Chemical Reactions and Equations — Exchange, Energy and Redox · Learner pages

Lesson 1 · Explain displacement and precipitation

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

Name or initials: ____________   Date: ____________

A reaction can rearrange atoms or ions while preserving each element count.

1. Use the supplied order. For this exercise, use Mg > Zn > Cu: magnesium is more reactive than zinc, and zinc more reactive than copper. The order is evidence provided with the task, not something inferred from colour.
2. Follow a displacement. Mg + CuSO₄ → MgSO₄ + Cu shows magnesium replacing copper in the compound. In the same model, copper cannot displace magnesium from MgSO₄ because it is less reactive. This is a prediction exercise, not a mixing activity.
3. Track the exchanged ions. In CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaCl(aq), calcium ions combine with carbonate ions. Sodium and chloride ions remain dissolved. The 2 before NaCl balances two Na and two Cl atoms.
4. Identify the insoluble product. CaCO₃(s) is the precipitate because calcium carbonate is insoluble under the stated aqueous conditions. Double displacement describes ion exchange; precipitation additionally identifies formation of an insoluble solid. Not every double-displacement example needs to produce a visible precipitate.

Worked example: Given Mg > Zn > Cu, predict Zn + CuSO₄ and Cu + ZnSO₄. Zinc is above copper, so Zn + CuSO₄ → ZnSO₄ + Cu is predicted. Copper is below zinc, so no displacement is predicted for Cu + ZnSO₄ in this model. Each decision follows the supplied order.

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

   1. Zn · 2. Cu · 3. Neither Mg nor Zn

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

   1. CaCl₂ · 2. CaCO₃ · 3. NaCl

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

   1. To match two Na and two Cl atoms on each side. · 2. To change sodium into calcium. · 3. To make chloride disappear.

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.

Lesson 2 · Reason about energy transfer

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

Name or initials: ____________   Date: ____________

Temperature is a measurement; energy transfer describes movement of energy between system and surroundings.

1. Choose a boundary. Call the reacting materials the system. The container, nearby air and the rest of the environment belong to surroundings, depending on the boundary chosen. Say which boundary is used before drawing an energy arrow.
2. Follow energy leaving. For an exothermic reaction, draw an energy arrow from system to surroundings. Surroundings may warm if other transfers do not hide the effect. A flame or heater used to start a reaction does not by itself tell whether the reaction overall releases energy.
3. Follow energy entering. For an endothermic reaction, draw the arrow into the system. Energy must be taken in overall. Light or electrical input may supply energy, so temperature alone is not the entire description. Pattern labels such as decomposition remain a separate classification.
4. Use temperature data carefully. A supplied record starts at 20 °C and rises to 28 °C with no external heater and comparable surroundings. The 8 °C rise supports energy release by the change. It is supporting evidence under these conditions, not a universal rule that all warming observations prove chemical reaction.

Worked example: Two teacher-provided records use comparable containers and surroundings with no heater. Record P changes 24 °C → 30 °C; record Q changes 24 °C → 19 °C. P rises 6 °C, supporting energy release. Q falls 5 °C, supporting energy absorption. These observations need the supplied context; they do not alone establish the chemical identities or exact energy in joules.

1. Which arrow represents an exothermic change?

   1. System → surroundings · 2. Surroundings → system only · 3. No energy transfer is possible

2. Which statement defines endothermic change?

   1. The system always produces a visible flame. · 2. The atom totals need not be conserved. · 3. The system takes in energy overall.

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

   1. −8 °C · 2. 8 °C · 3. 48 °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.

Lesson 3 · Connect oxidation, reduction and everyday effects

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

Name or initials: ____________   Date: ____________

A reaction can change how an element is combined without changing the element into another one.

1. Track oxygen between substances. Use the supplied equation CuO + CO → Cu + CO₂ as a paper model. Copper oxide loses oxygen while carbon monoxide gains it. The element counts remain Cu: 1, C: 1 and O: 2 on each side. Carbon monoxide is poisonous; this equation must not become a home activity.
2. Name the paired changes. CuO is reduced to Cu because it loses oxygen. CO is oxidised to CO₂ because it gains oxygen. In this example oxidation and reduction occur together. The oxygen/hydrogen definitions used here are a school-level model; not every redox reaction contains oxygen.
3. Explain corrosion prevention. Rusting of iron requires both oxygen and moisture in the usual classroom model. An intact coating can limit contact with them. If the coating is scratched, exposed iron may rust; a barrier reduces exposure rather than making iron permanently unreactive.
4. Explain rancidity and its limits. Oxidation can spoil fats and oils. Suitable airtight storage, lower temperature and protection from light can slow relevant changes, depending on the food. Such storage does not reverse spoilage or guarantee safety. Use a supplied storage record; do not taste or deliberately grow spoiled food.

Worked example: In a paper reaction model, Fe₂O₃ + 3CO → 2Fe + 3CO₂. Fe₂O₃ loses oxygen and is reduced; CO gains oxygen and is oxidised. Check Fe: 2 = 2, C: 3 = 3, O: 3 + 3 = 6 on the left and 3 × 2 = 6 on the right. Both changes belong to the same balanced reaction.

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

   1. CO₂ because it gained oxygen · 2. CuO · 3. CO

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

   1. It loses carbon. · 2. It becomes copper. · 3. It gains oxygen to form CO₂.

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

   1. Only light and dry oil · 2. Oxygen and moisture · 3. Only dry nitrogen

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.

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?

   Only water changes state. / The ions exchange partners. / One element splits into two new elements.

2. Which statement about precipitation is accurate?

   A precipitate must be a dissolved ion. / An insoluble solid forms from a solution reaction. / Every gas bubble is a precipitate.

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

   A 4 °C temperature rise. / Creation of new atoms. / Energy absorption by the change.

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

   The colour of the notebook. / The learner’s favourite subject. / Whether an external heater was operating.

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

   To limit oxygen exposure and help slow oxidation. / To reverse all existing spoilage. / To guarantee every food is safe forever.

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

   It creates carbon atoms. / It is reduced by losing oxygen. / It is oxidised by gaining oxygen.

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?

   Cu + MgSO₄ → CuSO₄ + Mg / Cu + ZnSO₄ → CuSO₄ + Zn / Mg + ZnSO₄ → MgSO₄ + Zn

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

   Only carbonate molecules / Na⁺ and Cl⁻ / Only solid calcium atoms

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

   Energy enters the system through light. / Energy must leave the system through light. / Light cannot transfer energy.

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

   Yes; all two-product equations prove heat absorption. / No; energy evidence or known reaction information is also needed. / Yes; any one-product equation proves heat release.

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

   Oxidation of CO occurs together with reduction of CuO. / Both substances gain oxygen. / Neither substance changes its oxygen content.

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

   Interpret supplied storage observations without tasting samples. / Taste unknown old oil to judge safety. / Deliberately eat spoiled food for a comparison.