Questions and tasks

1. What is common and different in the structure of chalkogenes atoms and of chrome subgroup elements atoms? Why do the latter show properties of metals? Make the electron formulas of atoms of chrome, molybdenum and tungsten.

2. How are chrome subgroup metals received in industry? Give the equations of the relevant reactions.

3. How does chrome react with hydrochloric, sulphuric and nitric acids? Give the equations of the relevant reactions.

4. What occurs at mixing of water solutions of: а) Cr₂(SO₄)₃ and Na₂S б) Cr(NO₃)₃ and Na₂CO₃? Give the equations of the relevant reactions in the molecular and ionic - molecular forms and explain the mechanism of these transformations.

5. Why at action of silver nitrate on water solutions of three isomers with identical composition CrCl₃6H₂O are precipitated one, two or three moles of silver chloride on each mol of isomer respectively?

6. What anions of chrome (VI) exist in water solutions and at what conditions are possible their mutual transformations? Give the ionic - molecular equations of the relevant reactions.

7. A gas of 200 mls volume had evolved at action of 1 g of water solution of hydrogen peroxide with excess amount of potassium dichromate and sulphuric acid at the temperature of 27° and at pressure of 100 kPa. Calculate weight fraction of hydrogen peroxide in the solution.

8. How are molybdenic and tungstenic acids received?

9. How and why do the ability to polymerisation, solubility and acid-base properties in a range H₂CrO₄ - H₂MoO₄ - H₂WO₄ change?

10. How does the thermal decomposition of ammonium chromate, molybdate and tungstenate proceed? Give the equations of the relevant reactions and explain the peculiarities of their proceeding.

Make up the equations o f the reactions Make up the equations of the reactions

Cr₂O₃ + Al =

Cr₂O₃ + C =

Cr + O₂ =

Cr + H =

Cr(OH)₃ + H =

Cr(OH)₃ + OH =

Cr₂O₃ + KOH

K₃[Cr(OH)₆] + Br₂ + KOH =

Cr₂(SO₄)₃ + H₂O₂ + KOH =

CrO + H₂O =

Cr₂O + H₂O =

K₂Cr₂O₇ + SO₂ + H₂SO₄ =

(NH₄)₂Cr₂O₇

CrO₃ + HCl =

CrO₂Cl + H₂O =

MoS₂ + O₂ =

MoO₃ + H₂ =

Mo + O₂ =

MoO₃ + KOH =

Mo + KNO₃ + KOH =

H₂MoO₄

MoO₂Cl₂ + H₂O =

WO₃ + H₂ =

W + O₂ =

WO₃ + KOH =

Experimental section

1. Materials and equipment: metallic chrome, metallic zinc, crystalline iodine, potassium iodide, potassium bromide, solutions of sulphuric acid, nitric acid, chrome (ІІІ) chloride, chrome (ІІІ) sulphate, ammonia, sodium hydroxide, sodium carbonate, sodium sulphide, potassium chromate, potassium dichromate, potassium iodide, sodium nitrite, sodium sulphite, ammonium molybdate, ammonium tungstenate, bromine water, hydrogen peroxide, concentrated hydrochloric acid, 100 mls beakers, test tubes, glass rods, clock glass.

2. Chemical properties of chrome

2.1. Put a slice of metallic chrome into a test tube, add 1-2 mls of 2N sulphuric acid solution and heat up slightly. What colour does the solution have? Then transfer the droplet of the solution on a clock glass. What occurs? Give the equations of the relevant reactions and explain the reason of strong reducing properties of chrome (ІІ).

2.2. Place 2-3 mls of 0,5N chrome (ІІІ) chloride solution into a test tube, add 3-4 drops of concentrated hydrochloric acid and 1-2 granules of metallic zinc. What occurs? Give the equations of the relevant reactions.

2.3. Place 2-3 drops of chrome (ІІІ) sulphate solution into each of two test tubes and add 1-3 drops of solution of ammonia to each one until precipitate forms. To this precipitate add 3-5 drops 2 M hydrochloric acid solution into the first test tube, the same amount of 2 M sodium hydroxide solution into the other. What can you observe? Give the equations of the relevant reactions and make a conclusion on acid-base properties of the obtained precipitate.

2.4. Measure with the help of the рH-meter hydrogen ionisation value of chrome (ІІІ) chloride solution. Then heat up the chrome salt solution and measure the value of pH again. What can you observe? Explain the difference of pH values of chrome salt solution at ambient temperature and when heated. Give the equations of the hydrolysis in the molecular and ionic forms.

2.5. Place 4-6 drops of chrome (ІІІ) chloride solution into a test tube. Note the colour of the solution. Then heat up the test tube until the solution changes colour. Stop heating and observe gradual reduction of the solution colour when cooled. Write down your observations and give the relevant explanation.

2.6. Place 3-4 drops of 0,5N chrome (ІІІ) chloride solution into each of two test tubes and add 4-6 drops of 0,5N solutions: of sodium carbonate into the first, of sodium sulphide into the other. What can you observe? Explain the proceeding of the processes and give the equations of the relevant reactions in the molecular and ionic forms.

2.7. Place 2-3 drops of 0,5N chrome (ІІІ) chloride solution, 4-6 drops of concentrated sodium hydroxide solution into each of two test tubes and add: the excess amount of bromine water into the first, the excess amount of hydrogen peroxide solution into the other. What can you observe? In what role does chrome (ІІІ) act in these reactions? Give the equations of the relevant reactions.

2.8. Place 2-3 drops of potassium dichromate solution into a test tube and add 2-3 drops of diluted alkali solution. What can you observe? Give the equation of the reaction.

2.9. Place 2-3 drops of potassium chromate solution into a test tube and add 2-3 drops of diluted hydrochloric acid. What can you observe? Give the equation of the reaction.

2.10. Place 6-7 drops of potassium dichromate solution into each of three test tubes and add in each 2-3 drops of diluted sulphuric acid. After that add 3-5 drops of 0,5N potassium iodide solution into the first test tube, the same amount of 0,5N sodium nitrite solution into the second, and the same amount of 0,5N sodium sulphite solution into the third. If necessary, heat up the test tubes slightly. What can you observe? Give the equations of the reactions.