Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define 'reactant' and 'product'.
• Identify reactants and products in a chemical reaction.
• Explain the Law of Conservation of Mass.
• Describe reactions using word equations.
• Distinguish between physical and chemical changes.

Lesson summary

Chemical reactions are happening all around us, all the time! From cooking your favourite pap and vleis to rusting iron roofs in coastal KwaZulu-Natal, chemical reactions are essential for life and industry. Understanding reactants and products is fundamental to understanding how the world works. Chemical reactions are more than just mixing things together; they involve a change in the substances and their properties. This week, we'll explore how to identify reactants and products and understand how they interact to form new substances. Why is this important?

Lesson notes

What is a Chemical Reaction? A chemical reaction is a process that involves the rearrangement of atoms and molecules to form new substances. It's not just about changing the shape or size of something (that's a physical change!). A chemical change results in the formation of new substances with different properties.

Reactants: The Starting Materials Reactants are the substances that you start with in a chemical reaction. They are the ingredients that are combined and rearranged to form the products. Reactants are written on the left side of a chemical equation.

Products: The New Substances Formed Products are the new substances that are formed as a result of a chemical reaction. They are the result of the rearrangement of the atoms and molecules of the reactants. Products are written on the right side of a chemical equation.

The Arrow: Showing the Transformation In a chemical equation, we use an arrow (→) to show the direction of the reaction, from reactants to products. It means "reacts to produce" or "yields".

Word Equations: Describing Reactions in Words We can describe chemical reactions using word equations. A word equation shows the names of the reactants and products, separated by an arrow.

For example: Methane + Oxygen → Carbon Dioxide + Water

Example: Burning Methane (Natural Gas) Methane (CH 4 ), the main component of natural gas used in many South African homes for cooking and heating, reacts with oxygen (O 2 ) in the air to produce carbon dioxide (CO 2 ) and water (H 2 O).

Reactants: Methane and Oxygen Products: Carbon Dioxide and Water

Example: Rusting of Iron Iron (Fe), found in many things from corrugated iron roofs to cooking pots, reacts with oxygen (O 2 ) in the presence of water (H 2 O) to form iron oxide (Fe 2 O 3 ), also known as rust.

Reactants: Iron, Oxygen, and Water Products: Iron Oxide (Rust)

Example: Photosynthesis Plants use carbon dioxide (CO 2 ) from the air and water (H 2 O) from the soil, along with sunlight, to produce glucose (C 6 H 12 O 6 ), a type of sugar, and oxygen (O 2 ). This is how plants make their own food!

Reactants: Carbon Dioxide and Water Products: Glucose and Oxygen Physical vs. Chemical Changes It's important to distinguish between physical and chemical changes.

Physical Change: Alters the form or appearance of a substance, but does not change its chemical composition.

Examples: melting ice, cutting paper, dissolving sugar in water. The chemical formula of the substance remains the same.

Chemical Change: Results in the formation of new substances with different chemical compositions and properties.

Examples: burning wood, rusting iron, cooking an egg. Law of Conservation of Mass A fundamental principle in chemistry is the Law of Conservation of Mass. This law states that mass is neither created nor destroyed in a chemical reaction. In other words, the total mass of the reactants must equal the total mass of the products. Why is this important? It means that the number of atoms of each element must be the same on both sides of the chemical equation. While we are not balancing equations yet, this understanding lays the foundation for future learning. Think of it like building with Lego blocks – you start with a certain number of blocks (reactants), and even though you build something different (products), you still have the same number of blocks! Guided Practice (With Solutions)

Question 1: Identify the reactants and products in the following word equation: Hydrogen + Oxygen → Water Solution: Reactants: Hydrogen and Oxygen Product: Water

Commentary: This is a straightforward application of the definitions of reactants and products. The substances on the left side of the arrow are the reactants, and the substance on the right side of the arrow is the product.

Question 2: In a simple experiment, magnesium metal (Mg) is burned in air. The magnesium reacts with oxygen (O 2 ) to form magnesium oxide (MgO). Identify the reactants and products.

Solution: Reactants: Magnesium and Oxygen Product: Magnesium Oxide

Commentary: Even though the information is presented in a sentence, we can still identify the reactants as the substances that "react" and the product as what is "formed".

Question 3: When baking bread, yeast (a type of fungus) consumes sugar (C 6 H 12 O 6 ) and produces carbon dioxide (CO 2 ) and ethanol (C 2 H 5 OH). Identify the reactants and products in this process.

Solution: Reactants: Sugar (C 6 H 12 O 6 )

Products: Carbon Dioxide (CO 2 ) and Ethanol (C 2 H 5 OH)

Commentary: In this case, the problem describes the sugar as being "consumed," which is a good indicator that it's a reactant. The carbon dioxide and ethanol are "produced," indicating that they are products. Note that there can be multiple reactants and multiple products.

Question 4: During the burning of wood, wood (primarily cellulose) reacts with oxygen in the air to produce ash, carbon dioxide, and water vapor. List the reactants and the products.