Lecture
Oxides and hydroxides of d-elements in different oxidation states differ sharply in their acid-base properties. If several oxidation states are possible for the atoms of an element, then as the oxidation state increases, the properties of their oxides (and the corresponding hydroxides) change from basic through amphoteric to acidic.
The change in the acid-base properties of the oxides and hydroxides of manganese and chromium in various oxidation states is shown in Table 34.3.
Table 34.3. Acid-base character of the oxides and hydroxides of manganese and chromium
| Manganese oxides |  |
 |
 |
 |
| Manganese hydroxides | Mn(OH)2 | MnO(OH) | Mn(OH)4 | HMnO4 |
| Properties | Basic | Amphoteric | Acidic | |
| Chromium oxides |  |
 |
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|
| Chromium hydroxides | Cr(OH)2 | Cr(OH)3 | H2CrO4, H2Cr2O7 | |
| Properties | Basic | Amphoteric | Acidic |
Chromium(VI) oxide CrO3 dissolves readily in water, forming a solution of either chromic acid H2CrO4 or dichromic acid H2Cr2O7. The latter is a strong acid. Molybdenum and tungsten oxides are insoluble in water, while the corresponding molybdic and tungstic acids are weak. Thus, down a group, the acidic properties of the hydroxides of d-elements weaken.
Many oxides and hydroxides of d-elements are characterized by amphoteric properties; for example, you are already familiar with the amphoteric hydroxide Zn(OH)2.
The acid-base properties of the oxides and hydroxides of d-elements depend on the oxidation state of their atoms. As the oxidation state increases, the properties of the oxides (and the corresponding hydroxides) of d-elements change from basic through amphoteric to acidic.
1. Compare the pattern of change in the acid-base properties of elements in the A- and B-groups. Suggest an explanation for the existing differences.
2. Using the d-elements of Group VIB as an example, indicate how the acid-base properties of a d-element change down the group.
3. Using the concept of the strength of the element—oxygen and oxygen—hydrogen bonds in the hydroxides of d-elements, explain the patterns of change in the acid-base properties of the hydroxides of chromium and manganese as their oxidation state increases.
4. Based on your knowledge of the patterns in the change of acid-base properties for manganese and chromium compounds in different oxidation states, write the equations of reactions that characterize these properties.
5. Cobalt(II) oxide reacts with a concentrated sodium hydroxide solution to form sodium tetrahydroxocobaltate. Write the corresponding equation for this reaction.
6. What is the medium (pH range) in a solution of copper(II) sulfate? Give an explanation.
7. The solubility of copper(II) sulfate at 20 °C is 17.2 g per 100 g of solution. Calculate the minimum volume of water in which 25 g of copper sulfate (blue vitriol) can be dissolved at this temperature.
8. The solubility of nickel sulfate at 0 °C is 20.7 g per 100 g of solution, and at 40 °C it is 33.5 g per 100 g of solution. Calculate the mass of a solution saturated at 40 °C from which, upon cooling to 0 °C, 28 g of NiSO4 ∙ 7H2O will be deposited.
9. A copper-gold alloy weighing 3.2 g was treated with an excess of concentrated nitric acid solution. This released 896 cm3 of gas. Determine the mass fraction of gold in the alloy.
10. To 12.8 g of copper filings, 50 cm3 of a hot concentrated sulfuric acid solution with a mass fraction of 94 % (ρ = 1.831 g/cm3) was added. Will the copper dissolve completely? Determine the volume of gas released and the mass fraction of CuSO4 in the resulting solution.
1. In the series of oxides , 
, the properties change:
2. As the oxidation state of manganese atoms increases, the properties of the hydroxides of this metal change:
3. Amphoteric properties are exhibited by:
4. The acidic properties of the oxides and hydroxides of d-elements:
5. In their highest oxidation states, chromium and manganese form compounds with oxygen that:
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