Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define and differentiate chemical and physical changes.
• Identify reactants and products in a word equation.
• Explain what happens to atoms during a reaction.
• Define the rate of reaction (fast vs. slow).
• Describe reactions at the particle level.

Lesson summary

This week, we begin exploring the fascinating world of chemical changes and how quickly these changes happen. Chemical changes are fundamental to life and industry. From the food we digest to the fuels that power our cars, chemical reactions are happening all around us. Understanding these reactions and the factors influencing their speed is crucial for various applications, from designing more efficient fertilizers for our farms to preventing spoilage of food in our homes. This topic is not just about memorizing facts; it’s about understanding how the world works at a molecular level.

Lesson notes

What is a Chemical Change? A chemical change (also called a chemical reaction) is a process that leads to the transformation of one set of chemical substances to another. During a chemical change, atoms are rearranged to form new substances. Importantly, the new substances formed have different properties than the original substances. This is a crucial distinction from physical changes. Physical Change vs.

Chemical Change: | Feature | Physical Change | Chemical Change | |----------------|---------------------------------------------------------|------------------------------------------------------------| | Substance | Same substance remains, only its form changes. | New substance(s) are formed. | | Reversibility | Often easily reversible. | Often difficult to reverse. | | Energy Change | Usually small energy changes. | Significant energy changes (heat, light). | | Examples | Melting ice, boiling water, tearing paper, dissolving sugar | Burning wood, rusting iron, cooking an egg, baking a cake | Examples Relevant to South Africa: Physical Change: Dissolving salt in water (the salt is still salt, just dispersed in water). Freezing water to make ice for drinks.

Chemical Change: Burning wood in a braai (new substances like ash and gases are formed). The souring of milk (lactic acid is formed). Rusting of a metal gate left out in the rain (iron oxide is formed). Reactants and Products In a chemical reaction, the substances that start the reaction are called reactants. The substances that are formed as a result of the reaction are called products. We represent chemical reactions using chemical equations. For example, in the rusting of iron: Iron + Oxygen → Rust (Iron Oxide)

In this equation: Reactants: Iron and Oxygen Product: Rust (Iron Oxide) Another example, related to cooking: Baking Soda + Vinegar → Carbon Dioxide + Water + Sodium Acetate Reactants: Baking Soda and Vinegar Products: Carbon Dioxide, Water, and Sodium Acetate What Happens at the Particle Level? At the particle level (atoms and molecules), chemical reactions involve the breaking and forming of chemical bonds. Reactants are molecules that collide with enough energy to break their existing bonds. Atoms are then rearranged and new bonds are formed to create the product molecules. Imagine LEGO bricks. In a chemical reaction, you dismantle one LEGO structure (the reactant) and use the individual bricks (atoms) to build a completely different LEGO structure (the product). Rate of Reaction The rate of reaction tells us how fast or slow a chemical reaction proceeds. It's a measure of how quickly reactants are converted into products. We can describe reaction rates qualitatively: Fast Reaction: Happens very quickly (e.g., explosion).

Slow Reaction: Happens very slowly (e.g., rusting of iron).

Moderate Reaction: Happens at a noticeable but not extremely fast or slow pace (e.g., the reaction between vinegar and baking soda).

Example of Rate of Reaction Differences: Consider two ways to cook an egg: Boiling: Takes approximately 10-15 minutes.

Frying: Takes approximately 3-5 minutes. Both processes involve chemical changes that denature the proteins in the egg.

However, frying is a faster reaction than boiling.

Worked example

Example 1: Identifying Reactants and Products

Consider the reaction of methane (a component of natural gas) burning in oxygen to produce carbon dioxide and water, used in many South African households for cooking and heating.

The word equation is:

Methane + Oxygen → Carbon Dioxide + Water

Reactants: Methane and Oxygen

Products: Carbon Dioxide and Water

Example 2: Describing Rate of Reaction