(a) When 1.0 mol dm⁻³ Na2S2O3(aq) is added to a solution containing Ag+(aq) ions, a linear complex, P, is formed. S2O32- ions are present in P as monodentate ligands.
- (a)(i)Define the term monodentate ligand.[2]
- (a)(ii)Give the formula of P, including its charge.[1]
- (b)(i)When 1.0mol dm⁻³ NaCN(aq) is added to a solution of P, a mixture which includes a second linear complex, Q, is formed. In this mixture the concentration of Q is much greater than the concentration of P. Write an equation for the reaction that occurs when NaCN(aq) is added to a solution of P.[1]
- (b)(ii)Suggest a reason why the concentration of Q is much greater than the concentration of P in the mixture.[1]
- (b)(iii)Name the type of reaction in which P forms Q.[1]
- (c)(i)Platinum forms a complex ion with the formula [Pt(CN)2Cl2]2-. In this complex ion the carbon atom of each CN- ligand bonds to the platinum ion. This complex shows stereoisomerism. There are only two isomers of this complex. Draw structures of these two isomers in the boxes below. [Figure shows two empty boxes labeled Pt][1]
- (c)(ii)Describe the geometry of [Pt(CN)2Cl2]2-.[1]
- (c)(iii)Name the type of stereoisomerism shown by [Pt(CN)2Cl2]2-.[1]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.
- [1] (a species) that donates one lone pair
- [1] to form a dative / coordinate to a central metal atom / metal ion
- [1] [Ag(S2O3)2]3-
- [1] [Ag(S2O3)2]3- + 2CN- → [Ag(CN)2]- + 2S2O32- OR Ag(S2O3)2]3- + 2NaCN → [Ag(CN)2]¯ + Na2S2O3 + S2O32-
- [1] Q is more stable / has a larger Kstab than P
- [1] ligand exchange / displacement / substitution
- [1] (c)(i) Cis isomer: Pt in center, two Cls adjacent, two CNs adjacent. Trans isomer: Pt in center, two Cls opposite, two CNs opposite. (both correct)
- [1] square planar
- [1] cis-trans OR geometric(al)
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