A-LevelChemistryEquilibria9701/42 · Feb/Mar 2018 · Q315 Marks

Hydrogen cyanide, HCN, is a weak acid in aqueous solution. HCN(aq) ⇌ H+(aq) + CN-(aq) K = 6.2 x 10-10 mol dm-3 Calculate the pH of 0.10 mol dm⁻³ HCN(aq). pH =

(a)(i)Hydrogen cyanide, HCN, is a weak acid in aqueous solution. HCN(aq) ⇌ H+(aq) + CN-(aq) K = 6.2 x 10-10 mol dm-3 Calculate the pH of 0.10 mol dm⁻³ HCN(aq). pH =[2]
(a)(ii)Draw a 'dot-and-cross' diagram to represent the bonding in the hydrogen cyanide molecule. Show the outer shell electrons only.[1]
(a)(iii)State the hybridisation of the carbon and nitrogen atoms in hydrogen cyanide, and give the H-C-N bond angle. hybridisation of C hybridisation of N H-C-N bond angle[2]
(a)(iv)Suggest structures for the organic products A and B in the following reactions. Assume that HCN reacts in a similar way to RCN. [Figure with HCN reacting to A (H₂ + Ni) and B (heat with HCl(aq))][2]
(b)(i)Adding a measured quantity of KCN to a solution of NiCl₂ produces the complex [Ni(CN)₂Cl₂]ˣ. The complex can exist as two separate isomers with the same geometry (shape) around the nickel ion. Deduce the overall charge, x, on this complex. x =[1]
(b)(ii)State the type of isomerism shown by these isomers.[1]
(b)(iii)If bromide ions are present in the solution, the complex [Ni(CN)₂ClBr]ˣ can form. Assuming that [Ni(CN)₂ClBr]ˣ has the same geometry as [Ni(CN)₂Cl₂]ˣ, state the number of isomers of [Ni(CN)₂ClBr]ˣ that could exist, and draw their structures in the box. • number of isomers of [Ni(CN)₂ClBr]ˣ structures of the isomers of [Ni(CN)₂ClBr]ˣ[3]
(c)An aqueous solution of KCN is gradually added to a solution of NiSO₄ until the KCN is in excess. The following series of reactions takes place. [Flowchart: NiSO₄ → C (green precipitate) with KCN; C → D (yellow solution) with more KCN; D → E (red solution) with KCN in excess] • The oxidation state of nickel does not change during these reactions. • None of C, D or E contains sulfur. • C contains no potassium. • The K: Ni ratio in D is 2:1. • The K: Ni ratio in E is 3:1. Use the information to suggest the formulae of C, D and E. C D E[3]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.

[2] [H+] = √(Ka. c) = √(6.2 × 10−10 × 0.1) [H+] = 7.9 × 10−6 pH = -log[H+] = 5.1(0)
[1] [Dot-and-cross diagram showing H-C≡N. H shares 1e with C, C shares 3e with N. C has 1 lone pair, N has 1 lone pair. All outer shell electrons shown correctly.]
[2] C: sp and N: sp angle 180°
[2] A is CH3NH2 B is HCO2H
[1] 2-
[1] geometrical / cis-trans
[3] 2 isomers [Structures showing cis- and trans-isomers for square planar [Ni(CN)2ClBr]²⁻, with Br and Cl in adjacent positions for cis and opposite for trans.]
[3] C is Ni(CN)2 D is K2Ni(CN)4 E is K3Ni(CN)5