions have stronger attractions, generally leading to higher lattice energies and magnitudes of formation enthalpy. Thus, smaller ions typically result in greater formation enthalpy.

Step 2: Rank the Halides by Size

The size of halide ions increases down the group in the periodic table. Therefore, the order from smallest to largest ion is: fluoride (F⁻), chloride (Cl⁻), bromide (Br⁻), iodide (I⁻). Smaller ions have stronger lattice energies.

Step 3: Determine Formation Enthalpy Order

Given the ion sizes and stronger lattice energies for smaller ions, fluoride will have the greatest enthalpy of formation followed by chloride, bromide, and iodide. The correct order in decreasing formation enthalpies is: fluoride > chloride > bromide > iodide.

Step 4: Match with Given Options

The order found (fluoride > chloride > bromide > iodide) corresponds with option (c). Therefore, option (c) is the correct answer.

Key Concepts

Alkali Metal Halides

Alkali metal halides are compounds formed between alkali metals (elements in Group 1 of the periodic table, such as lithium, sodium, and potassium) and halides, which are ions derived from halogen elements (elements in Group 17, like fluorine, chlorine, bromine, and iodine). These compounds are known for their ionic bonds, which result from the electrostatic attraction between the positive alkali metal ions and the negative halide ions.

Alkali metal halides are important in various industrial applications due to their stability, solubility, and chemical properties. One notable characteristic of these compounds is that they generally have high melting and boiling points. This is because their ionic bonds are quite strong due to the significant charge differences and the tight packing of the ions in a lattice structure. Understanding the formation and properties of these compounds helps in analyzing their uses, ranging from salt manufacturing to use in lighting technologies and chemical synthesis.

Lattice Energy

Lattice energy is a measure of the strength of the forces holding the ions together in an ionic solid. It is defined as the energy required to separate one mole of a solid ionic compound into its gaseous ions. The magnitude of lattice energy depends on two primary factors: the charge of the ions and the size of the ions.

High lattice energy indicates strong electrostatic attractions in the ionic compound, contributing to high melting points and solubility properties. In alkali metal halides, smaller ions tend to lead to higher lattice energies because the ions are more closely packed, allowing stronger electrostatic interactions. As a result, compounds made with smaller halide ions typically require more energy to break apart this lattice.

• Smaller ionic radius: Increases attraction between ions, raising lattice energy.
• Greater charge on ions: Leads to stronger ionic bonds, elevating lattice energy.

Understanding lattice energy helps explain the stability and reactivity of ionic compounds.

Halide Ion Size

Halide ion size is a critical factor in determining the properties of ionic compounds like alkali metal halides. As you move down the halogen group in the periodic table, ion size increases. This is due to the addition of electron shells, which make the ions larger.

The sequence of halide ions based on size from smallest to largest is: fluoride (F⁻), chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻). Smaller ionic sizes result in ions being more closely packed in the solid lattice of the ionic compound. This tight packing enhances the overall lattice energy of the compound.

• Fluoride ions: Smallest size, leading to high lattice energy.
• Chloride ions: Larger than fluoride, but still smaller than bromide and iodide.
• Bromide ions: Larger than chloride, resulting in less lattice energy.
• Iodide ions: Largest size, leading to the lowest lattice energy of the group.

Knowing the sizes of these ions helps predict and understand the formation enthalpy and other physical properties of the alkali metal halides.

Periodic Table Trends

The periodic table reveals essential trends that help explain chemical behavior, such as ion size and enthalpy changes. In general, elements within the same group have similar chemical properties but differ in size and reactivity as you move down the group.

For halogens:

• Atomic and ionic sizes increase down the group due to added electron shells.
• Reactivity among halogens generally decreases from fluorine to iodine because larger atoms hold onto their electrons less tightly.
• Enthalpy of formation of halides decreases as the ion's size increases because larger ions form weaker ionic bonds.

For alkali metals:

• Size increases down the group, but this trend affects other properties only subtly compared to halogens.
• Alkali metals are highly reactive, especially with halogens, due to their lone electrons in their outer shell.

Incorporating these trends helps students contextualize the properties and reactions of elements, which is crucial for understanding chemistry at an intermediate to advanced level.