A-Level Chemistry October/November 2024 Q2(c)(iv): Aqueous AgNO3 dissolved in ethanol reacts with an aqueous solution of CHCl3. State what…

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  Observation

  A white precipitate is formed.

  Explanation

  The aqueous ethanolic conditions cause the CHCl₃ to undergo hydrolysis. During this reaction, the covalent C-Cl bond is broken, which releases chloride ions (Cl⁻) into the solution. These aqueous chloride ions then react with the aqueous silver ions (Ag⁺) from the silver nitrate, forming an insoluble white precipitate of silver chloride, AgCl.

  Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

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  How the marks are awarded

  • M1 — Correctly stating the observation that a white precipitate (or white ppt) is formed.
  • M2 — Explaining the observation by stating that CHCl₃ is hydrolysed, which releases chloride ions (Cl⁻), and these ions then react with Ag⁺ to form silver chloride.

  Common mistakes

  • Stating 'no reaction' or 'no precipitate', incorrectly assuming the C-Cl bond in CHCl₃ is too strong to be hydrolysed under these conditions.
  • Giving the incorrect precipitate colour, such as 'cream' (for AgBr) or 'yellow' (for AgI), showing confusion in the tests for different halide ions.
  • Providing an incomplete explanation, such as 'silver chloride is formed', without explaining that the chloride ions are first released from CHCl₃ via hydrolysis.
  • Describing the reaction as instantaneous; the hydrolysis of CHCl₃ is relatively slow compared to iodoalkanes or bromoalkanes, so the precipitate forms slowly.

  Examiner tip: Always link the observation of a precipitate with aqueous silver nitrate back to the hydrolysis of the C-X bond, which releases the halide ion required for the precipitation reaction.

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