Chemical Reactions and Equations — Exchange, Energy and Redox · Answer guide

Lesson 1 · Explain displacement and precipitation

Use a supplied reactivity order and recognise an insoluble product in a double-displacement reaction.

1. Given Mg > Zn > Cu, which can displace copper from CuSO₄ in the stated model?

   Answer: Zn. Zinc is more reactive than copper in the supplied order.

2. In CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaCl(aq), which is the precipitate?

   Answer: CaCO₃. CaCO₃ is the new insoluble solid, marked (s).

3. Why is 2NaCl needed in the precipitation equation?

   Answer: To match two Na and two Cl atoms on each side.. The reactants supply two Na atoms and two Cl atoms; 2NaCl preserves both totals.

Discuss: Make paper cards labelled Ca²⁺, CO₃²⁻, Na⁺ and Cl⁻. Pair Ca²⁺ with CO₃²⁻ in a “solid” box and leave two Na⁺ and two Cl⁻ in a “solution” box. Check both atom counts and charge balance. Do not handle laboratory reagents.

Teaching note: An aqueous label means dissolved, not “liquid precipitate”. A precipitate is the new insoluble solid; it is not the entire solution. Open responses need discussion; they are not automatically marked.

Lesson 2 · Reason about energy transfer

Distinguish exothermic from endothermic change and judge temperature evidence with stated conditions.

1. Which arrow represents an exothermic change?

   Answer: System → surroundings. Exothermic means energy leaves the reacting system overall.

2. Which statement defines endothermic change?

   Answer: The system takes in energy overall.. Endothermic describes net energy absorption by the system.

3. A supplied no-heater record rises from 20 °C to 28 °C. What is the temperature increase?

   Answer: 8 °C. Final minus initial is 28 − 20 = 8 °C.

Discuss: On paper, draw the system boundary and an energy arrow for each supplied data card. Add one possible interfering factor, such as an external heater or heat exchange with room air. This is data interpretation, not a chemical experiment.

Teaching note: “Needs ignition” does not mean “endothermic overall”. Starting energy and the total energy change describe different things. Open responses need discussion; they are not automatically marked.

Lesson 3 · Connect oxidation, reduction and everyday effects

Track oxygen transfer and explain how exposure affects corrosion and rancidity.

1. In CuO + CO → Cu + CO₂, which substance is reduced?

   Answer: CuO. CuO loses oxygen to become Cu, which is reduction in the oxygen-transfer model.

2. In the same reaction, why is CO oxidised?

   Answer: It gains oxygen to form CO₂.. Carbon monoxide gains an oxygen atom in forming carbon dioxide.

3. Which pair is needed for ordinary rusting of iron in this model?

   Answer: Oxygen and moisture. The usual rusting model requires oxygen and moisture.

Discuss: Given a teacher’s photo or written record of a painted iron item with rust at a scratch, explain the exposed-surface mechanism. For food, compare only supplied packaging/storage data; never taste unknown or spoiled samples.

Teaching note: Oxidation does not mean “any change involving oxygen somewhere”. Identify which substance gains oxygen or loses hydrogen, and what happens to its reaction partner. Open responses need discussion; they are not automatically marked.

Final check · Set 1

Try one set after the lessons. Use the other set after reviewing any errors.

1. What makes the calcium chloride/sodium carbonate example double displacement?

   The ions exchange partners.. Calcium pairs with carbonate while sodium and chloride remain as dissolved ions.

2. Which statement about precipitation is accurate?

   An insoluble solid forms from a solution reaction.. Precipitation refers to formation of an insoluble solid.

3. With comparable surroundings and no heater, a record falls from 25 °C to 21 °C. What does it support?

   Energy absorption by the change.. Cooling by 4 °C supports energy being taken in under the stated conditions.

4. Which missing detail weakens a conclusion that a temperature rise was caused by reaction energy?

   Whether an external heater was operating.. An external heater could cause warming, so it is relevant to the inference.

5. What is a reasonable purpose of suitable airtight storage for an oil-rich food?

   To limit oxygen exposure and help slow oxidation.. Limiting oxygen can slow oxidative rancidity but does not reverse spoilage or guarantee safety.

6. In Fe₂O₃ + 3CO → 2Fe + 3CO₂, what happens to Fe₂O₃?

   It is reduced by losing oxygen.. The iron oxide loses oxygen and yields iron.

Final check · Set 2

Try one set after the lessons. Use the other set after reviewing any errors.

1. Given Mg > Zn > Cu, which proposed change agrees with the order?

   Mg + ZnSO₄ → MgSO₄ + Zn. Magnesium lies above zinc; the other proposals use a less reactive metal to displace a more reactive one.

2. Which ions remain dissolved after the stated CaCO₃ precipitation?

   Na⁺ and Cl⁻. Na⁺ and Cl⁻ remain in the aqueous solution while CaCO₃ forms the solid.

3. A light-driven reaction absorbs energy. Which description is consistent?

   Energy enters the system through light.. Light can supply energy to a reacting system.

4. Can the reaction pattern alone establish whether a change is exothermic?

   No; energy evidence or known reaction information is also needed.. Substance pattern and energy transfer are different classifications.

5. Which statement correctly describes the paired changes in CuO + CO → Cu + CO₂?

   Oxidation of CO occurs together with reduction of CuO.. Oxygen is transferred: one reactant gains it and the other loses it.

6. Which classroom task is appropriate for studying rancidity here?

   Interpret supplied storage observations without tasting samples.. Provided records support reasoning without exposing learners to unsafe food.