A-Level Chemistry October/November 2024 Q4(c)(i): Write an ionic equation for this reaction, using the formulae of the complex ions.

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  The reaction is a ligand exchange where cyanide ions displace ammonia ligands from the tetraamminecadmium(II) complex.

  The balanced ionic equation is:

  [Cd(NH3)4]2+ + 4CN- → [Cd(CN)4]2− + 4NH3

  Working:

  1. Identify Reactants and Products: The initial complex is [Cd(NH3)4]2+. It reacts with cyanide ions, CN-. This is a ligand substitution reaction, so the NH3 ligands are replaced by CN- ligands, forming a new complex and releasing NH3.
  2. Determine Oxidation State of Central Metal: In [Cd(NH3)4]2+, ammonia (NH3) is a neutral ligand (charge = 0). Therefore, the oxidation state of cadmium (Cd) is +2 to give the overall 2+ charge.
  3. Determine Formula of Product Complex: The oxidation state of Cd remains +2. Cyanide (CN-) is a uninegative ligand (charge = -1). Assuming the coordination number remains 4, the new complex will be [Cd(CN)4]. The charge is calculated as: (charge of Cd) + 4 * (charge of CN-) = (+2) + 4 * (-1) = -2. So the product complex is [Cd(CN)4]2-.
  4. Balance the Equation: To form one [Cd(CN)4]2- ion, four CN- ions are needed. These displace the four NH3 ligands from one [Cd(NH3)4]2+ ion. This gives the final balanced equation.

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  How the marks are awarded

  • B1 — The first mark is awarded for the fully correct and balanced ionic equation, showing correct formulae for all species ([Cd(NH3)4]2+, CN-, [Cd(CN)4]2−, NH3), correct charges, and correct stoichiometry (1:4:1:4).
  • B1 — The second mark is awarded for correctly identifying the formula and charge of the product complex ion as [Cd(CN)4]2−, which is a key component of the overall correct equation.

  Common mistakes

  • Incorrectly calculating the charge of the product complex, for example writing [Cd(CN)4]2+ by copying the reactant's charge, or [Cd(CN)4] with no charge.
  • Incorrect stoichiometry, such as not balancing the number of CN- ligands added and NH3 ligands displaced (e.g., using a 1:1 ratio).
  • Omitting the square brackets for the complex ions or the charges on the ions, which are essential components of the formulae.
  • Including spectator ions (e.g., Na+ from a NaCN reactant) in an equation that specifically asks for the ionic form.

  Examiner tip: Always calculate the charge of a product complex ion by summing the oxidation state of the central metal ion (which doesn't change in ligand exchange) and the total charge of the new ligands.

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