A-Level Chemistry May/June 2024 Q2(c): Tollens' reagent can be used to distinguish between aldehydes and ketones. Tollens' rea…

1 answer

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  Step 1

  The reaction between aqueous silver nitrate and aqueous sodium hydroxide forms a precipitate of silver(I) oxide.

  $2\text{AgNO}_3\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Ag}_2\text{O(s)} + 2\text{NaNO}_3\text{(aq)} + \text{H}_2\text{O(l)}$

  Step 2

  The silver(I) oxide precipitate dissolves in aqueous ammonia to form the soluble diamminesilver(I) hydroxide complex.

  $\text{Ag}_2\text{O(s)} + 4\text{NH}_3\text{(aq)} + \text{H}_2\text{O(l)} \rightarrow 2[\text{Ag(NH}_3)_2]\text{OH(aq)}$

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  How the marks are awarded

  • M1 — The first mark is for a correct, balanced equation for Step 1, showing the formation of silver(I) oxide from silver nitrate and sodium hydroxide. The ionic equation 2Ag+ + 2OH- → Ag2O + H2O is also acceptable.
  • M2 — The second mark is for a correct, balanced equation for Step 2, showing the dissolution of silver(I) oxide in aqueous ammonia to form the diamminesilver(I) complex. Water must be included as a reactant.

  Common mistakes

  • In Step 1, writing AgOH as the product instead of Ag2O and H2O. Silver(I) hydroxide is unstable and readily dehydrates.
  • In Step 2, incorrect balancing, particularly with the stoichiometry of ammonia and the final complex, e.g., Ag2O + 2NH3 + H2O → [Ag(NH3)2]OH.
  • Omitting water as a reactant in the equation for Step 2. The equation Ag2O + 4NH3 → 2[Ag(NH3)2]OH is not balanced for oxygen or hydrogen.
  • Writing the incorrect formula for the diamminesilver(I) complex, for example, [Ag(NH3)]+ or using an incorrect charge.

  Examiner tip: Memorise the key equations for the preparation and reactions of common laboratory reagents, including correct formulae, balancing, and state symbols.

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