6.5.1 Factors Affecting Lattice Energy

This topic explains the factors that affect the magnitude of lattice energy in ionic compounds. It describes how ionic charge and ionic radius influence lattice energy.

Definition

Lattice Energy: The change in energy that takes place when separated gaseous ions are packed together to form one mole of an ionic solid.

Charge Density: The ratio of ionic charge to the ionic radius of an ion.

Charge density = ionic chargeionic radius

Main Idea

• Ionic Charge
• Ionic Radius

Core Concept

• Ionic charge: As the magnitude of the ionic charge increases, the lattice energy also increases.
• Ionic radius: As the ionic radius increases, the lattice energy decreases.
• Charge density: Higher charge density → higher lattice energy.

Detailed Explanation

What is Charge Density?

The magnitude of lattice energy is influenced by the charge density of an ion. The charge density is defined as the ratio of ionic charge to the ionic radius of an ion.

Charge density = ionic chargeionic radius

There are two factors that have significant effect on lattice energy.

1. Ionic Charge

• The magnitude of the ionic charge is important in determining the lattice energy.
• As the magnitude of the ionic charge increases, the lattice energy also increases.
• Reason: The higher charges have strong electrostatic attractions, and they release higher energy when they come together to form an ionic lattice.
• Example: MgO has a higher lattice energy than NaCl. This is because Mg²⁺ and O²⁻ ions have a larger magnitude of charge as compared to the Na⁺ and Cl⁻ ions.

2. Ionic Radius

• Ionic radii also affect the lattice energy.
• As the ionic radius increases, the lattice energy decreases.
• Reason: Larger ions have higher electron-electron repulsions. This keeps opposite ions at a greater distance, leading to a weaker electrostatic attraction between the ions. Larger ions need less energy to separate them. This results in lower lattice energy.
• Example: Lithium fluoride (LiF) and potassium fluoride (KF) have the same ionic charge, but different ionic radii. Li⁺ ion is smaller than K⁺ ion, due to this LiF has higher electrostatic attraction between its ions as compared to KF. So, LiF has higher lattice energy than KF.

Summary Table

Concept Assessment Exercise 6.6

Which of the following compounds is expected to have higher lattice energy? Why?

• Answer: LiF has higher lattice energy than NaF.
• Reason: Li⁺ ion is smaller than Na⁺ ion. Smaller ion → higher charge density → stronger electrostatic attraction → higher lattice energy.

• Answer: MgCl₂ has higher lattice energy than NaCl.
• Reason: Mg²⁺ has a higher charge than Na⁺. Higher charge → stronger electrostatic attraction → higher lattice energy.

Intermediate Level

Factors Affecting Lattice Energy

Charge Density

Charge density = ionic chargeionic radius

- Higher charge density → higher lattice energy.

Concept Assessment Exercise 6.6

a. LiF or NaF - LiF has higher lattice energy because Li⁺ is smaller than Na⁺.

b. NaCl or MgCl₂ - MgCl₂ has higher lattice energy because Mg²⁺ has a higher charge than Na⁺.

Important Points

• 1. Lattice energy is influenced by charge density.
• 2. Charge density = ionic charge / ionic radius.
• 3. Ionic charge: Higher charge → higher lattice energy.
• 4. Ionic radius: Larger radius → lower lattice energy.
• 5. Example: MgO > NaCl (charge).
• 6. Example: LiF > KF (radius).
• 7. Concept Assessment: LiF > NaF; MgCl₂ > NaCl.

Remember

• - Charge density: Ionic charge / ionic radius
• - Ionic charge: Higher → higher lattice energy
• - Ionic radius: Larger → lower lattice energy
• - Example: MgO > NaCl
• - Example: LiF > KF

Common Mistakes

• 1. Forgetting the effect of ionic charge: Higher charge → higher lattice energy.
• 2. Forgetting the effect of ionic radius: Larger radius → lower lattice energy.
• 3. Confusing charge density with charge: Charge density = charge/radius.
• 4. Mixing up examples: MgO > NaCl; LiF > KF.

Quick Revision

• Lattice energy is influenced by charge density.
• Charge density = ionic charge / ionic radius.
• Ionic charge: Higher charge → higher lattice energy.
• Ionic radius: Larger radius → lower lattice energy.
• Example: MgO > NaCl (charge).
• Example: LiF > KF (radius).
• Concept Assessment: LiF > NaF; MgCl₂ > NaCl.

60-Second Revision

• Charge density: Ionic charge / ionic radius
• Ionic charge: Higher → higher lattice energy
• Ionic radius: Larger → lower lattice energy
• Example: MgO > NaCl
• Example: LiF > KF

QUALITY VALIDATION

• Topic: 6.5.1 Factors Affecting Lattice Energy
• Chapter: 6 (Energetics)

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