Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define chemical change and distinguish it from physical change.
• Identify reactants and products and write word equations.
• Describe the rate of a reaction and the factors that influence it.
• Explain the importance of controlling reaction rates in South Africa.
• Investigate the effect of surface area on reaction rate.

Lesson summary

Chemical changes are happening all around us, all the time! From the food we eat being digested in our bodies to the rusting of metal gates and the burning of fuel in cars, chemical changes are essential to life and our modern world. Understanding chemical changes helps us to control these processes, making our lives safer, healthier, and more efficient. For example, knowing how to prevent rusting can save South African communities money on infrastructure repairs. Understanding how to control the rate of reaction is crucial in many industries, from food production (slowing down spoilage) to mining (speeding up the extraction of valuable minerals).

Lesson notes

2.1 What is Chemical Change? A chemical change occurs when a substance is transformed into a new substance with different chemical properties. This involves the breaking and forming of chemical bonds between atoms. Unlike physical changes (like melting ice or dissolving salt in water), chemical changes are usually irreversible.

Evidence of Chemical Change: Change in Colour: A new colour appears that was not initially present. For example, iron turning reddish-brown as it rusts.

Formation of a Gas: Bubbles are produced (not boiling!). For instance, adding vinegar (acetic acid) to baking soda (sodium bicarbonate) produces carbon dioxide gas.

Formation of a Precipitate: A solid forms when two solutions are mixed.

Change in Temperature: Heat is either released (exothermic reaction) or absorbed (endothermic reaction). Burning wood releases heat (exothermic).

New Substance Forms: The original substance is gone, and something new is present. This is the most important evidence.

Examples of Chemical Changes: Rusting of Iron: Iron + Oxygen → Iron Oxide (Rust)

Burning of Wood: Wood + Oxygen → Carbon Dioxide + Water + Ash Cooking an Egg: The egg white and yolk change their chemical structure.

Photosynthesis: Plants use sunlight, carbon dioxide, and water to create glucose and oxygen.

Digestion of Food: Enzymes break down food molecules into smaller, usable components.

Explosion of Fireworks: A rapid combustion reaction that produces light, heat, and sound.

Examples of Physical Changes: Melting ice. Boiling water. Dissolving sugar in water. Crushing a rock. Cutting paper. 2.2 Reactants and Products In a chemical reaction, the substances that react are called reactants. The substances that are formed are called products. We represent chemical reactions using word equations.

Word Equations: These use words to describe the reactants and products. The arrow "→" indicates "reacts to form" or "produces".

Example 1: Methane + Oxygen → Carbon Dioxide + Water (Burning methane gas)

Example 2: Hydrogen + Oxygen → Water (Formation of water)

Example 3: Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen (Reaction of zinc with acid) 2.3 Rate of Reaction The rate of reaction is how quickly a chemical reaction proceeds. It is the speed at which reactants are converted into products. A fast reaction happens quickly, while a slow reaction takes a long time.

Factors Affecting Rate of Reaction: Temperature: Generally, increasing the temperature increases the rate of reaction. Think about cooking food – it cooks much faster on high heat!

Concentration: Increasing the concentration of reactants (i.e., more reactant molecules in a given volume) increases the rate of reaction. Imagine cleaning a dirty pot – using a stronger cleaning solution works faster.

Surface Area: For reactions involving solids, increasing the surface area increases the rate of reaction. A pile of wood shavings burns faster than a log because the shavings have more surface area exposed to the oxygen. This is crucial in gold mining in South Africa where the ore is ground to a fine powder to increase surface area for reaction with chemicals.

Catalysts: Catalysts are substances that speed up the rate of a reaction without being used up themselves. Enzymes in our bodies are biological catalysts.

Presence of Inhibitors: Inhibitors are substances that slow down the rate of reaction. Preservatives in food act as inhibitors to slow down spoilage. Guided Practice (With Solutions)

Question 1: Identify whether the following are physical or chemical changes and explain your reasoning: a) A candle burning. b) Sugar dissolving in water. c) Iron nail rusting.

Solution: a) Chemical Change. When a candle burns, the wax reacts with oxygen in the air to produce carbon dioxide, water, heat, and light. The wax is being transformed into new substances. b) Physical Change. When sugar dissolves in water, it is simply dispersing into the water. It's still sugar, and the water is still water. We can evaporate the water and get the sugar back. c) Chemical Change. When an iron nail rusts, the iron reacts with oxygen and water to form iron oxide (rust). The iron is being transformed into a new substance with different properties.

Question 2: Write a word equation for the reaction between magnesium and hydrochloric acid, which produces magnesium chloride and hydrogen gas.

Solution: Magnesium + Hydrochloric Acid → Magnesium Chloride + Hydrogen Question 3: Explain why burning wood shavings will ignite and burn faster than a log of wood. Relate your answer to one of the factors affecting the rate of reaction.

Solution: Burning wood shavings ignite and burn faster than a log of wood because of the increased surface area. The wood shavings have a much larger surface area exposed to the oxygen in the air compared to the log. Since the reaction (combustion) occurs at the surface of the wood, the greater the surface area, the faster the reaction.