A comprehensive assessment covering all 12 lessons of Module 3: chemical change, reaction types, precipitation, combustion, acid-base, redox, galvanic cells, collision theory, and rate factors.
Allow about 20 minutes. Select the best answer for each question. 1 mark each.
1. A student heats copper metal in air and observes the surface turning black. This is best described as a chemical change because:
2. What are the correct coefficients (in order) for the balanced equation:, Al +, O₂ →, Al₂O₃?
3. Which of the following ionic compounds is insoluble in water according to the solubility rules?
4. A hydrocarbon burns in a limited supply of oxygen, producing a yellow sooty flame. Which of the following best describes what has occurred?
5. Which of the following represents the balanced equation for the reaction between sulfuric acid and potassium hydroxide?
6. When cycad seeds are soaked in running water, water-soluble toxins (such as cycasin) leach out. When Fe₂O₃ reacts with HCl to form FeCl₃ and H₂O, this is a separate reaction. Which of the following correctly classifies BOTH processes?
7. A student places a strip of zinc metal in copper(II) sulfate solution. Based on the standard activity series (K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au), which prediction is correct?
8. In the reaction Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g), which of the following statements about oxidation and reduction is correct?
9. Which of the following correctly shows the two half-equations and the overall equation for the reaction in a zinc-copper galvanic cell?
10. Using standard reduction potentials: E°(Fe²⁺/Fe) = −0.44 V and E°(Ag⁺/Ag) = +0.80 V. A galvanic cell is constructed with an iron anode and a silver cathode. Which values are correct?
11. A zinc block is attached to the hull of a steel ship to prevent corrosion. Which statement correctly explains how this works?
12. At a given temperature, only a small fraction of collisions between reactant molecules result in a reaction. Which of the following best explains why?
13. A student investigates the reaction of limestone (CaCO₃) with HCl. She compares large limestone lumps with powdered limestone at the same concentration of HCl. Which of the following correctly identifies the effect of grinding on collision frequency and on the proportion of collisions that are effective?
14. On an energy distribution diagram (Maxwell-Boltzmann), adding a catalyst at constant temperature is shown by:
15. A galvanic cell operates at 25°C using the reaction Mg(s) + Fe²⁺(aq) → Mg²⁺(aq) + Fe(s). E°(Mg²⁺/Mg) = −2.37 V; E°(Fe²⁺/Fe) = −0.44 V. A student then adds a catalyst to the cell electrolyte. Which correctly predicts the effect on E°cell and the rate of reaction?
Allow about 25 minutes (these two extended responses include graph/data interpretation). Write extended answers in full sentences. Show all working for calculations.
Question 16 (10 marks)
A galvanic cell is constructed using the following half-reactions. Standard reduction potentials are provided.
| Half-reaction | E° (V) |
|---|---|
| Mg²⁺(aq) + 2e⁻ → Mg(s) | −2.37 |
| Fe²⁺(aq) + 2e⁻ → Fe(s) | −0.44 |
| Cu²⁺(aq) + 2e⁻ → Cu(s) | +0.34 |
| Ag⁺(aq) + e⁻ → Ag(s) | +0.80 |
(a) For a galvanic cell constructed from a magnesium electrode and a copper electrode:
(b) Explain why a salt bridge is required in this galvanic cell and describe the direction of ion flow through it. (2 marks)
(c) A new cell is constructed using a platinum electrode in a solution containing both Fe²⁺(aq) and Fe³⁺(aq) as one half-cell, and a silver electrode in AgNO₃(aq) as the other.
Question 17 (10 marks)
A student investigates the reaction: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
She measures the volume of CO₂ produced over time under four different conditions, changing one variable at a time from the control (marble chips, 1.0 mol/L HCl, 25°C, no catalyst).
(a) The student records the following data for the control condition:
| Time (s) | 0 | 30 | 60 | 90 | 120 | 150 | 180 |
|---|---|---|---|---|---|---|---|
| Volume CO₂ (mL) | 0 | 18 | 30 | 38 | 43 | 46 | 48 |
(b) The student repeats the experiment at 45°C (all other variables unchanged).
(c) In a separate experiment, a heterogeneous catalyst is added at 25°C.
Q16(a)(i): Anode = Mg (lower reduction potential, E° = −2.37 V; Mg is oxidised). Cathode = Cu (higher reduction potential, E° = +0.34 V; Cu²⁺ is reduced).
Q16(a)(ii): Anode: Mg(s) → Mg²⁺(aq) + 2e⁻. Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s). Overall: Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s).
Q16(a)(iii): E°cell = E°cathode − E°anode = +0.34 − (−2.37) = +2.71 V. Positive E°cell → reaction is spontaneous under standard conditions.
Q16(b): The salt bridge maintains electrical neutrality in both half-cell solutions. As the cell operates, Mg²⁺ ions build up in the anode compartment (making it positive) and Cu²⁺ ions are removed from the cathode compartment (making it negative). Without a salt bridge, this charge imbalance would stop the flow of electrons through the external circuit. The salt bridge allows ions to migrate: anions (e.g. Cl⁻ from KCl salt bridge) migrate toward the anode compartment; cations (e.g. K⁺) migrate toward the cathode compartment, maintaining charge balance.
Q16(c)(i): Platinum (an inert electrode) is used because the Fe²⁺/Fe³⁺ half-reaction involves only ions in solution, there is no solid metal electrode reactant or product. An active metal electrode would dissolve into the electrolyte and interfere with the cell reaction. Platinum conducts electrons without reacting.
Q16(c)(ii): At the platinum electrode, Fe²⁺ is oxidised: Fe²⁺(aq) → Fe³⁺(aq) + e⁻. This is the anode half-equation (oxidation occurs at the anode; anode E° = +0.77 V for the Fe³⁺/Fe²⁺ couple).
Q16(c)(iii): Anode = Pt|Fe²⁺/Fe³⁺ (E° = +0.77 V). Cathode = Ag⁺/Ag (E° = +0.80 V). E°cell = +0.80 − (+0.77) = +0.03 V. E°cell > 0 → spontaneous, but only marginally. The cell operates with iron(II) being oxidised at the platinum anode and silver ions being reduced at the silver cathode.
Q17(a)(i): Average rate = ΔV / Δt = (30 − 0) mL / (60 − 0) s = 30/60 = 0.50 mL/s.
Q17(a)(ii): The rate decreases because HCl is consumed as the reaction proceeds. As [HCl] decreases, there are fewer H⁺ ions per unit volume in solution. This means H⁺ ions collide with the CaCO₃ surface less frequently, collision frequency decreases. The activation energy is unchanged at constant temperature, so the proportion of collisions that are effective (energy ≥ Eₐ) is also unchanged. Therefore the number of effective collisions per second decreases → reaction rate decreases, approaching zero as one reactant is fully consumed.
Q17(b)(i): At higher temperature (45°C vs 25°C), the Maxwell-Boltzmann energy distribution changes as follows: (1) the curve shifts to the right, particles have higher average kinetic energy; (2) the peak of the curve decreases in height and becomes broader, the same number of particles is now spread over a wider range of energies; (3) the total area under the curve remains equal, the number of particles is unchanged. The activation energy line (Eₐ) does not move, it is fixed by the reaction mechanism. However, the area under the curve to the right of Eₐ is now significantly larger at 45°C. This means a much greater proportion of particles have kinetic energy ≥ Eₐ and can undergo effective collisions. More effective collisions per second → reaction rate increases significantly.
Q17(b)(ii): At 45°C compared to 25°C: (1) the initial rate is steeper / CO₂ is produced faster initially; (2) the total time to complete the reaction is shorter (reaction reaches plateau earlier). Note: the total amount of CO₂ produced is the same in both experiments (same mass of CaCO₃ and excess HCl).
Q17(c)(i): The heterogeneous catalyst provides an alternative reaction pathway with a lower activation energy. On a Maxwell-Boltzmann energy distribution diagram at 25°C, the curve is unchanged but a new Eₐ(cat) line is drawn to the left of the original Eₐ. A greater proportion of particles now have kinetic energy ≥ Eₐ(cat) → more effective collisions per second → rate increases. The catalyst is not consumed (it is regenerated at the end of each catalytic cycle).
Q17(c)(ii): The total amount of CO₂ produced will be unchanged. The total amount of CO₂ produced depends only on the amount of CaCO₃ (limiting reagent with excess HCl), once all CaCO₃ is consumed, the reaction stops. The catalyst only affects the rate (how quickly the reaction proceeds), not the thermodynamic outcome (what products are formed or how much). ΔH, the overall equation, and therefore the stoichiometric yield are all unchanged by the catalyst.
Review areas and next steps before Module 4: