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

Introduction

This tutorial focuses on Born-Haber cycle calculations, a crucial concept in A-level Chemistry that helps you understand the formation of ionic compounds from their constituent elements. This guide will break down the calculation process step-by-step, making it easier for you to tackle exam questions related to this topic.

Step 1: Understand the Born-Haber Cycle

• The Born-Haber cycle is a thermodynamic cycle that relates the lattice energy of an ionic compound to other energy changes occurring during the formation of that compound.
• Key components include:
  • Sublimation energy: Energy required to convert a solid to a gas.
  • Ionization energy: Energy needed to remove an electron from an atom.
  • Electron affinity: Energy change when an electron is added to an atom.
  • Lattice energy: Energy released when gaseous ions form a solid ionic compound.

Practical Tip: Familiarize yourself with these terms and their significance in calculations.

Step 2: Gather Necessary Data

• Start by collecting the following data for the ionic compound in question:
  • Sublimation energy of the metal.
  • Ionization energy of the metal.
  • Electron affinity of the non-metal.
  • Lattice energy of the compound (if not provided, you may need to calculate or estimate it).

Common Pitfall: Ensure that all energies are in the same units (usually kJ/mol) for accurate calculations.

Step 3: Set Up the Born-Haber Cycle

• Draw the Born-Haber cycle diagram, which typically includes:
  • A starting point at the elements in their standard states.
  • Arrows representing each energy change you need to account for.
  • The final formation of the ionic compound.

Practical Tip: Visualizing the cycle can help clarify the relationships between different energy changes.

Step 4: Perform the Calculation

• Use Hess's Law to calculate the lattice energy if it is not provided:

  1. Start with the total energy changes from the elemental state to the ionic state:
     • Total Energy = Sublimation Energy + Ionization Energy - Electron Affinity - Lattice Energy
  2. Rearrange the equation to solve for the lattice energy:
     • Lattice Energy = Sublimation Energy + Ionization Energy - Electron Affinity - Total Energy

• Insert the values you gathered:

  Lattice Energy = (Sublimation Energy) + (Ionization Energy) - (Electron Affinity) - (Total Energy)
  

Practical Tip: Double-check your calculations to avoid simple arithmetic mistakes.

Step 5: Interpret the Results

• Analyze the calculated lattice energy:
  • A larger (more negative) value indicates a more stable ionic compound.
  • Compare your results with theoretical values or values given in exam questions to assess accuracy.

Real-World Application: Understanding lattice energy can help in predicting the properties of ionic compounds, such as solubility and melting points.

Conclusion

Mastering Born-Haber cycle calculations is essential for A-level Chemistry success. By understanding the cycle, gathering necessary data, and performing calculations accurately, you can confidently tackle exam questions. Practice with various compounds to reinforce your understanding and improve your calculation skills. Consider reviewing additional resources or videos for further examples and explanations.