Oxidation, reduction and disproportionationEdexcel A-Level Chemistry: Subtopic test

1. 1
   A teacher demonstrates two reactions of s- and p-block elements: magnesium ribbon burning in oxygen, 2Mg + O₂ → 2MgO, and sodium burning in chlorine, 2Na + Cl₂ → 2NaCl.

   (a)

   In the reaction of magnesium with oxygen, which statement about oxygen is correct?

   [1 mark]

   • AIt is oxidised because it gains electrons
   • BIt is reduced because it loses electrons
   • CIt is a reducing agent because it gains electrons
   • DIt is an oxidising agent because it gains electrons

   (b)

   In the reaction of sodium with chlorine, which statement about sodium is correct?

   [1 mark]

   • AIt is oxidised because its oxidation number increases from 0 to +1
   • BIt is reduced because its oxidation number increases from 0 to +1
   • CIt is oxidised because it gains an electron
   • DIt is reduced because it gains an electron to form Na⁺

   (c)

   Explain, in terms of electron transfer, why magnesium acts as a reducing agent in its reaction with oxygen.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Chlorine gas is bubbled through aqueous potassium bromide. The solution turns orange as bromine forms. The ionic equation is Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.

   (a)

   Which species is the oxidising agent in this reaction?

   [1 mark]

   • ABr⁻
   • BCl₂
   • CCl⁻
   • DBr₂

   (b)

   Which statement describes what happens to the bromide ions?

   [1 mark]

   • AThey are reduced because they gain electrons
   • BThey are reduced because their oxidation number goes from −1 to 0
   • CThey are oxidised because they lose electrons and their oxidation number increases from −1 to 0
   • DThey are spectator ions and are neither oxidised nor reduced

   (c)

   Use oxidation numbers to show that this is a redox reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Chlorine is used to disinfect swimming pool water. When it dissolves in water it forms chloride ions and chlorate(I) ions, ClO⁻, which kill bacteria.

   (a)

   The reaction of chlorine with water is Cl₂ + H₂O → HCl + HClO. Use oxidation numbers to explain why this is a disproportionation.

   [3 marks]

   (b)

   Explain, using oxidation numbers and electrons, why chlorate(I) ions act as an oxidising agent when they are converted to chloride ions.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemistry teacher investigates the reactions of chlorine, a p-block element, with a Group 2 metal and with sodium hydroxide solution.

   (a)

   Calcium burns in chlorine to form calcium chloride, Ca + Cl₂ → CaCl₂. Explain, in terms of electron transfer and oxidation numbers, why this is a redox reaction, identify the oxidising agent and the reducing agent, and state the general pattern shown by metals and non-metals when they form ions.

   [6 marks]

   (b)

   Chlorine reacts with cold dilute sodium hydroxide: Cl₂ + 2NaOH → NaCl + NaClO + H₂O. With hot concentrated sodium hydroxide the reaction is 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O. Use oxidation numbers to show that both reactions are disproportionations and compare the extent of oxidation of chlorine in the two reactions.

   [6 marks]

   Total for question 4: 12 marks

End of questions