Shapes of complex ions and isomerismAQA A-Level Chemistry: Subtopic test

1. 1
   A student compares the shapes of four complex ions: [Ni(H₂O)₆]²⁺, [CoCl₄]²⁻, [Ag(NH₃)₂]⁺ and [Pt(NH₃)₂Cl₂].

   (a)

   What are the shape and bond angle of the complex ion [CoCl₄]²⁻?

   [1 mark]

   • AOctahedral, 90°
   • BSquare planar, 90°
   • CLinear, 180°
   • DTetrahedral, 109.5°

   (b)

   Which of these complex ions is linear?

   [1 mark]

   • A[Ni(H₂O)₆]²⁺
   • B[Ag(NH₃)₂]⁺
   • C[CoCl₄]²⁻
   • D[Pt(NH₃)₂Cl₂]

   (c)

   Explain why [Ni(H₂O)₆]²⁺ is octahedral but [CoCl₄]²⁻ is tetrahedral.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The octahedral complex ion [Co(NH₃)₄Cl₂]⁺ exists in two forms, one green and one violet. The complex ion [Ni(en)₃]²⁺, where en is the bidentate ligand H₂NCH₂CH₂NH₂, also exists in two forms.

   (a)

   What type of isomerism is shown by the two forms of [Co(NH₃)₄Cl₂]⁺?

   [1 mark]

   • AOptical isomerism
   • BChain isomerism
   • CCis-trans isomerism
   • DPositional isomerism

   (b)

   Which statement describes the trans isomer of [Co(NH₃)₄Cl₂]⁺?

   [1 mark]

   • AThe two Cl⁻ ligands are opposite each other, at 180°
   • BThe two Cl⁻ ligands are next to each other, at 90°
   • CIt is the non-superimposable mirror image of the cis isomer
   • DOne Cl⁻ ligand is bonded to the other

   (c)

   State what is meant by optical isomers and explain why [Ni(en)₃]²⁺ shows optical isomerism.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Platinum(II) forms the neutral complex Pt(NH₃)₂Cl₂, which has a square planar shape and exists as two isomers. One of them, cisplatin, is used as an anticancer drug.

   (a)

   State the shape of the complex, its co-ordination number and the arrangement of the chloride ligands in cisplatin.

   [3 marks]

   (b)

   Explain why square planar Pt(NH₃)₂Cl₂ shows cis-trans isomerism, but a tetrahedral complex of formula NiCl₂(NH₃)₂ would not.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is asked to deduce the structures of several complex ions: [Co(H₂O)₆]²⁺, [CoCl₄]²⁻, [Ag(NH₃)₂]⁺ (the active species in Tollens' reagent), [Cr(H₂O)₄Cl₂]⁺ and [Cr(C₂O₄)₃]³⁻, where C₂O₄²⁻ is the bidentate ethanedioate ion.

   (a)

   Describe and explain the shapes and bond angles of [Co(H₂O)₆]²⁺, [CoCl₄]²⁻ and [Ag(NH₃)₂]⁺.

   [6 marks]

   (b)

   Explain how [Cr(H₂O)₄Cl₂]⁺ and [Cr(C₂O₄)₃]³⁻ each show isomerism, naming the type of isomerism in each case.

   [6 marks]

   Total for question 4: 12 marks

End of questions