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 chemical equations.
• Describe how reactants become products.
• Represent reactions using word equations.
• Explain the importance of reactions with South African examples.

Lesson summary

Chemical reactions are happening all around us, all the time! From the food we digest to the air we breathe, and even the rusting of old corrugated iron roofs, chemical reactions are fundamental to life and the world we live in. In South Africa, understanding chemical reactions helps us understand everything from the optimal conditions for growing crops to the processes involved in mining and manufacturing. Knowing about reactants and products is the very first step in understanding how these reactions work. For example, understanding how cement is made (a chemical reaction!) is crucial for infrastructure development in South Africa.

Lesson notes

A chemical reaction is a process that involves the rearrangement of atoms and molecules to form new substances. Think of it like building something new from Lego bricks – you take the old bricks (atoms/molecules) and rearrange them into something completely different (new substances). Reactants are the substances that start a chemical reaction. They are the "ingredients" that are mixed or combined to create something new. They are written on the left side of a chemical equation. Products are the new substances that are formed as a result of a chemical reaction. They are the "finished product" that results from the rearrangement of the reactants. They are written on the right side of a chemical equation. The general format of a chemical reaction is: Reactants → Products The arrow (→) means "reacts to produce" or "yields". It indicates the direction of the reaction.

Example 1: Rusting of Iron Iron (Fe) reacts with oxygen (O₂) in the air in the presence of water (H₂O) to form rust (iron oxide, Fe₂O₃). Rust is the reddish-brown coating you see on old iron surfaces, like corrugated iron roofs common in many South African townships and rural areas. This is a chemical reaction that weakens the iron over time.

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

Reactants: Iron, Oxygen, Water Product: Rust (Iron Oxide)

Example 2: Photosynthesis Plants, including those crucial for agriculture in South Africa, use photosynthesis to create their own food. They take in carbon dioxide (CO₂) from the air and water (H₂O) from the soil and, using sunlight, convert them into glucose (C₆H₁₂O₆, a type of sugar) and oxygen (O₂).

Word equation: Carbon Dioxide + Water → Glucose + Oxygen Reactants: Carbon Dioxide, Water Products: Glucose, Oxygen Example 3: Burning Wood When you burn wood in a fire (like for a braai or for heating), the wood (which is mostly cellulose, a complex carbohydrate) reacts with oxygen (O₂) in the air to produce carbon dioxide (CO₂), water (H₂O), and energy in the form of heat and light.

Word equation: Wood + Oxygen → Carbon Dioxide + Water + Energy Reactants: Wood, Oxygen Products: Carbon Dioxide, Water, Energy Important Notes: Chemical reactions involve the breaking and forming of chemical bonds. This is what allows the atoms to rearrange and create new substances. Energy is either absorbed or released during a chemical reaction. Some reactions need energy to start (endothermic), while others release energy (exothermic). Burning wood is exothermic as heat and light are released. The total number of atoms of each element must be the same on both sides of the chemical equation. This is the Law of Conservation of Mass. Guided Practice (With Solutions)

Question 1: Identify the reactants and products in the following chemical reaction: Methane (CH₄) + Oxygen (O₂) → Carbon Dioxide (CO₂) + Water (H₂O)

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

Commentary: The reactants are the substances listed on the left side of the arrow, which are Methane and Oxygen. The products are listed on the right side of the arrow: Carbon Dioxide and Water.

Question 2: Write a word equation for the reaction where hydrogen gas (H₂) reacts with chlorine gas (Cl₂) to produce hydrogen chloride gas (HCl). Identify the reactants and products.

Solution: Word equation: Hydrogen + Chlorine → Hydrogen Chloride Reactants: Hydrogen, Chlorine Product: Hydrogen Chloride

Commentary: This question tests the ability to represent a reaction with a word equation. Remember to use the words "reacts with" or "plus" to separate reactants and the word "yields" or the arrow symbol (→) to separate reactants from products.

Question 3: In the chemical reaction: Sodium (Na) + Chlorine (Cl₂) → Sodium Chloride (NaCl) What are the reactants and the product? What common substance is Sodium Chloride?

Solution: Reactants: Sodium, Chlorine Product: Sodium Chloride Sodium Chloride is common table salt.

Commentary: This question adds a bit of real-world connection by linking a chemical reaction to a common substance. This also tests understanding of chemical formulas and names.

Question 4: A student mixes baking soda (sodium bicarbonate) and vinegar (acetic acid). The mixture fizzes and produces carbon dioxide gas, water, and sodium acetate. Identify the reactants and products. Write a word equation.

Solution: Reactants: Baking Soda (Sodium Bicarbonate), Vinegar (Acetic Acid)

Products: Carbon Dioxide, Water, Sodium Acetate Word equation: Baking Soda + Vinegar → Carbon Dioxide + Water + Sodium Acetate

Commentary: This illustrates a more complex chemical reaction that students might have seen or even performed as an experiment. Independent Practice (Questions Only)

Question 1: Identify the reactants and products in the following reaction: Magnesium + Oxygen → Magnesium Oxide Question 2: Write a word equation for the reaction where zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas.