Lesson Notes By Weeks and Term v4 - SHS 3
MATTER AND ITS PROPERTIES
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MATTER AND ITS PROPERTIES
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Subject: Chemistry
Class: SHS 3
Term: 1st Term
Week: 18
Grade code: 1.1.1.LI.2
Strand code: 1
Sub-strand code: 1
Content standard code: 1.1.1.CS.3
Indicator code: 1.1.1.LI.2
Theme: PHYSICAL CHEMISTRY
Subtheme: MATTER AND ITS PROPERTIES
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Welcome, future scientists and engineers! Today, we will explore the fascinating world of gases. Gases are all around us—the air we breathe, the LPG gas we use for cooking, the gas that inflates car tyres. Understanding how gases behave under different conditions of pressure, volume, and temperature is crucial not only for chemistry but for everyday life. For instance, why should you not leave an LPG cylinder in the hot sun? Why does a car tyre's pressure increase after a long journey from Accra to Kumasi? The Gas Laws provide the answers. We will study these laws, learn how to perform calculations with them, and understand the stories that graphs about gases can tell us.
A. The Kinetic Theory of Gases - The Foundation
Before we dive into the laws, let's remember the basic ideas about gases from the Kinetic Theory: Gas particles (atoms or molecules) are in continuous, rapid, and random motion. The volume of the particles themselves is negligible compared to the volume of the container they occupy. There are no significant forces of attraction or repulsion between the particles. Collisions between particles and with the walls of the container are perfectly elastic (no energy is lost). The average kinetic energy of the particles is directly proportional to the absolute temperature (in Kelvin).
These assumptions describe an ideal gas. Real gases behave very much like this under normal conditions. B. The Four Variables of a Gas
We describe the state of a gas using four key properties: Pressure (P): The force exerted by the gas per unit area on the walls of its container. Units: Pascal (Pa), atmospheres (atm), millimetres of mercury (mmHg). *Conversion:* 1 atm = 760 mmHg = 101,325 Pa. Volume (V): The space occupied by the gas, which is equal to the volume of its container. Units: cubic metres (m³), cubic decimetres (dm³), cubic centimetres (cm³). *Conversion:* 1 m³ = 1000 dm³ = 1,000,000 cm³. (Note: 1 dm³ is the same as 1 Litre). Temperature (T): A measure of the average kinetic energy of the gas particles. Crucial Point: In all gas law calculations, temperature MUST be in the Kelvin (K) scale. Conversion: K = °C + 273. Amount of Gas (n): The quantity of gas, measured in moles. C. Boyle's Law: The Pressure-Volume Relationship Statement: For a fixed mass of gas at a constant temperature, the volume is inversely proportional to the pressure. Meaning: If you increase the pressure on a gas, its volume will decrease proportionally. Think of squeezing a sealed 'pure water' sachet. Mathematical Expression: V ∝ 1/P or PV = k (where k is a constant) Formula for Calculations: P₁V₁ = P₂V₂ P₁ and V₁ are the initial pressure and volume. P₂ and V₂ are the final pressure and volume. Graphical Representation: A graph of P vs. V is a hyperbola (a downward curve). A graph of P vs. 1/V is a straight line through the origin.