Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Define and identify different types of intermolecular forces.
• Explain how these forces affect physical properties like boiling point and solubility.
• Analyze the relationship between forces and properties in specific compounds.
• Apply this knowledge to real-life scenarios.

Lesson summary

This lesson explores the invisible forces that exist *between* molecules, known as intermolecular forces. These forces are weaker than the bonds that hold atoms together *within* a molecule (like covalent or ionic bonds), but they are incredibly important. They are the reason why water is a liquid at room temperature while petrol evaporates quickly, why oil and water don't mix when preparing stew, and why some liquids are thick like honey while others are thin like water. By understanding these forces, we can predict and explain the physical properties of the substances all around us, from the water we drink to the fuel we use in our cars and 'tro-tros'.

Lesson notes

2.1. Intramolecular vs. Intermolecular Forces

It is crucial to understand the difference between these two types of forces. Intramolecular Forces: These are the strong forces *within* a molecule that hold the atoms together. They are the chemical bonds themselves (e.g., covalent bonds in H₂O, ionic bonds in NaCl). Breaking these forces involves a chemical reaction. Intermolecular Forces (IMFs): These are the weaker attractive forces that exist *between* separate molecules. When a substance melts or boils, we are overcoming these intermolecular forces, not the strong intramolecular bonds. The molecules themselves remain intact.

Analogy: Think of a bag of gari. The individual gari grains are the molecules. The forces holding the carbon, hydrogen, and oxygen atoms together *inside* one grain are intramolecular. The weak forces that make the grains clump together in the bag are intermolecular. 2.2. Types of Intermolecular Forces (From Weakest to Strongest)

There are three main types of IMFs we will study. The type of IMF present depends on the structure and polarity of the molecules.

Evaluation guide

• Explain how intermolecular forces affect physical properties of compounds .
• Talk for Learning:
• Discuss and explain how intermolecular forces affect physical properties such as solubility,
• density, viscosity, enthalpy of vaporisation, volatility, surface tension, melting point and