Credit will be given for strict adherence to the instructions, for observations precisely recorded and for accurate inferences. All tests, observations, and inferences must be clearly entered in your answer book, in ink, at the i.itne time they are made.
C is a sample of copper (II) tetraoxosulphate (VI) crystals. Carry out the following exercises on C. Record your observations and identify any gases evolved. State the conclusion you draw from the result of each test.
(a) Put half of C in a test tube and heat strongly
(b) Make solution of about 10 cm\(^{-3}\) with the sécond half of C and divide it into three portions
(i) To the first portion, add Sodium hydroxide solution in drops and then in excess. Heat the mixture
(ii) To the second portion, add aqueous ammonia in drops and then in excess followed by few drops of moderately concentrated HCI.
(iii) To the third portion, add all the zinc dust provided and stir thoroughly until there is a visible change.
C is hydrated copper(II) tetraoxosulphate(VI), \(CuSO_4\cdot5H_2O\). The expected observations and inferences are:
| Test | Observation | Inference / Conclusion |
|---|
| (a) Heat half of C strongly | The blue crystals turn white and colourless droplets condense on the cooler part of the tube; on stronger heating the residue turns black and a colourless gas of choking smell (\(SO_2/SO_3\)) is given off | Water of crystallisation is present (blue \(\to\) white anhydrous salt, water evolved); on strong heating it decomposes to black \(CuO\) |
| (b)(i) Solution + \(NaOH\) dropwise then excess, heat | A pale blue gelatinous precipitate forms, insoluble in excess; on heating it turns black | \(Cu^{2+}\) present (\(Cu(OH)_2\), which becomes black \(CuO\) on heating) |
| (b)(ii) Solution + \(NH_3\) dropwise then excess, then HCl | With a few drops of ammonia a pale blue precipitate forms; in excess ammonia it dissolves to a deep (royal) blue solution; adding \(HCl\) discharges the deep blue colour giving a pale blue/green solution | \(Cu^{2+}\) present, forming the soluble tetraamminecopper(II) complex \([Cu(NH_3)_4]^{2+}\), which is decomposed by acid |
| (b)(iii) Third portion + zinc dust, stir | The blue colour fades to colourless and a reddish-brown solid (copper) is deposited | Zinc displaces copper (Zn is above Cu in the activity series): \(Zn + CuSO_4 \to ZnSO_4 + Cu\) |
General conclusion: C contains \(Cu^{2+}\) and \(SO_4^{2-}\) ions and water of crystallisation, confirming hydrated copper(II) tetraoxosulphate(VI).
C is hydrated copper(II) tetraoxosulphate(VI), \(CuSO_4\cdot5H_2O\). The expected observations and inferences are:
| Test | Observation | Inference / Conclusion |
|---|
| (a) Heat half of C strongly | The blue crystals turn white and colourless droplets condense on the cooler part of the tube; on stronger heating the residue turns black and a colourless gas of choking smell (\(SO_2/SO_3\)) is given off | Water of crystallisation is present (blue \(\to\) white anhydrous salt, water evolved); on strong heating it decomposes to black \(CuO\) |
| (b)(i) Solution + \(NaOH\) dropwise then excess, heat | A pale blue gelatinous precipitate forms, insoluble in excess; on heating it turns black | \(Cu^{2+}\) present (\(Cu(OH)_2\), which becomes black \(CuO\) on heating) |
| (b)(ii) Solution + \(NH_3\) dropwise then excess, then HCl | With a few drops of ammonia a pale blue precipitate forms; in excess ammonia it dissolves to a deep (royal) blue solution; adding \(HCl\) discharges the deep blue colour giving a pale blue/green solution | \(Cu^{2+}\) present, forming the soluble tetraamminecopper(II) complex \([Cu(NH_3)_4]^{2+}\), which is decomposed by acid |
| (b)(iii) Third portion + zinc dust, stir | The blue colour fades to colourless and a reddish-brown solid (copper) is deposited | Zinc displaces copper (Zn is above Cu in the activity series): \(Zn + CuSO_4 \to ZnSO_4 + Cu\) |
General conclusion: C contains \(Cu^{2+}\) and \(SO_4^{2-}\) ions and water of crystallisation, confirming hydrated copper(II) tetraoxosulphate(VI).
