Chemistry SL
Reactivity 3: mechanisms of reaction — Reactivity 3
- 1.
Define an acid and a base according to the Brønsted-Lowry theory, in terms of proton transfer.
[2 marks] · no calculator - 2.
Marking analysis: A learner attempts the following task: “Define an acid and a base according to the Brønsted-Lowry theory, in terms of proton transfer.” Their response addresses only this point: “Defines a Brønsted-Lowry acid as a proton (H⁺) donor.” 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.
For the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, identify the two conjugate acid-base pairs.
[3 marks] · no calculator - 4.
Marking analysis: A learner attempts the following task: “For the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, identify the two conjugate acid-base pairs.” Their response addresses only this point: “Identifies NH₃ and NH₄⁺ as one conjugate pair, with NH₃ the base and NH₄⁺ its conjugate acid.” 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 - 5.
Calculate the pH of a 0.0500 mol dm⁻³ solution of a strong acid, HCl, assuming complete dissociation.
[3 marks] - 6.
Marking analysis: A learner attempts the following task: “Calculate the pH of a 0.0500 mol dm⁻³ solution of a strong acid, HCl, assuming complete dissociation.” Their response addresses only this point: “States that for a strong monoprotic acid, [H⁺] equals the acid concentration: [H⁺] = 0.0500 mol dm⁻³.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] - 7.
Define oxidation and reduction in terms of electron transfer, and use these definitions to explain why oxidation and reduction always occur together in a redox reaction.
[3 marks] · no calculator - 8.
Marking analysis: A learner attempts the following task: “Define oxidation and reduction in terms of electron transfer, and use these definitions to explain why oxidation and reduction always occur together in a redox reaction.” Their response addresses only this point: “Defines oxidation as the loss of electrons.” 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 - 9.
Deduce the oxidation state of chromium in the dichromate ion, Cr₂O₇²⁻.
[3 marks] · no calculator - 10.
Marking analysis: A learner attempts the following task: “Deduce the oxidation state of chromium in the dichromate ion, Cr₂O₇²⁻.” Their response addresses only this point: “States that oxygen has oxidation state −2, so 7 oxygens contribute −14.” 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 - 11.
Write the half-equations for the oxidation of Fe²⁺ to Fe³⁺ and the reduction of MnO₄⁻ to Mn²⁺ in acidic solution, then combine them into the overall balanced ionic equation.
[4 marks] · no calculator - 12.
Marking analysis: A learner attempts the following task: “Write the half-equations for the oxidation of Fe²⁺ to Fe³⁺ and the reduction of MnO₄⁻ to Mn²⁺ in acidic solution, then combine them into the overall balanced ionic equation.” Their response addresses only this point: “Writes the oxidation half-equation: Fe²⁺ → Fe³⁺ + e⁻.” 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 - 13.
Ethene, C₂H₄, reacts with hydrogen bromide, HBr, in an addition reaction. Outline the mechanism, identifying the type of mechanism involved.
[4 marks] · no calculator - 14.
Marking analysis: A learner attempts the following task: “Ethene, C₂H₄, reacts with hydrogen bromide, HBr, in an addition reaction. Outline the mechanism, identifying the type of mechanism involved.” Their response addresses only this point: “States that this is an electrophilic addition mechanism.” 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.
Methane reacts with chlorine in the presence of ultraviolet light in a substitution reaction. State the type of mechanism, and outline the initiation step.
[2 marks] · no calculator - 16.
Marking analysis: A learner attempts the following task: “Methane reacts with chlorine in the presence of ultraviolet light in a substitution reaction. State the type of mechanism, and outline the initiation step.” Their response addresses only this point: “States that this is a free-radical substitution mechanism.” 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 - 17.
Explain why sulfuric acid, H₂SO₄, is described as a diprotic acid, and state the number of moles of NaOH required to fully neutralise 1 mole of H₂SO₄.
[3 marks] · no calculator - 18.
Marking analysis: A learner attempts the following task: “Explain why sulfuric acid, H₂SO₄, is described as a diprotic acid, and state the number of moles of NaOH required to fully neutralise 1 mole of H₂SO₄.” Their response addresses only this point: “States that a diprotic acid can donate two protons (H⁺ ions) per molecule.” 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.
Describe a chemical test to distinguish between a solution containing Fe²⁺ ions and one containing Fe³⁺ ions, in terms of a redox reaction with acidified potassium manganate(VII).
[3 marks] · no calculator - 20.
Marking analysis: A learner attempts the following task: “Describe a chemical test to distinguish between a solution containing Fe²⁺ ions and one containing Fe³⁺ ions, in terms of a redox reaction with acidified potassium manganate(VII).” Their response addresses only this point: “States that acidified potassium manganate(VII) (purple) is added dropwise to each solution.” 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 - 21.
Ethanoic acid has Ka = 1.8 × 10⁻⁵. Calculate the pH of a 0.0500 mol dm⁻³ solution of ethanoic acid, stating one assumption made.
[4 marks] - 22.
Marking analysis: A learner attempts the following task: “Ethanoic acid has Ka = 1.8 × 10⁻⁵. Calculate the pH of a 0.0500 mol dm⁻³ solution of ethanoic acid, stating one assumption made.” Their response addresses only this point: “Sets up Ka ≈ [H⁺]²/[HA]₀, assuming [H⁺] is small compared to the initial acid concentration.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 23.
Bromoethane reacts with aqueous sodium hydroxide in a nucleophilic substitution reaction to form ethanol. Outline the mechanism, identifying the nucleophile and stating why the carbon-bromine bond breaks heterolytically.
[4 marks] · no calculator - 24.
Marking analysis: A learner attempts the following task: “Bromoethane reacts with aqueous sodium hydroxide in a nucleophilic substitution reaction to form ethanol. Outline the mechanism, identifying the nucleophile and stating why the carbon-bromine bond breaks heterolytically.” Their response addresses only this point: “States that the hydroxide ion, OH⁻, acts as the nucleophile, attracted to the slightly positive (δ+) carbon atom bonded to the electronegative bromine.” 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.
State what is measured by a standard electrode potential, and state the reference value assigned to the standard hydrogen electrode.
[2 marks] · no calculator - 26.
Marking analysis: A learner attempts the following task: “State what is measured by a standard electrode potential, and state the reference value assigned to the standard hydrogen electrode.” Their response addresses only this point: “States that a standard electrode potential measures the tendency of a half-cell to undergo reduction, relative to a standard reference, under standard conditions.” 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.
State what is meant by a buffer solution, and outline how a mixture of a weak acid and its conjugate base can resist changes in pH when a small amount of acid is added.
[3 marks] · no calculator - 28.
Marking analysis: A learner attempts the following task: “State what is meant by a buffer solution, and outline how a mixture of a weak acid and its conjugate base can resist changes in pH when a small amount of acid is added.” Their response addresses only this point: “States that a buffer solution resists changes in pH when small amounts of acid or base are added.” 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.
State what is meant by an amphiprotic species, and explain, using equations, how the hydrogencarbonate ion, HCO₃⁻, can act as both a Brønsted-Lowry acid and a base.
[3 marks] · no calculator - 30.
Marking analysis: A learner attempts the following task: “State what is meant by an amphiprotic species, and explain, using equations, how the hydrogencarbonate ion, HCO₃⁻, can act as both a Brønsted-Lowry acid and a base.” Their response addresses only this point: “States that an amphiprotic species can act as either an acid (proton donor) or a base (proton acceptor), depending on what it reacts with.” 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