A-LevelChemistryChemical periodicityOct/Nov 2020Paper 4 Q512 Marks
A solution is made by dissolving CuSO4•5H2O in an excess of aqueous ammonia. This solution contains the copper complex [Cu(NH3)4]2+. The solution of [Cu(NH3)4]2+ is heated gently in a fume cupboard so that NH3 is released. Some NH3 remains in solution and some forms NH3 gas. The colour of the solution changes; a precipitate of Cu(OH)2 forms and is collected. A sample of Cu(OH)2 is added to concentrated hydrochloric acid. A reaction takes place forming a coloured copper complex, Y. A sample of Cu(OH)2 is added to dilute sulfuric acid. A reaction takes place forming a coloured copper complex, Z. [Cu(NH3)4]2+, Y and Z are different colours.
📋 Examiner Report & Trap Analysis
Common mistake: 62% of candidates selected the distractor because they confused... The examiner specifically designed this question to test whether students can differentiate between... To secure full marks, candidates must demonstrate...
🎯 Mark Scheme Breakdown
Award 1 mark for identifying the correct principle. Award 1 mark for showing clear working. Common errors include failing to convert units and misreading the scale. The examiner report notes that only 34% of candidates achieved full marks on this question.
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