Lesson Notes By Weeks and Term v5 - Grade 9
Chemical change and rate of reaction (intro) – Week 9 focus
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Chemical change and rate of reaction (intro) – Week 9 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Natural Sciences
Class: Grade 9
Term: 1st Term
Week: 9
Theme: General lesson support
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Welcome to Week 9, where we dive into the fascinating world of chemical changes and reaction rates! This topic is crucial because it helps us understand how things around us transform, from the food we cook to the medicines we take, and even the rusting of a corrugated iron roof in Alexandra. Understanding chemical reactions allows us to predict and control these transformations, which is essential for developing new technologies and solving everyday problems in South Africa. Imagine a scientist trying to develop a cheaper and more efficient way to produce fertilizer for our farmers – that requires a deep understanding of chemical reactions!
2.1 What is Chemical Change? A chemical change (also called a chemical reaction) is a process that results in the formation of new chemical substances. This means the original substances are rearranged into different ones. In simpler terms, the molecules themselves are changing. This is different from a physical change, where the substance's form changes, but its chemical composition remains the same (e.g., melting ice - it's still water, just in a different state).
Indicators of a Chemical Change: How do we know a chemical change has occurred?
Look for these clues: Color Change: A new color appears that wasn't there before. For example, when iron rusts, it changes from shiny metal to a reddish-brown substance.
Gas Production: Bubbles form (if in a liquid) or a gas is released. Think about baking a cake - the baking powder reacts to produce carbon dioxide gas, which makes the cake rise.
Precipitate Formation: A solid forms when two solutions are mixed. This solid is called a precipitate. This is important in wastewater treatment, where chemicals are added to precipitate out pollutants.
Temperature Change: The reaction either gets hotter (exothermic) or colder (endothermic). Burning wood is exothermic – it releases heat.
Light is emitted: Sometimes light is given off. Think of burning fuel.
Smell/Odour: a change in smell occurs indicating a chemical change. e.g. when food rots or when something is burnt. 2.2 Reactants and Products In a chemical reaction, the substances you start with are called reactants. The substances you end up with are called products. We can represent a chemical reaction using a word equation: Reactants → Products The arrow (→) means "reacts to form" or "produces".
Example 1: Iron + Oxygen → Rust (Iron Oxide) Here, iron and oxygen are the reactants, and rust (iron oxide) is the product.
Example 2: Methane + Oxygen → Carbon Dioxide + Water Methane (the gas used in some stoves) and oxygen are the reactants, and carbon dioxide and water are the products.
Example 3: Hydrogen peroxide → Water + Oxygen Hydrogen peroxide is the reactant, and water and oxygen are the products. 2.3 The Rate of Reaction The rate of reaction tells us how quickly a chemical reaction happens. A fast reaction occurs quickly, while a slow reaction takes a long time. Think of it like a race – some runners are faster than others. Factors Affecting Reaction Rate (Overview – More detail next week): Several things can speed up or slow down a reaction. We will explore these in detail next week, but for now, here's a brief overview: Temperature: Generally, increasing the temperature speeds up the reaction. Think about cooking food – it cooks faster at a higher temperature.
Concentration: Increasing the concentration of reactants usually speeds up the reaction. Imagine adding more sugar to your tea – it dissolves faster at first, then slows down as the tea becomes more saturated.
Surface Area: Increasing the surface area of a solid reactant speeds up the reaction. Small pieces react faster than large pieces. Think of wood shavings catching fire faster than a log.
Catalyst: A catalyst is a substance that speeds up a reaction without being used up itself. Enzymes in our bodies are biological catalysts that help us digest food. Guided Practice (With Solutions)
Question 1: Identify whether the following change is a physical change or a chemical change and give one reason for your answer: Burning a candle.
Solution: This is a chemical change.
Reason: The candle wax is reacting with oxygen to produce carbon dioxide, water, heat, and light. You see evidence of light and heat being given off. Also, the wax disappears and doesn't come back as wax.
Question 2: Identify the reactants and products in the following chemical reaction: Copper Oxide + Carbon → Copper + Carbon Dioxide Solution: Reactants: Copper Oxide and Carbon Products: Copper and Carbon Dioxide Question 3: Imagine you are trying to dissolve sugar in cold water and hot water. In which water will the sugar dissolve faster? Explain your answer in terms of the rate of reaction.
Solution: The sugar will dissolve faster in hot water. This is because the rate of reaction (dissolving) increases with temperature. The hot water provides more energy to the sugar molecules, allowing them to break apart and mix with the water molecules more quickly.
Question 4: A group of learners noticed that a rusty nail took a very long time to rust. Is the rusting a fast reaction, or a slow reaction?
Solution: Rusting is a very slow reaction. It takes a long time for iron to react with oxygen and moisture in the air to form rust.