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 properties like boiling point and solubility.
• Predict physical properties of compounds based on their intermolecular forces.
• Relate these concepts to real-life applications in Ghana.

Lesson summary

This lesson explores the "hidden" forces that hold molecules together, known as intermolecular forces. These forces are weaker than the bonds *within* a molecule (like covalent or ionic bonds), but they are incredibly important. They determine why water is a liquid at room temperature while the gas in our cooking cylinders is a gas. They explain why oil and water don't mix when preparing stew, and why *akpeteshie* can be separated from fermented palm wine through distillation. Understanding these forces allows us to predict and explain the physical properties of the substances all around us, from the water we drink to the fuels we use.

Lesson notes

A. Intramolecular vs. Intermolecular Forces: A Critical Distinction

First, let's clarify two very important terms. INTRAmolecular Forces: These are the strong forces *within* a single molecule. They are the chemical bonds that hold atoms together. Examples: Covalent bonds in H₂O, Ionic bonds in NaCl. Breaking these requires a chemical reaction. INTERmolecular Forces (IMFs): These are the weaker forces of attraction *between* separate molecules. When you boil water, you are not breaking the O-H covalent bonds; you are overcoming the intermolecular forces holding the H₂O molecules together to allow them to escape into the gas phase.

Analogy: Think of a school. The strong bonds holding the atoms of a student together (bones, skin) are like intramolecular forces. The friendships and interactions between different students in the compound are like intermolecular forces. It's much easier to separate the students from each other than to break a student's bones. B. Types of Intermolecular Forces (IMFs)

We will study three main types, arranged from weakest to strongest. London Dispersion Forces (LDF) Who has them? ALL molecules and atoms have LDF. They are the only forces present in non-polar molecules (e.g., CH₄, CO₂, O₂, Cl₂). How do they form? Electrons in a molecule are in constant, random motion. At any given instant, the electrons might accumulate on one side of the molecule, creating a temporary, instantaneous dipole (a momentary δ⁻ and δ⁺). This temporary dipole can then induce a similar temporary dipole in a neighbouring molecule, leading to a weak, short-lived attraction. Strength Factors: The strength of LDF depends on two main things: Number of Electrons (Molar Mass): More electrons mean a larger, more "polarisable" electron cloud, leading to stronger temporary dipoles and thus stronger LDF. Molecular Shape: Long, chain-like molecules have a larger surface area for contact than compact, spherical molecules, leading to stronger LDF. Example: The Halogens (Group 17) All halogen molecules (F₂, Cl₂, Br₂, I₂) are non-polar. The only IMFs they have are LDF.

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