JUNIOR COLLEGE CHEMISTRY DEPARTMENT EXPERIMENT 22 SECOND YEAR PRACTICAL Name: Group: Date: THE CHEMISTRY OF CHROMIUM AND MANGANESE CHROMIUM 1. To a solution of Cr 3+ ions add aqueous sodium hydroxide until in excess. Green precipitate soluble in excess Green ppt is Cr(OH) 3, gives Cr(OH) 4 (aq): NaOH giving a green solution. Cr 3+ (aq)+ 3 OH (aq) Cr(OH) 3(s) Cr(OH) 3(s)+ OH (aq) Cr(OH) 4 (aq) 2. Add dilute sulphuric to some potassium dichromate solution, then a few cm 3 of ether followed by hydrogen peroxide. Shake gently. A deep blue (cov.) compound extracts Blue covalent, short lived, CrO 5 forms into the ether layer. Colour slowly It extracts into the ether layer but slowly changes to a light blue one in water. converts to the more stable, water sol. Cr 3+ 3. To a solution of Cr 3+ ions add aqueous ammonia. Green precipitate, insoluble in excess Green ppt is Cr(OH) 3 Cr 3+ (aq)+ 3 OH (aq) Cr(OH) 3(s) 4. Add a small quantity of aqueous sodium hydroxide to some potassium dichromate solution. Keep the mixture for test 5. Solution turns from orange to Orange Cr 2O 7 2, stable in, converts yellow to the CrO 4 2, stable in alkaline solution Now fill in the following: 2 Cr 2 O 7 aq H 2 O 2 2CrO 4 aq 2H + aq O.N. of Cr...+ 6... O.N. of Cr...+ 6... Colour..orange... Colour...yellow... Expt 22 Page 1
5. Acidify the mixture obtained in test 4 with dilute sulphuric. Yellow solution changes back to an In, the orange dichromate, Cr 2O 2 7 orange one is found 6. Acidify some potassium dichromate solution with dilute sulphuric, than add some zinc dust. Warm gently until no further colour changes occur. The colour of the solution changes to The unstable turquoise Cr 2+ (aq) forms turquoise initially but changes to blue Cr 3+ in presence of Ox. agents such as air. 7. Add some silver nitrate solution to some aqueous potassium chromate. A brickred precipitate is formed The characteristically coloured Ag 2CrO 4 precipitates out in this. 2Ag + (aq) + CrO 2 4 (aq) Ag 2CrO 4(s) 8. Examine the s of CHROMIUM 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 No Visible change Effervescence gas burns with a pop Effervescence gas burns with a pop Cr is coated by a passive layer in c.hno 3 Expt 22 Page 2
MANGANESE 9. To a solution of Mn 2+ ions add aqueous sodium hydroxide. Note the appearance of the precipitate on forming and after some time. Buff precipitate, insoluble in excess, Buff ppt is Mn(OH) 2 but this is oxidized to forms. Ppt goes dark brown, almost black, on standing in air. Dark brown/black MnO 2 in air. Mn 2+ (aq) + 2OH (aq) Mn(OH) 2(s) MnO 2(s) 10. To a solution of Mn 2+ ions add aqueous ammonia. Note the colour of the precipitate on forming and after some time. Buff ppt, insoluble in excess, is. obtained Ppt turns dark brown on standing Comments as above. 11. Add a few drops of concentrated nitric to about 2 cm 3 of the solution of Mn 2+ ions and then add a very small quantity of solid sodium bismuthate. A purple coloured solution forms on The bismuthate oxidizes the Mn 2+ ions to the addition of the Mn 2+ (aq) ions to a Purple MnO 4 : 14H + + 5BiO 3 + 2Mn 2+ Suspension of the BiO 3 in HNO 3 5Bi 3+ + 2MnO 4 + 7H 2O 12. Place 2 cm 3 of the solution of Mn 2+ ions in a small beaker, add an equal volume of peroxodisulphate(vi) solution, a few drops of aqueous silver nitrate and boil. Dark brown/black precipitate Mn 2+ ions were oxidized to MnO 2 formed. What is the function of the silver nitrate?...the silver ions act as a catalyst in this... 13. Acidify some potassium manganate(vii) solution with an equal volume of dilute sulphuric and then add about 1 cm 3 of aqueous hydrogen peroxide. The MnO 4 Is decolourised and a Oxygen is released when H 2O 2 is oxidized colourless gas that relights a By the stronger oxidising agent the MnO 4 glowing splint was given off. 2MnO 4+5H 2O 2+6H + 2Mn 2+ +5O 2+8H 2O Expt 22 Page 3
14. Add 2 pellets of sodium hydroxide to about 1 cm 3 of the potassium manganate(vii) solution and warm gently. Look for any bubbles of gas evolved, and note the colour of the resulting solution. N.B. When you finish this test, carefully pour away the contents of the tube amid a stream of water and rinse the test tube well. A green solution was observed The green mangante (VI) MnO 2 4 ion was Produced in this. 15. Add 1 cm 3 of dilute sulphuric to an equal volume of dilute potassium manganate(vii) solution. Mix well. Add 1 cm 3 of ethanedioic solution and warm gently. The manganate (VII) was Ethanedioic is oxidized to CO 2 while the decolourised. (Some bubbles of a Purple MnO 4 turns to the colourless Mn 2+ Colourless gas are released) 16H + + 2MnO 4 + 5C 2O 4 2 2Mn 2+ + 8H 2O + 10CO 2 16. TEACHER DEMONSTRATION Put a spatula load of manganese (IV) oxide, 3 pellets of sodium hydroxide and a half spatula load of potassium nitrate (V) into a crucible. Heat directly over a Bunsen flame and when the pellets melt, carefully stir the molten mass with a glass rod. Allow to cool back to room temperature and note the colour of the solid. Dissolve the solid in a small quantity of water and note the colour of the solution and any precipitate. A bright green solution was seen Green solution contains the MnO 4 2 ion, only To form. Dilution with water gave stable in alkaline conditions A purple solution, and precipitated Disproportionates to MnO 4 and MnO 2 Out a black solid 3MnO 4 2 +2H 2O 2MnO 4 +MnO 2 +4OH What type of takes place when the solid is dissolved in water? A disproportionation occurs when green MnO 4 2 is dissolved in water as it gives MnO 4 and MnO 2. Expt 22 Page 4
17. Examine the s of MANGANESE 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 H 2 gas evolved H 2 gas evolved Brown gas evolved (Gas is NO 2) Expt 22 Page 5