Born - Haber Calculations | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial will guide you through the Born-Haber cycle, a crucial concept in A-level Chemistry. Understanding this cycle helps you calculate lattice enthalpy and understand the relationship between theoretical and experimental values in ionic compounds. This guide will break down the key components of the Born-Haber cycle and provide practical steps for performing relevant calculations.

Step 1: Understanding the Born-Haber Cycle

• The Born-Haber cycle is a thermochemical cycle that relates the lattice enthalpy of an ionic compound to other thermodynamic quantities.
• It demonstrates how to derive lattice enthalpy from a series of known enthalpy changes, including:
  • Sublimation
  • Ionization energy
  • Electron affinity
  • Formation enthalpy

Practical Tip

Familiarize yourself with the various thermodynamic terms before diving into calculations, as this will make understanding the cycle easier.

Step 2: Constructing the Generic Born-Haber Cycle

• Draw a diagram with the following steps in order:
  1. Sublimation of the solid to form gaseous atoms.
  2. Ionization of the gaseous atoms to form cations.
  3. Gain of electrons by gaseous atoms to form anions.
  4. Formation of the solid ionic compound from the gaseous ions.

Common Pitfall

Ensure each step is correctly represented with its respective enthalpy change, as missing or mislabeling can lead to incorrect calculations.

Step 3: Finding Lattice Enthalpy

• Use the Born-Haber cycle to calculate the lattice enthalpy using Hess’s law. The equation is:

  [ \Delta H_f = \Delta H_{\text{sublimation}} + \Delta H_{\text{ionization}} + \Delta H_{\text{electron affinity}} + \Delta H_{\text{lattice}} ]

• Rearrange the equation to solve for lattice enthalpy:

  [ \Delta H_{\text{lattice}} = \Delta H_f - (\Delta H_{\text{sublimation}} + \Delta H_{\text{ionization}} + \Delta H_{\text{electron affinity}}) ]

Practical Advice

Collect accurate data for each enthalpy change from reliable sources or textbooks before performing the calculations.

Step 4: Finding Other Quantities

• In addition to lattice enthalpy, you can derive other thermodynamic quantities such as:

  • Enthalpy of formation
  • Ionization energies
  • Electron affinities

• Use the known values and relationships within the Born-Haber cycle to calculate these quantities.

Real-World Application

Understanding these calculations is essential for predicting the stability of ionic compounds and their formation in real-world applications, such as in materials science and chemistry.

Step 5: Comparing Theory with Experiment

• After calculating lattice enthalpy, compare your theoretical value to experimental data.
• Discuss any discrepancies and possible reasons for differences, such as:
  • Assumptions in the model
  • Limitations in measurement techniques

Key Consideration

Always consider the context of your calculations and experimental conditions when interpreting results.

Conclusion

The Born-Haber cycle is a foundational concept in A-level Chemistry that provides insight into the stability of ionic compounds. By following the steps outlined in this tutorial, you can effectively calculate lattice enthalpy and understand its significance in thermodynamics. Make sure to practice these calculations with various ionic compounds to enhance your understanding and proficiency in this area.