1. 1
   Aqueous ammonia reacts with hydrochloric acid: NH₃(aq) + HCl(aq) → NH₄⁺(aq) + Cl⁻(aq). A 25.0 cm³ sample of 0.0400 mol dm⁻³ hydrochloric acid has a hydrogen ion concentration of 0.0400 mol dm⁻³.

   (a)

   In the forward reaction, which species is the Brønsted–Lowry base?

   [1 mark]

   • AHCl
   • BNH₃
   • CNH₄⁺
   • DCl⁻

   (b)

   What is the pH of the hydrochloric acid sample?

   [1 mark]

   • A1.40
   • B0.0400
   • C2.40
   • D12.60

   (c)

   Deduce the formula of the conjugate acid of NH₃ and the conjugate base of HCl in this reaction, explaining how you identified each.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A voltaic cell is set up using a zinc electrode in 1.00 mol dm⁻³ zinc sulfate solution, connected by a wire and a salt bridge to a copper electrode in 1.00 mol dm⁻³ copper(II) sulfate solution. The cell produces a steady current, and the overall cell reaction is Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s).

   (a)

   Which electrode is the anode, and what happens to its mass as the cell operates?

   [1 mark]

   • AZinc; its mass decreases
   • BZinc; its mass increases
   • CCopper; its mass decreases
   • DCopper; its mass increases

   (b)

   In which direction do electrons flow in the external circuit, and in which direction do anions in the salt bridge move?

   [1 mark]

   • AFrom Cu to Zn in the wire; anions move towards the copper half-cell
   • BFrom Zn to Cu in the wire; anions move towards the zinc half-cell
   • CFrom Zn to Cu in the wire; anions move towards the copper half-cell
   • DFrom Cu to Zn in the wire; anions move towards the zinc half-cell

   (c)

   Deduce the oxidation state of copper in CuSO₄ and in Cu(s), and state whether copper is oxidised or reduced in this cell.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Butane, CH₃CH₂CH₂CH₃, reacts with chlorine gas in the presence of ultraviolet light. Initiation occurs when a small number of Cl₂ molecules absorb UV radiation and split to form chlorine radicals. These radicals then react with butane in a chain reaction that produces a mixture of monochlorinated products, together with some further-substituted by-products.

   (a)

   Write an equation, including radical dots, for the initiation step, and identify the type of bond-breaking involved.

   [3 marks]

   (b)

   Butane has two chemically distinct types of hydrogen atom. Explain, in terms of propagation and termination steps, why the reaction produces a mixture of products rather than a single pure compound, using 1-chlorobutane and 2-chlorobutane as examples.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   2-bromopropane, (CH₃)₂CHBr, is heated under reflux with aqueous sodium hydroxide. The hydroxide ion acts as a nucleophile, substituting the bromine to form propan-2-ol: (CH₃)₂CHBr + OH⁻ → (CH₃)₂CHOH + Br⁻. The hydrobromic acid, HBr(aq), formed if instead an acidic (non-nucleophilic) work-up is used is a strong acid, fully ionised in water.

   (a)

   Using a description in words of electron-pair movement, describe and explain the mechanism of this nucleophilic substitution reaction, and identify the type of bond-breaking that occurs at the C–Br bond.

   [6 marks]

   (b)

   If instead an acidic work-up produces HBr(aq) as a by-product, explain, in terms of Brønsted–Lowry theory and the extent of ionisation, why HBr(aq) has a much lower pH than an ethanoic acid solution of the same concentration, and deduce an equation for the reaction of this HBr(aq) with excess solid calcium carbonate.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   But-1-ene, CH₂=CHCH₂CH₃, is bubbled through orange bromine water. The alkene reacts by electrophilic addition, and the colour of the bromine water fades from orange to colourless as 1,2-dibromobutane forms: CH₂=CHCH₂CH₃ + Br₂ → CH₂BrCHBrCH₂CH₃.

   (a)

   Which species acts as the electrophile in this reaction?

   [1 mark]

   • AThe alkene, because it has a high electron density
   • BH₂O, because it can act as a nucleophile
   • CBr₂, because the high electron density of the C=C bond induces a dipole in the non-polar Br₂ molecule, making one bromine atom electron-deficient
   • DBr₂, because bromine is more electronegative than carbon

   (b)

   What happens to the Br–Br bond as the reaction proceeds?

   [1 mark]

   • AIt undergoes homolytic fission, forming two bromine radicals
   • BIt breaks only after the addition reaction is otherwise complete
   • CIt does not break; bromine adds as a single Br₂ unit to one carbon atom
   • DIt undergoes heterolytic fission, with both electrons going to the bromine atom furthest from the alkene

   (c)

   Explain why but-1-ene decolourises bromine water rapidly, whereas butane (with no C=C double bond) does not react with bromine water under the same conditions.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Molten lead(II) bromide, PbBr₂(l), is electrolysed using inert graphite electrodes connected to a direct current supply. Lead metal collects at one electrode and orange-brown bromine vapour is released at the other.

   (a)

   At which electrode does the lead form, and what type of electrode process occurs there?

   [1 mark]

   • AThe anode; oxidation
   • BThe anode; reduction
   • CThe cathode; reduction
   • DThe cathode; oxidation

   (b)

   Deduce the half-equation for the process occurring at the positive electrode.

   [1 mark]

   • APb²⁺ + 2e⁻ → Pb
   • BBr₂ + 2e⁻ → 2Br⁻
   • CPb → Pb²⁺ + 2e⁻
   • D2Br⁻ → Br₂ + 2e⁻

   (c)

   Explain how current is conducted through the molten lead(II) bromide, and why solid lead(II) bromide does not conduct electricity.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A gardener uses dilute nitric acid, HNO₃(aq), to lower the pH of an alkaline compost heap. The compost contains particles of solid calcium carbonate and solid magnesium hydroxide, both of which react with the acid.

   (a)

   Deduce balanced equations, including state symbols, for the reactions of dilute nitric acid with (i) solid calcium carbonate and (ii) solid magnesium hydroxide.

   [3 marks]

   (b)

   Describe, in words, the general shape of the pH curve obtained if a strong acid were added gradually to a large excess of strong alkali, from the start of the experiment until excess acid had been added, and state the pH at the equivalence point for a strong acid–strong base reaction.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   2-Methylpropane, (CH₃)₃CH, is reacted with bromine in the presence of ultraviolet light, giving a mixture of monobrominated products. The major product, 2-bromo-2-methylpropane, (CH₃)₃CBr, is then separated and heated under reflux with aqueous sodium hydroxide, which converts it into 2-methylpropan-2-ol, (CH₃)₃COH.

   (a)

   Explain, using the ideas of radical chain reactions, why 2-bromo-2-methylpropane is formed as the major product rather than one of the other possible monobrominated isomers, and write an equation for the initiation step of this reaction.

   [6 marks]

   (b)

   Using a description in words of electron-pair movement, describe and explain the mechanism by which hydroxide ions convert 2-bromo-2-methylpropane into 2-methylpropan-2-ol, and explain why potassium hydroxide, rather than potassium chloride, is used to bring about this reaction.

   [6 marks]

   Total for question 8: 12 marks

End of questions