Problem 104 - Chemistry Principles and Practice

Question

Match the following three lattice energies with the three compounds LiF, KCl, and CsI. Explain your answer. \(715 \mathrm{~kJ} / \mathrm{mol} \quad 582 \mathrm{~kJ} / \mathrm{mol}\) \(1036 \mathrm{~kJ} / \mathrm{mol}\)

Step-by-Step Solution

Answer

1036 kJ/mol for LiF, 715 kJ/mol for KCl, 582 kJ/mol for CsI.

Step 1: Understand the Relationship

Lattice energy is related to the charges of the ions and the distance between them. Higher charges and smaller ionic radii typically result in higher lattice energy.

Step 2: Rank Compounds by Expected Lattice Energy

Considering that all compounds have monovalent ions, we compare their ionic sizes: LiF (smallest ions), KCl (moderate-sized ions), and CsI (largest ions). Thus, the order of lattice energy from highest to lowest is LiF, KCl, CsI.

Step 3: Match Lattice Energies to Compounds

The highest lattice energy (1036 kJ/mol) corresponds to LiF due to its smallest ionic sizes. The moderate lattice energy (715 kJ/mol) matches KCl. The lowest lattice energy (582 kJ/mol) corresponds to CsI due to its largest ion sizes.

Key Concepts

Ionic Compounds

Ionic compounds are formed when atoms transfer electrons between each other, resulting in the creation of charged ions. This typically occurs between metals and non-metals.

• Metals, which are good electron donors, become positively charged ions (cations).
• Non-metals, which are good electron acceptors, become negatively charged ions (anions).

The formation of these ions creates a strong electrostatic force of attraction that holds the ions together, forming a lattice structure.
The strength of this attraction is a key factor in determining the compound's lattice energy. Ionic compounds usually have high melting and boiling points due to the strong interactions between ions. They are also generally soluble in water, where the attraction between the ions in the compound and water molecules helps dissolve the compound.

Ionic Radii

Ionic radii refer to the effective distance from the nucleus of an ion to the outermost electron shell. It plays a critical role in understanding the properties of ionic compounds.

• Smaller ionic radii usually indicate stronger attractions between ions.
• This is because ions can come closer together, increasing the electrostatic attraction.

In the example of LiF, KCl, and CsI, the ionic radii vary significantly. LiF has the smallest ions, meaning that its ions are more tightly packed in the lattice, leading to a higher lattice energy.
In contrast, CsI has the largest ions, where the distance between them reduces the strength of the attractions.
Therefore, as ionic radii increase, lattice energy tends to decrease, influencing the stability and properties of the compound.

Ion Charges

Ion charges, represented by the magnitude of the positive or negative charge on an ion, significantly affect the lattice energy of ionic compounds. The lattice energy is directly proportional to the product of the charges of the ions.
For instance, if each ion in a compound carries a charge of +1 or -1, as in the case with LiF, KCl, and CsI, the lattice energies are generally moderate.

• However, as the absolute value of the charges increases (e.g., if ions carry a +2 or -2 charge), lattice energy significantly increases.

This concept forms the basis for understanding why compounds with higher charged ions have significantly higher lattice energy, as a result of stronger electrostatic forces.
Therefore, when ion charges are the same, ionic radii become the dominant factor in determining lattice energy.