Thermodynamics | Part 1 of 2 | Born-Haber Cycles | A level Chemistry Explained

Introduction

This tutorial provides a comprehensive overview of Born-Haber cycles, a key concept in thermodynamics relevant to A-Level Chemistry. Understanding this topic is essential for explaining how ionic compounds form and the energy changes associated with these processes. By following this guide, you will learn about enthalpy changes, lattice enthalpy, and how to construct Born-Haber cycles for different ionic compounds.

Step 1: Review Enthalpy Changes

• Understand the concept of enthalpy, which refers to the total heat content of a system.

Recap the different types of enthalpy changes, including

• Enthalpy of formation
• Enthalpy of combustion
• Lattice enthalpy

Step 2: Explore Lattice Enthalpy

• Define lattice enthalpy as the energy required to separate one mole of an ionic solid into its gaseous ions.

Recognize that lattice enthalpy can be either

• Lattice formation enthalpy (energy released when ions combine)
• Lattice dissociation enthalpy (energy required when ions are separated)

Step 3: Compare Lattice Enthalpies

Learn how to compare lattice enthalpies using factors such as

• Ionic charge (greater charge results in higher lattice enthalpy)
• Ionic radius (smaller ions lead to higher lattice enthalpy)
• Use the trend to predict stability and properties of ionic compounds.

Step 4: Understand Enthalpy of Solution

• Define the enthalpy of solution as the energy change when one mole of a solute dissolves in a solvent.
• Be aware that this can be endothermic (heat absorbed) or exothermic (heat released).

Step 5: Apply Hess's Law to Enthalpy of Solution

• Recall Hess's law, which states that the total enthalpy change during a chemical reaction is the same, regardless of the route taken.
• Use Hess's law to calculate enthalpy changes in solution processes.

Step 6: Construct Born-Haber Cycles

• Understand that a Born-Haber cycle is a thermodynamic cycle that relates the lattice enthalpy of an ionic compound to other enthalpy changes.

Follow these steps to construct a Born-Haber cycle

1. Identify the ionic compound and its components (e.g., NaCl).

List the relevant enthalpy changes

• Sublimation of the metal
• Ionization energy of the metal
• Bond dissociation energy (if applicable)
• Electron affinity of the non-metal
• Lattice enthalpy
2. Arrange the enthalpy changes in a cycle.
3. Calculate the lattice enthalpy using Hess's law.

Step 7: Analyze Different Examples

Practice constructing Born-Haber cycles for various compounds

• Sodium Chloride (NaCl)
• Sodium Oxide (Na2O)
• Calcium Sulfide (CaS)
• Discuss how polarisation affects covalent character and the limitations of the perfect ionic model.

Step 8: Distinguish Theoretical and Experimental Lattice Enthalpy

Understand the difference

• Theoretical lattice enthalpy is calculated based on ionic models.
• Experimental lattice enthalpy is determined through measurements.
• Analyze discrepancies between the two and their implications for ionic character.

Conclusion

In this tutorial, you learned about the fundamentals of Born-Haber cycles, including the definitions and comparisons of various enthalpy changes. You practiced constructing cycles for different ionic compounds and explored the significance of lattice enthalpy in understanding ionic bonding. As a next step, consider reviewing real-world applications of these concepts in predicting ionic compound properties and stability.