Intermolecular Forces | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial provides a comprehensive overview of intermolecular forces, a fundamental concept in A-level Chemistry. Understanding these forces is crucial for explaining the physical properties of substances, such as boiling and melting points. This guide will walk you through the types of intermolecular forces, how they work, and their real-world implications.

Step 1: Understand Intramolecular vs. Intermolecular Forces

• Intramolecular Forces: These are the forces that hold atoms together within a molecule, including ionic and covalent bonds.
• Intermolecular Forces: These are the forces of attraction between different molecules. They do not involve the transfer of electrons and arise from the movement of electrons within atoms.

Step 2: Learn About the Types of Intermolecular Forces

There are three main types of intermolecular forces:

London Forces

• Proposed by Fritz London, these forces arise from the constant movement of electrons, leading to temporary dipoles.
• Key characteristics:
  • Electron density is usually symmetrical but can be temporarily uneven.
  • This temporary dipole can induce dipoles in neighboring atoms, resulting in an attractive force.
• Practical Tip: London forces are present in all substances, but they are the only type of intermolecular force in non-polar molecules.

Permanent Dipole-Dipole Interactions

• Occur between molecules that have permanent dipoles due to differences in electronegativity.
• When aligned, these molecules attract each other, but random movement can disrupt this alignment.
• Types include:
  • Permanent Dipole-Permanent Dipole
  • Permanent Dipole-Induced Dipole

Hydrogen Bonds

• Special strong dipole-dipole interactions that occur when hydrogen is bonded to highly electronegative elements like nitrogen, oxygen, or fluorine.
• Characteristics:
  • Involves an electron-deficient hydrogen and the lone pair of a highly electronegative atom.
  • Examples include interactions in water and ammonia.
• Practical Tip: Hydrogen bonds significantly influence properties such as boiling points and melting points.

Step 3: Explore the Properties Caused by Intermolecular Forces

• Boiling Points: The strength of intermolecular forces affects boiling points. For example, substances with hydrogen bonds typically have higher boiling points than those with only London forces.
• Density of Ice vs. Water: Hydrogen bonding causes ice to be less dense than liquid water due to an open lattice structure formed in ice.
• Surface Tension: Strong hydrogen bonds in water contribute to high surface tension, allowing insects to walk on its surface.

Step 4: Analyze Trends in Intermolecular Forces

• Boiling points of hydrides (elements from Groups 4-7) show trends based on type and strength of intermolecular forces:
  • Noble gases exhibit gradual increases in boiling points due to increasing London forces as the number of electrons increases.

Conclusion

Understanding intermolecular forces is essential for grasping key concepts in chemistry, particularly their influence on the physical properties of substances. From the types of forces—London forces, permanent dipole-dipole interactions, and hydrogen bonds—to their implications on boiling points and other properties, this knowledge forms a critical foundation for further study in chemistry. As a next step, consider exploring specific examples of substances and their intermolecular forces to see these concepts in action.