The stability of compounds in the series Mn(VII) — Tc(VII) — Re(VII) is considerably increased but oxidising properties are decreased.

Tc₂O₇ and Re₂O₇ can be prepared by the direct synthesis reaction:

4E + 7O₂ = 2E₂O₇

These oxides are well soluble in water. Pertechnetic and perrhenic acids are formed:

E₂O₇ + H₂O = 2HEO₄.

These acids can be also obtained by oxidation in diluted nitric acid, for instance:

3Tc + 7HNO₃ = 3HTcO₄ + 7NO + 2H₂O

HTcO₄ and HReO₄ are strong acids. In the series HMnO₄, HTcO₄, HReO₄ the strength of acids drops a little as a result of the growth of atomic radii.

Oxidising activity of +7 in the series Mn—Tc—Re is lowered due to the growth of stability of the highest oxidation state of these elements. In contrary to HMnO₄, HTcO₄ and HReO₄ don’t oxidize H₂S. In this case, exchange reaction takes place and sulfides of Re (Tc) are formed, for example:

2HReO₄ + 7H₂S ® Re₂S₇ + 8H₂O.

Е₂S₇ are black solid substances that are soluble in oxidant-acids:

Re₂S₇ + HNO_3(conc.) = HReO₄ + SO₂ + NO₂ + H₂O

There are some other representatives of Re(VII) salts. Mesorhenic acid H₃ReO₅ and orthorhenic acid H₅ReO₄ salts are also known.

Questions and tasks

1. How are manganese subgroup metals received in industry? Give the equations of the relevant reactions.

2. What electronic structure do manganese subgroup elements atoms have? What oxidation states can these elements have in compounds? Give examples.

3. What is the position of manganese, technetium and rhenium in the electromotive series of metals? With what acids do they react? Give the equations of the relevant reactions.

4. How do the acid-base properties of manganese oxides and hydroxides change? How do they react with acids and alkalis?

5. Give examples of reactions, in which manganese (IV) oxide acts as an oxidising agent, reducing agent and does not change an oxidation state.

6. How and why do oxidising properties of permanganate-ion depend on acidity of solution? Give the equations of relevant reactions.

7. What volume of 0,01 N potassium permanganate solution can be reduced by 100 mls of hydrogen sulfide at the temperature of 0° and at the pressure of 101,3 kPa in the presence of excess amount of sulfuric acid?

8. Give the equations of reactions of potassium permanganate interaction with sulfurous acid and with hydrogen peroxide in acidic, neutral and alkaline medium.

9. How and why do the thermal stability, acid-base and oxidising properties in a range of acids HMnO₄ - HTcO₄ - HReO₄ change?

Дата добавления: 2015-07-25