Chemistry HL
Reactivity 3: electron transfer (HL) — Reactivity 3 HL
- 1.
Define oxidation and reduction in terms of electron transfer.
[2 marks] · no calculator - 2.
Marking analysis: A learner attempts the following task: “Define oxidation and reduction in terms of electron transfer.” Their response addresses only this point: “States that oxidation is the loss of electrons by a species.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 3.
Using standard electrode potentials E°(Cu²⁺/Cu) = +0.34 V and E°(Zn²⁺/Zn) = −0.76 V, calculate E°cell for the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), stating which half-cell is the cathode.
[4 marks] - 4.
Marking analysis: A learner attempts the following task: “Using standard electrode potentials E°(Cu²⁺/Cu) = +0.34 V and E°(Zn²⁺/Zn) = −0.76 V, calculate E°cell for the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), stating which half-cell is the cathode.” Their response addresses only this point: “Identifies the Cu²⁺/Cu half-cell as the cathode, since it has the more positive (less negative) standard electrode potential.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 5.
State how the sign of E°cell indicates whether a redox reaction is spontaneous under standard conditions, and hence state whether the reaction between Zn(s) and Cu²⁺(aq) is spontaneous, given E°cell = +1.10 V.
[2 marks] · no calculator - 6.
Marking analysis: A learner attempts the following task: “State how the sign of E°cell indicates whether a redox reaction is spontaneous under standard conditions, and hence state whether the reaction between Zn(s) and Cu²⁺(aq) is spontaneous, given E°cell = +1.10 V.” Their response addresses only this point: “States that a positive E°cell indicates a spontaneous (feasible) reaction under standard conditions, while a negative E°cell indicates a non-spontaneous one.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 7.
A current of 2.00 A is passed through molten lead(II) bromide, PbBr₂(l), for 1930 seconds using inert electrodes. Calculate the number of moles of electrons transferred. (F = 96500 C mol⁻¹)
[4 marks] - 8.
Marking analysis: A learner attempts the following task: “A current of 2.00 A is passed through molten lead(II) bromide, PbBr₂(l), for 1930 seconds using inert electrodes. Calculate the number of moles of electrons transferred. (F = 96500 C mol⁻¹)” Their response addresses only this point: “States the formula for charge: Q = It.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 9.
Using n(e⁻) = 0.0400 mol from the electrolysis of molten PbBr₂, calculate the mass of lead deposited at the cathode. (Pb²⁺ + 2e⁻ → Pb; Ar(Pb) = 207)
[3 marks] - 10.
Marking analysis: A learner attempts the following task: “Using n(e⁻) = 0.0400 mol from the electrolysis of molten PbBr₂, calculate the mass of lead deposited at the cathode. (Pb²⁺ + 2e⁻ → Pb; Ar(Pb) = 207)” Their response addresses only this point: “States that 2 moles of electrons are required per mole of Pb deposited, from Pb²⁺ + 2e⁻ → Pb.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] - 11.
Explain why the standard hydrogen electrode (SHE) is assigned a standard electrode potential of exactly 0.00 V, and outline why this matters for tabulating other electrode potentials.
[3 marks] · no calculator - 12.
Marking analysis: A learner attempts the following task: “Explain why the standard hydrogen electrode (SHE) is assigned a standard electrode potential of exactly 0.00 V, and outline why this matters for tabulating other electrode potentials.” Their response addresses only this point: “States that the SHE is an arbitrary but universally agreed reference electrode.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 13.
Aqueous copper(II) sulfate is electrolysed using inert platinum electrodes under the usual laboratory conditions. Identify the cathode and anode products, explain why water rather than sulfate is oxidised at the anode, and write a balanced anode half-equation.
[4 marks] · no calculator - 14.
Marking analysis: A learner attempts the following task: “Aqueous copper(II) sulfate is electrolysed using inert platinum electrodes under the usual laboratory conditions. Identify the cathode and anode products, explain why water rather than sulfate is oxidised at the anode, and write a balanced anode half-equation.” Their response addresses only this point: “States that copper metal, Cu(s), is deposited at the cathode, since Cu²⁺ is preferentially reduced over H⁺ (Cu²⁺/Cu has the more positive standard electrode potential).” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator - 15.
Two electrolytic cells are connected in series: one contains molten PbBr2 and the other aluminium oxide dissolved in molten cryolite. Assume 100% current efficiency for metal deposition in each. Deduce which cell deposits more moles of metal for the same charge and explain why. Use Pb2+ + 2e- -> Pb and Al3+ + 3e- -> Al.
[3 marks] · no calculator - 16.
Marking analysis: A learner attempts the following task: “Two electrolytic cells are connected in series: one contains molten PbBr2 and the other aluminium oxide dissolved in molten cryolite. Assume 100% current efficiency for metal deposition in each. Deduce which cell deposits more moles of metal for the same charge and explain why. Use Pb2+ + 2e- -> Pb and Al3+ + 3e- -> Al.” Their response addresses only this point: “States that cells connected in series carry the same current, so the same quantity of charge (and hence the same number of moles of electrons) passes through both.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 17.
State the Nernst equation. For Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s) at fixed temperature, explain how increasing only [Cu2+] above 1 mol dm^-3 affects cell potential when [Zn2+] remains at 1 mol dm^-3. Treat ion activities as proportional to concentration.
[3 marks] · no calculator - 18.
Marking analysis: A learner attempts the following task: “State the Nernst equation. For Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s) at fixed temperature, explain how increasing only [Cu2+] above 1 mol dm^-3 affects cell potential when [Zn2+] remains at 1 mol dm^-3. Treat ion activities as proportional to concentration.” Their response addresses only this point: “States the Nernst equation, E = E° − (RT/nF) ln Q, relating cell potential under non-standard conditions to the standard cell potential and the reaction quotient Q.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 19.
A current of 1.5 A is passed through molten aluminium oxide to deposit aluminium metal at the cathode: Al³⁺ + 3e⁻ → Al. Calculate the time required to deposit 2.70 g of aluminium. (Ar(Al) = 27, F = 96500 C mol⁻¹)
[5 marks] - 20.
Marking analysis: A learner attempts the following task: “A current of 1.5 A is passed through molten aluminium oxide to deposit aluminium metal at the cathode: Al³⁺ + 3e⁻ → Al. Calculate the time required to deposit 2.70 g of aluminium. (Ar(Al) = 27, F = 96500 C mol⁻¹)” Their response addresses only this point: “Calculates moles of Al = 2.70 / 27 = 0.100 mol.” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[5 marks] - 21.
State one practical reason for electroplating an object with a metal such as chromium or silver, and outline the role of the object being plated in the electrolytic cell.
[3 marks] · no calculator - 22.
Marking analysis: A learner attempts the following task: “State one practical reason for electroplating an object with a metal such as chromium or silver, and outline the role of the object being plated in the electrolytic cell.” Their response addresses only this point: “States a valid reason, e.g. to protect the underlying metal from corrosion, or to improve its appearance.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 23.
Outline how a hydrogen-oxygen fuel cell generates electrical energy, including the half-equations at each electrode in alkaline conditions, and state one advantage of a fuel cell compared with burning hydrogen directly as a fuel.
[4 marks] · no calculator - 24.
Marking analysis: A learner attempts the following task: “Outline how a hydrogen-oxygen fuel cell generates electrical energy, including the half-equations at each electrode in alkaline conditions, and state one advantage of a fuel cell compared with burning hydrogen directly as a fuel.” Their response addresses only this point: “States the anode (negative electrode) half-equation: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator - 25.
Distinguish between a primary cell and a secondary cell, giving one example of each.
[2 marks] · no calculator - 26.
Marking analysis: A learner attempts the following task: “Distinguish between a primary cell and a secondary cell, giving one example of each.” Their response addresses only this point: “States that a primary cell cannot be recharged, since its electrode reactions are not easily reversible, e.g. a standard alkaline battery.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 27.
Write the standard cell notation (cell diagram) for a voltaic cell made from a Zn(s)/Zn²⁺(aq) half-cell and a Cu(s)/Cu²⁺(aq) half-cell, given that zinc is the more easily oxidised metal.
[3 marks] · no calculator - 28.
Marking analysis: A learner attempts the following task: “Write the standard cell notation (cell diagram) for a voltaic cell made from a Zn(s)/Zn²⁺(aq) half-cell and a Cu(s)/Cu²⁺(aq) half-cell, given that zinc is the more easily oxidised metal.” Their response addresses only this point: “States that the anode (oxidation, the more easily oxidised metal) is written on the left: Zn(s) | Zn²⁺(aq).” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 29.
Explain, in terms of electron transfer, why iron rusts when exposed to both oxygen and water, and state one method of preventing rusting based on this mechanism.
[4 marks] · no calculator - 30.
Marking analysis: A learner attempts the following task: “Explain, in terms of electron transfer, why iron rusts when exposed to both oxygen and water, and state one method of preventing rusting based on this mechanism.” Their response addresses only this point: “States that iron atoms are oxidised (lose electrons) to form Fe²⁺ ions: Fe → Fe²⁺ + 2e⁻, acting as an anode region.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator