Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define 'chemical reaction', 'reactant', and 'product'.
• Identify reactants and products in a word equation.
• Describe the observable signs of a chemical reaction.
• Explain the Law of Conservation of Mass.
• Write simple word equations for reactions.

Lesson summary

This week, we delve into the fascinating world of chemical reactions, focusing specifically on reactants and products. Understanding chemical reactions is crucial because they're happening all around us, all the time! From cooking a simple meal to the rusting of a gate, or even the processes happening inside our bodies, chemical reactions are at play. In South Africa, understanding these reactions can help us better understand issues like water purification, the manufacture of fertilizers for agriculture, and even the production of energy.

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 simpler terms, it's when substances interact and change into something different. These changes involve the breaking and forming of chemical bonds. Chemical reactions cannot be undone by physical changes (like melting or boiling). For instance, you can melt ice (a physical change) back into water and freeze it again, but you can't un-burn a piece of wood (a chemical change).

Reactants: The Starting Ingredients Reactants are the substances that start a chemical reaction. They are the "ingredients" that are combined and transformed.

Think of baking a cake: the flour, sugar, eggs, and butter are all reactants.

Products: What You Get at the End Products are the new substances that are formed as a result of a chemical reaction. Continuing with the cake example, the baked cake is the product of the reaction between the ingredients.

Representing Chemical Reactions: Word Equations We use word equations to represent chemical reactions in a simple and understandable way. A word equation shows the names of the reactants and products, separated by an arrow (→) that indicates the direction of the reaction. The arrow can be read as "reacts to form" or "produces".

Example 1: Rusting of Iron In South Africa, we see rusting quite often, especially near the coast where the air is salty. Rusting is a chemical reaction where iron reacts with oxygen in the presence of water to form iron oxide (rust).

Word Equation: Iron + Oxygen + Water → Iron Oxide (Rust) Here, Iron, Oxygen, and Water are the reactants, and Iron Oxide (Rust) is the product.

Example 2: Burning Wood Burning wood in a braai is a common practice in South Africa. The wood (which is mainly carbon-based) reacts with oxygen in the air to produce carbon dioxide, water, and energy (heat and light).

Word Equation: Wood (Carbon) + Oxygen → Carbon Dioxide + Water + Energy Here, Wood (Carbon) and Oxygen are the reactants, and Carbon Dioxide, Water, and Energy are the products.

Example 3: Neutralization Reaction Vinegar (acetic acid) reacts with baking soda (sodium bicarbonate) to produce carbon dioxide, water, and sodium acetate. This is often used in baking or for cleaning.

Word Equation: Acetic Acid + Sodium Bicarbonate → Carbon Dioxide + Water + Sodium Acetate Here, Acetic Acid and Sodium Bicarbonate are the reactants, and Carbon Dioxide, Water, and Sodium Acetate are the products. Indications of a Chemical Reaction Several observable changes can indicate that a chemical reaction has occurred.

These include: Colour Change: The reactants and products have different colours. For example, when copper sulfate solution reacts with iron, the blue colour of the copper sulfate fades, and a reddish-brown solid (copper) forms.

Gas Production: Bubbles of gas are formed. For example, when you mix vinegar and baking soda, carbon dioxide gas is released, causing fizzing.

Precipitate Formation: A solid (precipitate) forms when two solutions are mixed. For example, when you mix silver nitrate solution with sodium chloride solution, a white solid (silver chloride) forms.

Temperature Change: Heat is either released (exothermic reaction) or absorbed (endothermic reaction). For example, burning wood releases heat (exothermic), while dissolving ammonium nitrate in water absorbs heat (endothermic). Law of Conservation of Mass A fundamental principle governing chemical reactions is the Law of Conservation of Mass. This law states that in a closed system, the total mass of the reactants must equal the total mass of the products. In other words, matter cannot be created or destroyed in a chemical reaction, only rearranged. This means that the number and types of atoms present in the reactants are the same as those present in the products. Guided Practice (With Solutions)

Question 1: Identify the reactants and products in the following word equation: Methane + Oxygen → Carbon Dioxide + Water Solution: Reactants: Methane and Oxygen Products: Carbon Dioxide and Water

Commentary: This question directly tests the understanding of the definitions of reactants and products. Methane and Oxygen are the substances that start the reaction, while Carbon Dioxide and Water are the substances formed.

Question 2: What are two common signs (observations) that a chemical reaction has taken place when baking a cake?

Solution: Temperature Change: The cake batter is baked in an oven, which provides heat (energy), leading to a rise in temperature.

Gas Production: Baking powder releases carbon dioxide gas, which causes the cake to rise and become fluffy.

Commentary: This question connects the concepts to a real-life scenario (baking). It assesses the ability to identify observable changes associated with a chemical reaction.

Question 3: Consider the reaction where hydrogen reacts with oxygen to form water.