Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define and differentiate chemical and physical changes.
• Identify reactants and products in a reaction.
• Explain the concept of 'rate of reaction'.
• Identify factors that affect the rate of reaction.
• Recognise chemical reactions in everyday life.

Lesson summary

This week, we delve into the fascinating world of chemical changes and the factors that influence how quickly they occur. Understanding chemical changes is crucial because they are happening all around us, from the food we digest to the medicines we take. Chemical changes are the basis of many industrial processes that provide us with essential resources. Understanding the rate of chemical reactions helps us control these processes, making them more efficient and safe. Imagine the consequences if we couldn't control the rate of combustion in our power plants or the rate of decay of waste in our landfills!

Lesson notes

2.1 What is a Chemical Change? A chemical change, also known as a chemical reaction, is a process that results in the formation of new substances. This means that the original substances are transformed into something completely different with different chemical properties. This involves the breaking and forming of chemical bonds. This is very different from a physical change, where the substance changes its appearance or state but not its chemical composition.

Examples of Chemical Changes: Burning wood: Wood reacts with oxygen in the air to produce ash, carbon dioxide, water, and heat. The wood is gone, and new substances have been formed.

Rusting of iron: Iron reacts with oxygen and water to form rust (iron oxide), which is a reddish-brown substance very different from shiny iron.

Cooking an egg: The proteins in the egg undergo a chemical change (denaturation) when heated, causing the egg to solidify.

Photosynthesis: Plants use sunlight, carbon dioxide, and water to produce glucose (sugar) and oxygen.

Digestion of food: Our bodies use enzymes to break down complex food molecules into simpler ones that can be absorbed.

Examples of Physical Changes: Melting ice: Solid water (ice) changes to liquid water, but it is still water (H₂O).

Boiling water: Liquid water changes to gaseous water (steam), but it is still water (H₂O).

Dissolving sugar in water: Sugar disappears, but it is still sugar, just dispersed throughout the water. You can evaporate the water and get the sugar back.

Crushing a can: The can changes shape, but it's still the same material. 2.2 Reactants and Products In a chemical reaction, the substances that react together are called reactants, and the substances that are formed are called products. We can represent a chemical reaction with a simple equation: Reactants → Products Example 1: Consider the rusting of iron: Iron + Oxygen + Water → Rust (Iron oxide)

In this reaction: Reactants: Iron, Oxygen, Water Product: Rust (Iron oxide)

Example 2: Burning methane gas (CH₄), a common fuel: Methane + Oxygen → Carbon Dioxide + Water In this reaction: Reactants: Methane, Oxygen Products: Carbon Dioxide, Water Symbolic Representation: Chemical reactions can also be represented using chemical formulas. For example, the burning of methane can be written as: CH₄ + 2O₂ → CO₂ + 2H₂O 2.3 Rate of Reaction The rate of reaction refers to how quickly a chemical reaction occurs. It is a measure of how fast the reactants are converted into products. A fast reaction happens quickly, while a slow reaction takes a long time. Factors Affecting Rate of Reaction (Introduction): Several factors influence the rate of a chemical reaction. This week, we will introduce three key factors: Concentration: The concentration of a reactant is the amount of that reactant present in a given volume. Generally, increasing the concentration of a reactant increases the rate of reaction. Think of it like having more "stuff" available to react.

Temperature: Increasing the temperature generally increases the rate of reaction. Higher temperatures mean the molecules have more energy and are more likely to collide and react.

Surface Area: For reactions involving solids, the surface area of the solid reactant affects the rate of reaction. Increasing the surface area (e.g., using powdered reactants instead of large chunks) increases the rate of reaction because more of the solid is exposed to the other reactants.

Think of it this way: Imagine a group of learners trying to pack books into boxes.

Concentration:* More learners packing (higher concentration) means the books are packed faster.

Temperature:* If they are energized and moving quickly (higher temperature), they will pack faster.

Surface Area:* If the books are already neatly arranged (higher surface area exposure), it's easier to pack them quickly compared to if they're scattered all over the place. Guided Practice (With Solutions)

Question 1: Identify the reactants and products in the following chemical reaction: Hydrogen + Chlorine → Hydrogen Chloride Solution: Reactants: Hydrogen, Chlorine Product: Hydrogen Chloride

Commentary: This question tests the basic understanding of identifying reactants and products in a simple word equation.

Question 2: Which of the following changes is a chemical change and which is a physical change? Explain your answer. a) Freezing water into ice. b) Burning a candle.

Solution: a)

Physical Change: Freezing water is a physical change because the substance remains water (H₂O). It only changes its state from liquid to solid. b)

Chemical Change: Burning a candle is a chemical change because the wax reacts with oxygen to produce carbon dioxide, water, light, and heat. The wax is transformed into entirely new substances.

Commentary: This question tests the ability to distinguish between physical and chemical changes based on the formation of new substances.