Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define and identify reactants and products.
• Use word and chemical equations to represent reactions.
• Differentiate between physical and chemical changes.
• Explain the Law of Conservation of Mass.
• Identify reactants and products in everyday examples.

Lesson summary

Chemical reactions are happening all around us, all the time! From the food we cook in our kitchens to the air we breathe and even the rusting of metal, chemical reactions play a vital role in our lives and the environment. In South Africa, understanding chemical reactions helps us to better understand many phenomena, such as the safe use of cleaning products, the processes involved in industries like mining (where chemicals are used to extract valuable minerals), and the changes that occur in our environment due to pollution and climate change (e.g., acid rain reacting with buildings).

Lesson notes

2.1 Chemical Reactions: A Definition A chemical reaction is a process that involves the rearrangement of atoms and molecules to form new substances. In other words, some substances disappear, and new substances are formed. This involves the breaking and forming of chemical bonds. Chemical reactions are different from physical changes which only change the appearance or state of a substance (e.g., melting ice) but do not create a new substance. 2.2 Reactants Reactants are the substances that are present at the beginning of a chemical reaction. They are the "ingredients" that are mixed or combined to undergo a chemical change. Reactants are written on the left-hand side of a chemical equation. For example, in the reaction of wood burning, wood and oxygen are the reactants. 2.3 Products Products are the new substances that are formed as a result of a chemical reaction. They are the "outcome" of the reaction. Products are written on the right-hand side of a chemical equation. For example, in the reaction of wood burning, ash, carbon dioxide, and water vapor are some of the products. 2.4 Chemical Equations A chemical equation is a symbolic representation of a chemical reaction. It uses chemical formulas and symbols to show the reactants, products, and the direction of the reaction. The arrow (→) indicates the direction of the reaction, pointing from the reactants to the products. A "+" sign is used to separate multiple reactants or products. Word equations are a simple way to describe chemical reactions using the names of the substances involved.

For example: Methane + Oxygen → Carbon Dioxide + Water This equation shows that methane (a gas used in some stoves) reacts with oxygen in the air to produce carbon dioxide and water. Chemical equations use chemical formulas to represent the substances: CH₄ + 2O₂ → CO₂ + 2H₂O This equation shows the same reaction as above, but with chemical formulas for each substance. 2.5 Law of Conservation of Mass One of the fundamental laws of chemistry 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 be equal to the total mass of the products. In simpler terms, whatever you start with, you have to end with – atoms just rearrange themselves. This is why it is important to balance chemical equations. 2.6 Examples Example 1: Rusting of Iron Rusting is a common chemical reaction, especially in South Africa's humid coastal regions. Iron (Fe) reacts with oxygen (O₂) in the air, in the presence of water (H₂O), to form iron oxide (rust, Fe₂O₃).

Word equation: Iron + Oxygen + Water → Rust Chemical Equation: 4Fe + 3O₂ + 2H₂O → 2Fe₂O₃.2H₂O (Hydrated Iron Oxide)

In this reaction: Reactants: Iron, Oxygen, Water Product: Rust Example 2: Photosynthesis Plants use photosynthesis to create their food. They absorb carbon dioxide (CO₂) from the air and water (H₂O) from the soil, and in the presence of sunlight, they produce glucose (C₆H₁₂O₆ - a type of sugar) and oxygen (O₂).

Word equation: Carbon Dioxide + Water → Glucose + Oxygen Chemical Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ In this reaction: Reactants: Carbon Dioxide, Water Products: Glucose, Oxygen This is an important reaction in South Africa and the rest of the world since it produces oxygen and is the basis for food webs.

Example 3: Neutralization of Acid Soil Many soils in South Africa are acidic. Farmers often add lime (calcium carbonate, CaCO₃) to neutralize the soil. The lime reacts with the acid in the soil (e.g., sulfuric acid, H₂SO₄) to form calcium sulfate (CaSO₄), water (H₂O) and carbon dioxide (CO₂).

Word Equation: Calcium Carbonate + Sulfuric Acid → Calcium Sulfate + Water + Carbon Dioxide Chemical Equation: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂ In this reaction: Reactants: Calcium Carbonate, Sulfuric Acid Products: Calcium Sulfate, Water, Carbon Dioxide Guided Practice (With Solutions)

Question 1: Identify the reactants and products in the following word equation: Hydrogen + Chlorine → Hydrogen Chloride Solution: Reactants: Hydrogen, Chlorine Product: Hydrogen Chloride Explanation: The substances on the left side of the arrow are the reactants (the starting materials), and the substance on the right side of the arrow is the product (what is formed).

Question 2: Methane (CH₄) reacts with oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O). Write the word equation for this reaction and identify the reactants and products.

Solution: Word Equation: Methane + Oxygen → Carbon Dioxide + Water Reactants: Methane, Oxygen Products: Carbon Dioxide, Water Explanation: The information given in the question directly tells you what the reactants and products are. Writing the word equation is simply representing this information in equation form.