Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define chemical reactions, reactants, and products.
• Identify reactants and products in a reaction.
• Write simple chemical reactions as word equations.
• Explain how atoms are rearranged in a reaction.
• Describe everyday examples of chemical reactions.

Lesson summary

Chemical reactions are happening all around us, all the time! From cooking your favourite meal like pap and stew to the rusting of a car, chemical reactions are fundamental processes that change substances into new ones. Understanding chemical reactions is crucial because it helps us understand how the world works, develop new technologies, and solve problems related to health, environment, and industry. In South Africa, understanding chemical reactions is important in various industries, including mining (extracting precious metals), agriculture (producing fertilizers), and manufacturing (creating everyday products).

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. In other words, it's a process where some substances (the reactants) are transformed into different substances (the products). Chemical reactions often involve breaking and forming chemical bonds.

Reactants: Reactants are the substances that you start with in a chemical reaction. They are the "ingredients" that react together. Reactants are written on the left side of a chemical equation.

Products: Products are the new substances that are formed as a result of the chemical reaction. They are the "results" of the reaction. Products are written on the right side of a chemical equation.

Word Equations: A word equation is a simple way to represent a chemical reaction using the names of the reactants and products. An arrow (→) is used to show the direction of the reaction, pointing from the reactants to the products. The word "and" or a plus sign (+) is often used to separate multiple reactants or products.

Example 1: Rusting of Iron Iron (Fe) reacts with oxygen (O₂) in the air to form iron oxide (rust, Fe₂O₃). This is a slow chemical reaction that happens over time.

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

Word Equation: Iron + Oxygen → Iron Oxide Example 2: Burning Wood (Combustion) Wood, which primarily contains cellulose (a complex carbohydrate), reacts with oxygen (O₂) during burning to produce carbon dioxide (CO₂) and water (H₂O). Heat and light are also released.

Reactants: Wood (Cellulose) and Oxygen Products: Carbon Dioxide and Water Word Equation: Wood + Oxygen → Carbon Dioxide + Water Example 3: Photosynthesis (Important for plants!) Plants use carbon dioxide (CO₂) and water (H₂O) in the presence of sunlight to produce glucose (a sugar, C₆H₁₂O₆) and oxygen (O₂). Glucose provides energy for the plant to grow.

Reactants: Carbon Dioxide and Water Products: Glucose and Oxygen Word Equation: Carbon Dioxide + Water → Glucose + Oxygen Rearrangement of Atoms: During a chemical reaction, the atoms of the reactants are rearranged to form the products. This means that the atoms themselves don't disappear or change into different atoms. They simply bond together in new ways. The number of atoms of each element remains the same before and after the reaction.

Worked example

Let's look at the reaction between hydrogen (H₂) and oxygen (O₂) to form water (H₂O).

Reactants: Hydrogen (H₂) and Oxygen (O₂)

Products: Water (H₂O)

Word Equation: Hydrogen + Oxygen → Water

What happens at the atomic level? Two molecules of hydrogen (each containing two hydrogen atoms) react with one molecule of oxygen (containing two oxygen atoms). The hydrogen and oxygen atoms rearrange themselves to form two molecules of water, each containing two hydrogen atoms and one oxygen atom. The bonds between the hydrogen atoms and the oxygen atoms in the original molecules are broken, and new bonds are formed between hydrogen and oxygen atoms to create water molecules. The number of hydrogen and oxygen atoms remains the same throughout the reaction.

Guided Practice (With Solutions)

Question 1:

Identify the reactants and products in the following reaction: Methane (CH₄) burns in oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O). Write the word equation.