Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define and differentiate chemical and physical change.
• Identify reactants and products in a chemical reaction.
• Explain what is meant by 'rate of reaction'.
• List factors that affect the rate of a reaction.
• Relate reaction rates to everyday phenomena.

Lesson summary

Chemical reactions are the heart of our world. From the food we digest to the air we breathe, to the rusting of a corrugated iron roof, chemical changes are constantly happening around us. Understanding these changes is crucial not just for success in science, but also for making informed decisions about our health, environment, and the technology we use. In South Africa, understanding the rusting process, for instance, is critical for building durable infrastructure (bridges, buildings, etc.) and maintaining vehicles exposed to diverse weather conditions.

Lesson notes

What is Chemical Change? A chemical change (also known as a chemical reaction) occurs when substances combine or break apart to form new substances. These new substances have different chemical properties than the original substances. This means the arrangement of atoms within the molecules has changed. Importantly, chemical changes are usually irreversible without further chemical reactions. In contrast, a physical change involves a change in the form or appearance of a substance, but not its chemical composition. For example, melting ice is a physical change because it's still H2O, just in a different state.

Examples in a South African Context: Chemical Change: Burning wood for cooking on an open fire. The wood (fuel) reacts with oxygen in the air to produce ash, carbon dioxide, and water vapor. These new substances are very different from the original wood.

Chemical Change: The rusting of a metal gate. Iron in the gate reacts with oxygen and water to form iron oxide (rust).

Physical Change: Crushing a rock to use as gravel on a driveway. The rock pieces are smaller, but they are still the same rock material.

Physical Change: Boiling water to make tea. The water changes from liquid to gas (steam), but it's still H2

O. Reactants and Products: In a chemical reaction, the substances that react together are called reactants. The substances that are formed as a result of the reaction are called products. We can represent chemical reactions using chemical equations.

For example: `Methane + Oxygen → Carbon Dioxide + Water` In this equation: Reactants: Methane and Oxygen Products: Carbon Dioxide and Water More formally: `CH4 + 2O2 → CO2 + 2H2O` What is the Rate of Reaction? The rate of reaction is a measure of how quickly a chemical reaction occurs. It tells us how fast the reactants are being used up and how fast the products are being formed. A faster reaction means the reactants disappear quickly and the products appear quickly. Think of it like baking a cake. If you increase the oven temperature, the cake will bake faster – the reaction (cooking) happens at a higher rate.

Factors Affecting the Rate of Reaction: Several factors can influence how fast a chemical reaction occurs. We will explore these factors in more detail in the coming weeks, but here's an introduction: Temperature: Generally, increasing the temperature increases the rate of reaction. Molecules move faster at higher temperatures and collide more frequently with enough energy to react.

Concentration: Increasing the concentration of the reactants (more reactants in the same space) usually increases the rate of reaction. More molecules are available to collide and react.

Surface Area: Increasing the surface area of a solid reactant increases the rate of reaction. This is because more of the reactant is exposed and available to react. Think of kindling (small pieces of wood) versus a large log when starting a fire. The kindling has a much higher surface area and catches fire much more quickly.

Catalyst: A catalyst is a substance that speeds up a chemical reaction without being used up itself. Enzymes in our bodies are biological catalysts that help us digest food.

Worked example

Example 1: Consider the reaction of iron with oxygen to form rust. In coastal areas of KwaZulu-Natal, rust forms much faster than in the dry Karoo. Explain why.

Explanation: The reaction is: `Iron + Oxygen + Water → Rust`. The reaction requires both oxygen and water. Coastal areas have higher humidity (more water vapor in the air) than the Karoo.

Therefore, the presence of more water accelerates the rusting process.

Example 2: Wood shavings burn faster than a log of wood of the same mass. Explain why.

Explanation: The reaction is: `Wood + Oxygen → Carbon Dioxide + Water + Energy (Heat)`. The rate of reaction depends on the surface area of the wood exposed to oxygen. Wood shavings have a much larger surface area than a log of wood, allowing more oxygen to react with the wood at the same time, causing it to burn faster.

Example 3: Think about food spoiling faster in summer than in winter. Explain this in terms of reaction rate.

Explanation: The spoilage of food is a result of chemical reactions driven by microorganisms. Microorganisms such as bacteria and fungi thrive in warmer temperatures. These organisms catalyse the decomposition of food, which happens much faster at higher temperatures in the summer than in the cooler winter.

Guided Practice (With Solutions)