Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define 'reactants' and 'products'.
• Identify reactants and products in a reaction.
• Write simple word equations.
• Explain the Law of Conservation of Mass.
• Apply these concepts to everyday examples.

Lesson summary

Chemical reactions are fundamental processes that constantly occur around us and within us. From cooking food in our kitchens to the rusting of metal, understanding chemical reactions helps us explain and predict how substances change. Specifically, understanding reactants (the starting materials) and products (the substances formed) is essential. This knowledge is vital for understanding everything from how fertilizers help our crops grow to how pollutants in the air affect our health. Consider the combustion of wood in a braai – a familiar South African tradition.

Lesson notes

2. 1. 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 a physical change (like melting ice), where the substance stays the same. In a chemical reaction, new substances with different properties are formed. Signs of a chemical reaction often include: Change in colour Formation of a gas (bubbles) Formation of a precipitate (a solid forming in a liquid) Change in temperature (heat released or absorbed) Emission of light 2.

2. Reactants and Products: The Players in a Chemical Reaction Reactants: These are the substances that you start with in a chemical reaction. They are the "ingredients" that are mixed or combined to undergo a change. Reactants are always written on the left-hand side of a chemical equation.

Products: These are the new substances that are formed as a result of the chemical reaction. They are the "outcome" of the reaction. Products are always written on the right-hand side of a chemical equation. 2.

3. Representing Chemical Reactions: Word Equations We can represent chemical reactions using word equations. A word equation describes the reaction using the names of the reactants and products, separated by an arrow (→) that shows the direction of the reaction.

General Form: Reactant 1 + Reactant 2 + ... → Product 1 + Product 2 + ...

Example 1: Burning Wood (Braai) Wood reacts with oxygen to produce ash, carbon dioxide, water, and heat.

Word Equation: Wood + Oxygen → Ash + Carbon Dioxide + Water + Heat Example 2: Rusting of Iron (Affecting South African Infrastructure) Iron reacts with oxygen in the presence of water to form rust (iron oxide). This process is detrimental to our infrastructure, like bridges and buildings.

Word Equation: Iron + Oxygen + Water → Rust (Iron Oxide)

Example 3: Baking a Cake (A Common Household Reaction) Flour, sugar, eggs, and baking powder are mixed and heated to produce a cake.

Word Equation: Flour + Sugar + Eggs + Baking Powder + Heat → Cake 2.

4. The Law of Conservation of Mass One of the most important principles related to chemical reactions is the Law of Conservation of Mass. This law states that matter cannot be created or destroyed in a chemical reaction. This means that the total mass of the reactants must equal the total mass of the products. Although the substances change, the total amount of "stuff" stays the same. It is essential for understanding stoichiometry and chemical equations in later grades. 2.

5. Why is This Important? Understanding reactants and products allows us to: Predict the outcome of chemical reactions. Control chemical reactions for specific purposes (e.g., industrial processes, cooking). Understand the chemical changes happening around us every day. Develop new technologies and solutions to problems (e.g., creating new medicines, developing sustainable energy sources). Guided Practice (With Solutions)

Question 1: Identify the reactants and products in the following chemical reaction: When methane gas burns in the presence of oxygen, it produces carbon dioxide and water.

Solution: Reactants: Methane, Oxygen Products: Carbon Dioxide, Water

Commentary: The question explicitly states that methane gas burns "in the presence of oxygen", indicating these are the starting materials (reactants). The question also says it "produces carbon dioxide and water", indicating these are the final products.* Question 2: Write a word equation for the following reaction: When vinegar (acetic acid) is mixed with baking soda (sodium bicarbonate), it produces carbon dioxide, water, and sodium acetate.

Solution: Vinegar + Baking Soda → Carbon Dioxide + Water + Sodium Acetate

Commentary: The wording "is mixed with" implies the combination of reactants. The products are explicitly stated to be "carbon dioxide, water, and sodium acetate".* Question 3: In photosynthesis, plants use carbon dioxide and water in the presence of sunlight to produce glucose (sugar) and oxygen. Identify the reactants and products.

Solution: Reactants: Carbon Dioxide, Water Products: Glucose, Oxygen

Commentary: Photosynthesis is a fundamental process we learn about in early grades. Knowing this helps identify the initial inputs (reactants) and the resulting outputs (products).* Question 4: Magnesium metal reacts with hydrochloric acid to produce magnesium chloride and hydrogen gas. Write the word equation.

Solution: Magnesium + Hydrochloric Acid → Magnesium Chloride + Hydrogen

Commentary: Pay attention to the words used to describe the process. "Reacts with" indicates reactants. "Produces" indicates products.* Independent Practice (Questions Only) When iron sulfide reacts with hydrochloric acid, hydrogen sulfide gas and iron chloride are produced. Identify the reactants and products. Write a word equation for the following reaction: When hydrogen gas reacts with chlorine gas, it produces hydrogen chloride gas.