Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define and distinguish between chemical and physical changes.
• Identify reactants and products in a reaction.
• Define 'rate of reaction' and its importance.
• Describe how temperature affects reaction rate using particle theory.
• Identify examples of slow and fast reactions.

Lesson summary

This week, we delve into the fascinating world of chemical changes and how fast these changes occur – a concept we call the rate of reaction. Understanding chemical changes is crucial because they are happening all around us, constantly transforming the world. From the ripening of fruit (a chemical change making the fruit sweeter) to the combustion of fuel in a taxi (powering our transport) to the rusting of a metal gate (a common problem affecting our homes and infrastructure), chemical changes directly impact our daily lives in South Africa.

Lesson notes

2.1 What is a Chemical Change? A chemical change is a process that involves the rearrangement of atoms and molecules to form new substances. This means the original substance is transformed into something completely different, with new chemical properties. Chemical changes are irreversible in many cases, or require another chemical reaction to reverse them.

Examples of Chemical Changes: Burning Wood: Wood is made up of complex organic molecules. When burned, these molecules react with oxygen in the air, producing carbon dioxide, water, ash, and releasing heat and light.

Rusting of Iron: Iron reacts with oxygen and water in the air to form iron oxide (rust), which is a reddish-brown substance that weakens the iron. This is a major problem in South Africa where infrastructure and buildings are exposed to the elements.

Cooking an Egg: The proteins in the egg undergo a change in their structure when heated, causing the egg to solidify.

Photosynthesis: Plants use sunlight, water, and carbon dioxide to produce glucose (sugar) and oxygen. 2.2 What is a Physical Change? A physical change is a process that alters the form or appearance of a substance but does not change its chemical composition. The molecules are still the same; only their arrangement or state changes. Physical changes are usually reversible.

Examples of Physical Changes: Melting Ice: Ice (solid water) turns into liquid water. The water molecules are still H₂O, just arranged differently.

Boiling Water: Liquid water turns into steam (gaseous water). Again, the water molecules remain H₂

O. Dissolving Sugar in Water: The sugar molecules spread out among the water molecules, but they are still sugar molecules (C₁₂H₂₂O₁₁). You can get the sugar back by evaporating the water.

Cutting Paper: The paper is simply cut into smaller pieces; it's still paper. Distinguishing Between Chemical and Physical Changes: | Feature | Chemical Change | Physical Change | |-----------------|-------------------------------------------------------|--------------------------------------------------------| | Substance | New substance(s) are formed | No new substance is formed | | Composition | Change in the chemical composition | No change in the chemical composition | | Reversibility | Often irreversible | Usually reversible | | Energy Changes | Significant energy changes (heat, light) may occur | Relatively small energy changes | | Examples | Burning, rusting, cooking, digestion, photosynthesis | Melting, boiling, dissolving, cutting, changing shape | 2.3 Reactants and Products In a chemical reaction, the substances that react with each other are called reactants, and the substances that are formed are called products.

Example: Iron + Oxygen → Iron Oxide (Rust) Here, Iron and Oxygen are the reactants, and Iron Oxide is the product. 2.4 Rate of Reaction The rate of reaction is a measure of how quickly a chemical reaction proceeds. It is often expressed as the amount of reactant consumed or product formed per unit time. In simpler terms, it tells us how fast a chemical change is happening.

Importance of Rate of Reaction: Industry: Many industrial processes rely on controlling the rate of reaction to maximize efficiency and yield. For example, in the fertilizer industry, speeding up the reaction to produce ammonia is crucial.

Food Preservation: Slowing down the rate of spoilage reactions helps preserve food for longer. Refrigeration slows down the growth of bacteria and the rate of enzymatic reactions that cause food to rot.

Medicine: Understanding reaction rates is important for drug development and delivery.

Environmental Protection: Controlling the rate of harmful reactions, such as those that contribute to acid rain, is essential for protecting the environment. 2.5 Factors Affecting the Rate of Reaction Several factors can influence the rate of a chemical reaction.

We'll focus on two key ones: Temperature and Concentration.

Temperature: Generally, increasing the temperature increases the rate of reaction. This is because at higher temperatures, the particles (atoms or molecules) have more kinetic energy and move faster. This leads to more frequent and more energetic collisions between reactant particles, increasing the likelihood of a successful reaction. Think of trying to light a fire on a cold day versus a hot day – it's usually easier to get a fire going when it's warmer.

Concentration: Increasing the concentration of a reactant (i.e., having more reactant particles in a given volume) also increases the rate of reaction. This is because there are more reactant particles available to collide with each other, leading to more frequent collisions and a higher chance of a successful reaction. Imagine a crowded taxi rank – the more people there are, the more likely they are to bump into each other. Similarly, the more reactant particles there are, the more likely they are to collide and react.