JUNIOR COLLEGE CHEMISTRY DEPARTMENT EXPERIMENT 21 SECOND YEAR PRACTICAL. Name: Group: Date: THE CHEMISTRY OF COPPER AND IRON

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1 JUNIOR COLLEGE CHEMISTRY DEPARTMENT EXPERIMENT 21 SECOND YEAR PRACTICAL Name: Group: Date: COPPER THE CHEMISTRY OF COPPER AND IRON 1. To a solution of Cu 2+ ions add aqueous sodium hydroxide and heat the mixture Blue ppt. insoluble in excess. Precipitate turned to a black solid on heating Blue ppt is Cu(OH) 2 which turns to black CuO on heating Cu 2+ (aq) + 2OH - (aq) Cu(OH) 2(s) CuO (s) + H 2O (l) 2. (a) To a solution of Cu 2+ ions add aqueous ammonia slowly until no further change occurs. Keep this mixture for test (b). Blue ppt. soluble in excess giving a deep Blue ppt is Cu(OH) 2 while the deep blue solution blue solution. contains the water soluble Cu(NH 3) 4 2+ ion. Cu 2+ (aq)+2oh - (aq) Cu(OH) 2(s) Cu(NH 3) 4 2+ (aq) +2OH - (b) To the mixture obtained in (a) add aqueous sodium salicylate (2-hydroxybenzoate). Keep this mixture for test (c). The deep blue solution changed to a Ligand exchange gives the formation of the green dark green solution on addition of copper (II) salicylate complex : sodium salicylate solution Cu(NH 3) 4 2+ (aq) +2Salic -- (aq) Cu(salic) 2(aq) +4NH 3(aq) 3. To a solution of Cu 2+ ions add concentrated hydrochloric slowly until no further change occurs. The light blue solution turned to a Formation of the yellow CuCl 4 2- (aq) complex ion in a green solution. blue solution shows mixture as green. Cu(H 2O) 4 2+ (aq) + 4Cl - (aq) CuCl 4 2- (aq) + 4 H 2O(l) Expt 21 Page 1

2 4. To a solution of Cu 2+ ions add aqueous EDTA The light blue solution changed to a The hexadentate ligand EDTA replaced the water slightly different shade of light blue. ligands in Cu(H 2O) 2+ 4 Cu(H 2O) 4 2+ (aq) + EDTA 4- (aq) CuEDTA 2- (aq)+ 4H 2O(l) 5. To a solution of Cu 2+ ions add aqueous potassium hexacyanoferrate (II) A red-brown precipitate formed on The red-brown precipitate with hexacyanoferrate(ii) on addition of the hexacyanoferrate(ii) is characteristic of Cu 2Fe(CN) 6 solution Cu 2+ (aq) + Fe(CN) 6 4- (aq) Cu 2Fe(CN) 6(s) 6 (a) To about 1 cm 3 of Fehling s solution A, add 1 cm 3 of Fehling s solution B slowly. Add 2 cm 3 of glucose solution and warm the mixture in a beaker of boiling water. The mixture should just boil for about 30 seconds. (Fehling s solution contains copper (II) ions). Decant the liquid and keep the precipitate for the next test A very fine red-brown precipitate Classically a test for reducing sugars, the Fehling settled to the bottom of the test tube. test provides a method for forming red-brown Cu 2O. Cu 2+ (aq) Cu 2O (s) (b) To the reddish-brown precipitate obtained in (a), add concentrated hydrochloric and warm the mixture. The precipitate dissolves to give a colourless solution. The red-brown (/orange) Cu 2O dissolves to give the colourless water-soluble complex : CuCl 2 - (aq) Cu 2O (s) + 4HCl (aq) 2CuCl 2 - (aq) + H 2O (l)+2h + (aq) 7. Dissolve some copper(ii) chloride in some distilled water in a boiling tube, add some concentrated HCl, and some copper powder. Boil the mixture until a brown solution forms. Then pour the contents into some water in a small beaker. A very dark brown (black?) solution Dark brown solution contains CuCl 4 2- (aq) (brown forms at first but it goes colourless. White ppt. forms in excess water. when conc) and CuCl 4 3- (aq) ions. White ppt is CuCl (s) Cu+Cu 2+ (aq) +c.hcl CuCl 2 - (aq) CuCl (s) in water. Expt 21 Page 2

3 8. To some copper(i) oxide in a test tube add (a) concentrated HCl and warm The red-brown (orange) solid The colourless CuCl 2 - (aq) forms in conc. HCl but dissolves to form a colourless solution dilution with water precipitates out the CuCl (s) Dilution gives a white precipitate Cu 2O (s) + c.hcl CuCl 2 - (aq) CuCl (s) in water. (b) dilute sulphuric and warm The solid dissolved but the solution The Cu 2O dissolved in the sulphuric but the gradually turned blue and the red- unstable Cu + (aq) disproportionate to Cu (s) and Cu 2+ brown solid reappeared Cu 2O (s) + 2H + (aq) H 2O + 2Cu + (aq) Cu (s)+ Cu 2+ (aq) 9. To a solution of Cu 2+ ions add aqueous potassium iodide, followed by gradual addition of aqueous sodium thiosulphate. Adding I - to Cu 2+ gave a light brown Iodide reduces Cu 2+ to Cu + (which precipitates as a ppt in a dark brown solution. Only the white ppt remained with S 2O white solid) and I 2(aq) (brown solution) forms. Cu 2+ (aq)+4i - (aq) 2CuI (s) + I 2(aq) I 2 + S 2O 3 2-2I - +S 4O Examine the s of COPPER METAL with the following reagents in the cold. If no occurs, boil carefully (do not boil the conc. HNO 3 ). Note and test for any gases evolved. Enter your observations in the table below. Where no occurs enter no visible change. water aqueous sodium hydroxide dilute hydrochloric dilute sulphuric concentrated nitric Vigorous evolving a brown gas and forming a green solution Expt 21 Page 3

4 IRON 1. To a solution of Fe 2+ ions add aqueous sodium hydroxide. Note the colour of the precipitate on standing. Green precipitate, insoluble in Green precipitate is Fe(OH) 2 but turns to brown excess. Ppt. turned rusty brown on Fe(OH) 3 in air. Fe 2+ (aq) + 2OH -- Fe(OH) 2(s) standing. Fe(OH) 2(s) + OH (aq)+o 2 Fe(OH) 3(s) Fe 2O 3 2. To a solution of Fe 2+ ions add aqueous ammonia. Note the colour of the precipitate on standing. Green precipitate, insol. In xs. NH 3 As above. Green ppt is Fe(OH) 2 Brown ppt is Precipitate turned rusty brown on Fe(OH) 3 turning to Fe 2O 3 with loss of water. standing 3. To a solution of Fe 2+ ions add aqueous potassium thiocyanate Light brown solution results on addition of the KCNS solution. Colour is due to the presence of Fe 3+ impurities as the complex Fe(H 2O) 5CNS] + is colourless 2+ (aq) + CNS (aq) [Fe(H 2O) 5CNS] + (aq) + H 2O (l) 4. To a few cm 3 of aqueous Fe 2 add a few drops of the potassium thiocyanate solution followed by a few cm 3 of hydrogen peroxide. Note the colour of the solution and add tin(ii) chloride solution until a colour change occurs. Addition of H 2O 2 to the light brown Colourless [Fe(H 2O) 5CNS] + is oxidized to blood- solution gave a blood-red solution. This turned colourless with SnCl 2. red [Fe(H 2O) 5CNS] 2+ with H 2O 2 and reduced again with SnCl 2 solution 5. To a solution of Fe 2+ ions add 1 cm 3 concentrated nitric. Note the colour of the solution. A brown solution was noted. NO, formed by of HNO 3 with Fe 2+, Solution gradually turned light forms the brown complex [Fe(H 2O) 5NO] 2+ green. 2+ (aq) + NO [Fe(H 2O) 5NO] 2+ (aq) Expt 21 Page 4

5 6. To a solution of Fe 2+ ions add aqueous potassium hexacyanoferrate (II) Light blue precipitate. (Reaction of little significance) Fe 2+ (aq) + Fe(CN) 4 6 (aq) Fe 2Fe(CN) 6(s) 7. To a solution of Fe 2+ ions add aqueous potassium hexacyanoferrate (III) A very dark blue, almost black, Important Charac. coloured ppt formed characteristic precipitate is formed. 3Fe 2+ (aq) + 2Fe(CN) 6 3 (aq) Fe 3[Fe(CN) 6] 2(s) 8. To a solution of Fe 2+ ions add aqueous ified potassium manganate(vii). The purple MnO -- 4 colour is Reaction between Fe 2+ and MnO 4 produces the discharged. (almost) colourless Mn 2+ and Fe 3+ 5Fe 2+ +MnO H + 5Fe 3+ +Mn 2+ +4H 2O 9. DO THIS TEST IN A FUME CUPBOARD. Heat some FeSO 4.7H 2 O crystals in a dry test tube, first gently and then strongly. The pale green crystals turn white but form a brown solid on strong FeSO 4.7H 2O (s) FeSO 4 + 7H 2O (l) on heating 2FeSO 4(s) Fe 2O 3(s) + SO 2(g) + SO 3(g) on heating. Acidic gas is also released further (stronger) heating. THE FOLLOWING TESTS REFER TO IRON(III). 10. To a solution of Fe 3+ ions add aqueous sodium hydroxide. A dark brown precipitate, insoluble Ppt is Fe(OH) 3 in excess, is produced Fe 3+ (aq) + 3OH (aq) Fe(OH) 3(s) Fe(OH) 3(s) Fe 2O 3.xH 2O 11. To a solution of Fe 3+ ions add aqueous ammonia. Observations as (10) above Comments as above Expt 21 Page 5

6 12. To a solution of Fe 3+ ions add aqueous potassium thiocyanate. A blood-red solution is formed when The bright red coloured complex (shown) forms KCNS is added to Fe 3+ (aq) 3+ (aq) + CNS (aq) Fe(H 2O) 5CNS] 2+ (aq) + H 2O (l) 13. To a solution of Fe 3+ ions add aqueous potassium thiocyanate followed by aqueous NaF. Addition of KCNS gives blood red Ligand exchanges give the colour changes : solution which turns colourless with aqueous KF. 3+ (aq)+cns Fe(H 2O) 5CNS] 2+ (aq) Fe(H 2O) 5CNS] 2+ (aq)+6f -- (aq) FeF 6 3 (c less) 14. To a solution of Fe 3+ ions add a few cm 3 of sodium bicarbonate solution. Gentle effervescence a brown precipitate is also formed. Aqueous solutions of Fe 3+ are ic, releasing CO 2 from HCO 3-. Hydrated iron(iii) hydroxide is also precipitated 15. To a solution of Fe 3+ ions add aqueous potassium iodide. A dark brown solution results when aq. KI is added to aq Fe 3+. Brown solution is due to aqueous iodine (I 2(aq)) 2Fe 3+ (aq) + 2I (aq) 2Fe 2+ (aq) + I 2(aq) Why is there no iron(iii) iodide? The highly polarizing Fe 3+ causes an oxidation of the Iodide (I - ) ions to Iodine (I 2) Examine the s of IRON METAL with the following reagents in the cold. If no occurs, boil carefully (do not boil the conc. HNO 3 ). Note and test for any gases evolved. Enter your observations in the table below. Where no occurs enter no visible change. Expt 21 Page 6

7 water change in time given. aqueous sodium hydroxide change dilute hydrochloric Vigorous effervescence - evolution of H 2 (burns with a pop) Fe (s) + 2H + (aq) Fe 2+ (aq) + H 2 dilute sulphuric Effervescenceevolution of H 2. Fe (s) + 2H + (aq) Fe 2+ (aq) + H 2 concentrated nitric Vigorous brown gas(no 2) released. Fe (s) + HNO 3(aq) Fe 2+ (aq) + NO 2(g) + 2OH NB Sometimes metal just turns black with no apparent Expt 21 Page 7