FYUGP B.Sc Chemistry Semester 1: Unit 3 - Chemical Bonding I (Ionic Interaction)
This unit focuses on the nature of ionic compounds, their energetics, and the principles governing ionic interactions. Topics include lattice energy, Born-Lande equation, Kapustinski equation, and the Born-Haber cycle. These notes are designed to help students understand the theoretical and practical aspects of ionic bonding.
1. General Characteristics of Ionic Compounds
Ionic compounds are formed by the electrostatic attraction between positively charged cations and negatively charged anions. Key characteristics include:
- High Melting and Boiling Points: Due to strong electrostatic forces.
- Solubility in Polar Solvents: Ionic compounds dissolve in water and other polar solvents.
- Electrical Conductivity: Conduct electricity in molten or aqueous states.
- Crystalline Structure: Form well-defined crystal lattices.
2. Lattice Energy and Solvation Energy
Lattice Energy: The energy released when one mole of an ionic compound is formed from its gaseous ions.
Solvation Energy: The energy change when ions are dissolved in a solvent. It is influenced by the solvent's polarity and the ion's charge density.
3. Born-Lande Equation
The Born-Lande equation calculates the lattice energy of an ionic crystal:
- U = Lattice energy
- NA = Avogadro's number
- M = Madelung constant
- Z+, Z- = Charges of the ions
- e = Electron charge
- ε0 = Permittivity of free space
- r0 = Distance between ion centers
- n = Born exponent (related to ion compressibility)
4. Kapustinski Equation
The Kapustinski equation is a simplified version of the Born-Lande equation, useful for estimating lattice energy when detailed crystal data is unavailable:
- ν = Number of ions in the formula unit
- r+, r- = Radii of the cation and anion
5. Madelung Constant
The Madelung constant (M) accounts for the geometric arrangement of ions in a crystal lattice. It depends on the crystal structure:
- For NaCl (rock salt structure), M ≈ 1.7476.
- For CsCl (cesium chloride structure), M ≈ 1.7627.

Fig 1: Madelung Constant for NaCl and CsCl Structures
6. Born-Haber Cycle
The Born-Haber cycle is a thermodynamic cycle used to calculate lattice energy indirectly. It involves the following steps:
- Formation of Gaseous Atoms: Sublimation of solid metal and dissociation of diatomic non-metal.
- Ionization: Formation of gaseous cations and anions.
- Lattice Formation: Combination of gaseous ions to form the ionic solid.
Example: For NaCl:
- ΔHf = Enthalpy of formation
- ΔHsub = Enthalpy of sublimation
- ΔHion = Ionization energy
- ΔHdiss = Dissociation energy
- ΔHEA = Electron affinity
- U = Lattice energy

Fig 2: Born-Haber Cycle for NaCl
Practical Applications
- Predicting Solubility: Lattice energy and solvation energy help predict the solubility of ionic compounds.
- Material Design: Understanding ionic interactions is crucial for designing materials with specific properties, such as high melting points or conductivity.
- Thermodynamic Calculations: The Born-Haber cycle is widely used in thermodynamics to calculate unknown quantities.
These notes provide a comprehensive understanding of ionic interactions and their applications. Practice problems and thermodynamic calculations will help reinforce these concepts.
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