Skip to content
OCR A Jun 2016 Paper 5 Q7
Answers available below
Show answer
Question
1
2
3
4
5
6
7
8
![7 Hydroxide ions, OH, and cyanide ions, CN, can react with some aqueous solutions of transition metal compounds. 16 (a) When nickel(II) sulfate is dissolved in water, a pale green solution forms containing a six-coordinate complex ion C. Aqueous potassium hydroxide is added to aqueous nickel(II) sulfate. A green solid D forms. An excess of aqueous potassium cyanide is added to aqueous nickel(II) sulfate. A yellow solution forms containing a four-coordinate complex ion E that contains only nickel, carbon and nitrogen. (i) In C, D and E, nickel has the +2 oxidation state. Suggest the formulae of C, D and E. Complex ion C:Solid D:Complex ion E:[3] (ii) Write equations, and name the types of reaction, for the formation of D and E. Formation of solid D from aqueous nickel(II) sulfate. Equation:Type of reaction:Formation of complex ion E from complex ion C. Equation:Type of reaction:[4]OCR 2016<br />
(b) In some gold mines, cyanide is used to extract gold from its ore. 17 Gold metal in the ore reacts with cyanide ions, water and oxygen to form a water-soluble complex ion, [Au(CN)2], with a bond angle of 180. Hydroxide ions are also formed. (i) Name the shape of [Au(CN)2].[1] (ii) Using oxidation numbers, show that a redox reaction takes place.[2] (iii) Construct the overall equation for this reaction. State symbols are not required.[2] (iv) Some owners of gold mines remove cyanide ions from waste by adding oxidising agents, such as chlorate(I) ions, before discharge into watercourses. The overall equation is shown below. CN(aq) + ClO(aq) + 2H2O(l) NH4 +(aq) + CO3 2(aq) + Cl (aq) The oxidation half-equation is: CN + 3H2O + + CO3 NH4 2 + 2H+ + 2e Construct the reduction half-equation. State symbols are not required.[1]OCR 2016 [Total: 13] Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JUN16_question_7-0.jpg)
![F325 Question (i) 7 (a) Answer IGNORE any charges shown within complexes (treat as rough working) Complex ion C: [Ni(H2O)6]2+ (cid:57) Solid D: Complex ion E: [Ni(CN)4]2 (cid:57) Ni(OH)2 (cid:57) Mark scheme Marks June 2016 Guidance ALLOW +2 and 2 for charges Square brackets required ALLOW Ni(H2O)4(OH)2 (H2O)4 and (OH)2 in any order IGNORE any square brackets Square brackets required TAKE CARE for round brackets within complex ion, i.e. (H2O), (OH) and (CN) 22<br />
F325 Question 7 (a) Answer (ii) Mark independently of 7(a)(i) ALLOW +2 and 2 for charges IGNORE any charges shown within complexes (treat as rough working) Ni2+ + 2OH (cid:111) Ni(OH)2 (cid:57) Type of reaction: precipitation (cid:57) INDEPENDENT of equation [Ni(H2O)6]2+ + 4CN (cid:111) [Ni(CN)4]2 + 6H2O(l) (cid:57) Type of reaction: ligand substitution (cid:57) INDEPENDENT of equation (b) (i) linear (cid:57) June 2016 Guidance For equations: IGNORE state symbol (even if wrong) Square brackets not required for Ni(OH)2 ALLOW [Ni(H2O)6]2+ + 2OH (cid:111) [Ni(H2O)4(OH)2] + 2H2O ALLOW [Ni(H2O)6]2+ + 2OH (cid:111) Ni(OH)2 + 6H2O ALLOW NiSO4(aq) + 2OH(aq) (cid:111) Ni(OH)2(s) + SO4 2(aq) ALLOW NiSO4(aq) + 2KOH(aq) (cid:111) Ni(OH)2(s) + K2SO4(aq) ALLOW acid/base OR neutralisation OR deprotonation ONLY IF [Ni(H2O)6]2+ AND [Ni(H2O)4(OH)2] used ALLOW precipitate ALLOW [Ni(H2O)6]2+ + 4KCN (cid:111) [Ni(CN)4]2 + 6H2O + 4K+ LOOK at formulae for E from 7(a)(i) (copied at bottom) ALLOW ECF in 7aii Equation for no round brackets around CN, i.e. [NiCN4]2 in 7a(i) This is the only ECF allowed from 7ai structures. ALLOW ligand exchange IGNORE planar Mark scheme Marks 23<br />
F325 Question 7 (b) Answer (ii) Au/Gold has been oxidised from 0 to +1 (cid:57) O/Oxygen/O2 has been reduced from 0 to 2 (cid:57) Mark scheme Marks (b) (iii) IGNORE any charges shown within complexes (treat as rough working) 4Au + 8CN + 2H2O + O24[Au(CN)2] + 4OH (cid:57)(cid:57) First mark for all 6 species Second mark for balancing June 2016 Guidance IF Ag referred to, rather than Au, treat as a slip and apply BOD ALLOW 0 to 1 (i.e. no + sign for +1) ALLOW 1 mark for ALL oxidation numbers correct with no oxidised or reduced OR oxidation and reduction wrong way round, e.g. Au goes from 0 to +1 and O goes from 0 to 2 (cid:57) Au is reduced from 0 to +1 and O is oxidised from 0 to 2 (cid:57) IF Ag referred to, rather than Au, treat as a slip and apply BOD IGNORE state symbols CARE: In [Au(CN)2],sign is OUTSIDE square brackets For 1st mark, IGNORE e present ALLOW 1 mark for balanced equation with CN missing, i.e. 4Au + 2H2O + O24Au+ + 4OH ALLOW 1 mark rogue e on either side ALLOW multiples, e.g. 2Au + 4CN + H2O + O22[Au(CN)2] + 2OH Au + 2CN +H2O +O2[Au(CN)2] + OH IGNORE state symbols ALLOW e for electron ALLOW multiples (b) (iv) ClO + 2H+ + 2e (cid:111) Cl + H2O (cid:57) Total 13 24<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JUN16_answer_7-0.jpg)
