Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Differentiate between physical and chemical changes.
• Identify reactants and products in a reaction.
• Represent reactions using balanced chemical equations.
• Translate word equations into balanced chemical equations.
• Explain the law of conservation of mass.

Lesson summary

Chemical changes are fundamental to understanding the world around us. From the burning of fuels that power our cars to the rusting of iron in our homes and the digestion of food in our bodies, chemical changes are constantly occurring. In South Africa, understanding chemical change is especially relevant to industries like mining (extraction of metals), agriculture (fertilizers and pesticides), and energy production (coal burning and renewable energy). Many of the environmental challenges we face, such as air and water pollution, are also directly linked to chemical reactions.

Lesson notes

2.1 Physical Change vs.

Chemical Change Physical Change: A change that alters the form or appearance of a substance but does not change its chemical composition. The substance is still the same, just in a different form. Physical changes are usually easily reversible.

Examples: Melting ice: H₂O(s) → H₂O(l) – The substance is still water (H₂O), just in a liquid state.

Boiling water: H₂O(l) → H₂O(g) – Water changing to steam.

Dissolving salt in water: NaCl(s) → NaCl(aq) – The salt is dispersed throughout the water, but it's still sodium chloride.

Crushing a rock: It is still the same rock, just smaller pieces.

Cutting wood: the wood remains wood, just in different shapes or sizes.

Chemical Change: A change that results in the formation of new substances with different chemical compositions and properties. These changes involve the breaking and/or forming of chemical bonds. Chemical changes are often irreversible or require significant energy input to reverse.

Examples: Burning wood: Wood + Oxygen → Carbon dioxide + Water + Ash – New substances (CO₂, H₂O, Ash) are formed.

Rusting of iron: Iron + Oxygen → Iron oxide (Rust) – A new compound, iron oxide, is formed. This is a significant problem in South Africa, especially in coastal areas.

Cooking an egg: The proteins in the egg undergo denaturation and change their structure, leading to a permanent change.

Baking bread: Yeast produces carbon dioxide through fermentation, causing the dough to rise and changing its texture and composition.

Neutralization of acid with a base: Hydrochloric acid + Sodium hydroxide -> Sodium chloride + Water - new substances, salt, and water are created. Distinguishing between Physical and Chemical Changes: | Feature | Physical Change | Chemical Change | |-----------------|-----------------------------------|------------------------------------------| | Composition | Remains the same | Changes | | New substances | No | Yes | | Reversibility | Usually reversible | Often irreversible | | Energy Changes | Small energy changes are common. | Significant energy changes are common (heat/light released or absorbed). | | Key Indicators | Change of state, size, or shape. | Formation of a precipitate, gas evolution, colour change, heat/light produced. | 2.2 Representing Chemical Change: Chemical Equations A chemical equation is a symbolic representation of a chemical reaction using chemical formulas and symbols.

Reactants: The substances that react together. They are written on the left side of the equation.

Products: The new substances that are formed. They are written on the right side of the equation. Arrow (→): Indicates the direction of the reaction. It is read as "reacts to form" or "yields". Plus sign (+): Separates different reactants or products.

State symbols: Indicate the physical state of the substance: (s) = solid (l) = liquid (g) = gas (aq) = aqueous (dissolved in water)

Example: Hydrogen gas reacts with oxygen gas to form water (in the gas state at higher temperatures).

Word equation: Hydrogen + Oxygen → Water Unbalanced chemical equation: H₂ (g) + O₂ (g) → H₂O (g) 2.3 Balancing Chemical Equations: Law of Conservation of Mass The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction. This means the number of atoms of each element must be the same on both sides of the chemical equation. Balancing ensures this.

Steps for Balancing Chemical Equations: Write the unbalanced equation. Count the number of atoms of each element on both sides of the equation. Start balancing by placing coefficients (numbers in front of the chemical formulas) to equalize the number of atoms of one element at a time. Start with elements that appear in only one reactant and one product. Balance polyatomic ions as a single unit if they appear unchanged on both sides. If necessary, multiply all coefficients by a common factor to obtain the simplest whole-number ratio. Double-check that all atoms are balanced.

Example (Balancing): Balance the following equation: Methane (CH₄) burns in oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O).

Unbalanced equation: CH₄ (g) + O₂ (g) → CO₂ (g) + H₂O (g)

Count atoms: Left side: C = 1, H = 4, O = 2 Right side: C = 1, H = 2, O = 3 Balance hydrogen: Place a coefficient of 2 in front of H₂O: CH₄ (g) + O₂ (g) → CO₂ (g) + 2H₂O (g)

Now: C = 1, H = 4, O = 2 (left) and C = 1, H = 4, O = 4 (right)

Balance oxygen: Place a coefficient of 2 in front of O₂: CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)

Now: C = 1, H = 4, O = 4 (left) and C = 1, H = 4, O = 4 (right)

Balanced equation: CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)

Example 2: Reaction of Iron with Hydrochloric Acid Word Equation: Iron + Hydrochloric acid → Iron(II) chloride + Hydrogen gas Unbalanced equation: Fe(s) + HCl(aq) → FeCl₂(aq) + H₂(g)

Balancing: Iron is already balanced (1 on each side).

Balance chlorine: Place a coefficient of 2 in front of HCl: Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g) Now hydrogen is also balanced.