Ib chemistry illustrating the oxidation states of vanadium

IB CHEMISTRY Illustrating the oxidation states of vanadium

Aim To make samples of the less common oxidation states of vanadium

Requirements and materials

safety goggles disposable gloves conical flask, 100 ml spatula ammonium metavanadate solution measuring cylinder, 25 cm^3 sulphuric acid, dilute, 1 mol dm^-3 sulphuric acid, concentrated 5 test tubes and holder

zinc powder, Bunsen burner filter funnel and paper potassium manganate(VII) solution, 0.002 mol dm^-3 sodium sulphite potassium iodide solution, 0.05 mol dm^-3 sodium thiosulphate solution, 0.1 mol dm^-3 distilled water test tube rack

SAFETY MESSAGES:

• 
  Wear goggles
  
• 
  Sodium hydroxide is corrosive, even in dilute solution.
  
• 
  Ammonium vanadate is toxic so the solution with dilute sulphuric acid has already been made for step 1.
  
• 
  Dilute sulphuric acid 1mol dm^-3 is an irritant.
  
• 
  Manganese(IV) oxide and manganese(II) sulphate are harmful if swallowed.
  

• 
  Concentrated sulphuric acid solution is corrosive and reacts violently with water. Therefore you must:
  
• 
  wear safety goggles and gloves;
  
• 
  leave it in the fumehood
  
• 
  only use the pipette
  
• 
  inform the teacher of any spillages
  

Procedure

You will need to carry out the following steps and make and record appropriate observations. Use the table at the bottom to identify the oxidation state of vanadium.

1. 
   Place 1 spatula measure of ammonium metavanadate(V) in a conical flask and add about 25 ml of dilute sulphuric acid. Carefully add about 5 cm³ of concentrated sulphuric acid and swirl the flask until you obtain a clear yellow solution of vanadium(V) solution.
   

2. 
   Pour about 2 ml of this vanadium(V) solution into two test tubes ready for later tests (steps 6 and 7).
   

3. 
   To the conical flask from step 1 add 1 spatula measure of zinc powder, a little at each time. Swirl the flask at intervals. Record.
   

4. 
   When the solution has become violet (you may need to heat the flask for this final change), filter about 2 ml into two test tubes for steps 5 and 6.
   

5. 
   To one test tube in step 4 add, a little each time, an excess of acidified potassium manganate(VII) solution, shaking after each addition, until no further change is observed. Record.
   

6. 
   To one of the test tubes containing vanadium(V) (from step 2) add a little sodium sulphite and shake. Filter if cloudy. Now boil carefully (at a fume cupboard) to remove excess sulphur dioxide and add about the same volume of the vanadium(II) solution from step 4. Record.
   

7. 
   To the second of the tubes containing vanadium(V) (from step 2), add about 2 ml potassium iodide solution and mix. Then add about 2 ml of sodium thiosulphate solution. Record.
   

Colour vanadium ions and their corresponding oxidation states

V^5+ (VO2^+)	V^4+ (VO^2+)	V^3+	V^2+
yellow	blue	green	violet