Interaction of permanganate and chromium(III) compounds
The thermodynamic feasibility of redox reactions between chromium and manganese compounds is determined by their standard potentials. Since the potential of the permanganate ion (+1.51 V) is higher than that of the dichromate ion (+1.33 V), potassium permanganate can act as an oxidizing agent toward chromium(III) salts.
When acidified solutions of potassium permanganate and chromium(III) sulfate are mixed, the reaction proceeds spontaneously. The purple color of permanganate disappears, and the solution turns orange, which is characteristic of the resulting dichromate.
In a strongly acidic medium, the reaction products are potassium dichromate, dichromic acid, and manganese(II) sulfate. The reverse reaction—the oxidation of manganese(II) ions by dichromate—does not occur under standard conditions, because dichromate is a weaker oxidizing agent and requires harsh conditions (e.g., boiling or the presence of specific catalysts).
Transformations of chromium(III) hydroxo complexes
Chromium(III) in an alkaline medium is capable of forming soluble hydroxo complexes, for example, a green solution of potassium tetrahydroxodiaquachromate(III). The chemical properties of this complex depend on the added reagent:
- Action of hydrogen peroxide. When H₂O₂ is added to the hydroxo complex in an alkaline medium, chromium(III) is oxidized to chromium(VI). Visually, this is manifested by a change in the solution color from green to yellow, indicating the formation of chromate ions.
- Action of acids. The addition of an excess of sulfuric acid leads to the complete destruction of the complex ion. As a result, the medium becomes acidic, and the solution changes color to purple (or gray-green), indicating the formation of free hydrated chromium(III) ions in the form of chromium sulfate.
Redox properties of copper(II)
Copper(II) ions also exhibit interesting redox and complexation properties. When a copper(II) sulfate solution reacts with potassium iodide, copper acts as an oxidizing agent.
The copper(II) ion oxidizes the iodide ion to free molecular iodine while being reduced to copper(I). The resulting copper(II) iodide is extremely unstable and instantly decomposes into copper(I) iodide and free iodine. The precipitation of a white precipitate of copper(I) iodide is observed, which is often masked by the brown color of the solution due to the presence of released molecular iodine.
In addition, copper in salt solutions can be displaced by more active metals. For example, an iron coin placed in a copper sulfate solution becomes coated with a red layer of metallic copper, since iron is located further to the left in the electrochemical series. Silver, being a less active metal, does not react with copper sulfate.
Coordination theory using complexes as an example
A classic example of hydrate isomerism is chromium(III) chloride hexahydrate. Its structure is easily proven using the precipitation reaction of outer-sphere chloride ions with silver nitrate:
- Green isomer. When silver nitrate is added to one mole of this substance, only 0.1 mole of silver chloride precipitates (during the reaction of equimolar amounts in a 1:1 ratio). This proves that only one chlorine ion is in the outer sphere. The formula of the complex is dichlorotetraaquachromium(III) chloride dihydrate.
- Purple isomer. The reaction with silver nitrate yields a precipitate in a 1:3 molar ratio; consequently, all three chloride ions are in the outer sphere. The complex is hexaaquachromium(III) chloride.
Coordination properties are also characteristic of copper(II), which forms complexes with a coordination number of 4. Examples include compounds with monodentate methylamine and bidentate ethylenediamine, which forms particularly stable inner-complex chelate rings.
Briefly: answers to topical questions
What products are formed during the reduction of potassium permanganate in various media (acidic, neutral, alkaline)?
The products of potassium permanganate reduction depend on the pH of the medium, although sources describe reactions only for acidic and neutral media.
- Acidic medium — manganese(II) salts are formed, specifically the $Mn^{2+}$ cation (for example, in the form of manganese sulfate $MnSO_4$).
- Neutral medium — a precipitate of manganese(IV) oxide ($MnO_2$) drops out.
- Alkaline medium — the use of potassium hydroxide ($KOH$) to create the medium is mentioned, but the specific products of permanganate reduction are not given in the text.
To remember the colors of chromium compounds, use the association: "Chromate is like a sun (yellow, alkaline medium), dichromate is like an orange (orange, acidic medium)."
Class and exam questions
Why doesn't potassium dichromate oxidize manganese sulfate under normal conditions?
Due to the difference in oxidation potentials. The reduction potential of permanganate (+1.51 V) is higher than that of dichromate (+1.33 V), so dichromate is not a strong enough oxidizing agent to convert Mn²⁺ to MnO₄⁻ without catalysts.
What color precipitate is formed in the reaction of copper(II) sulfate with potassium iodide?
A precipitate of copper(I) iodide is formed, which itself is white, but in solution it is masked by the brown color of the released molecular iodine.
How do the isomers of chromium(III) chloride hexahydrate differ from each other?
They differ in the distribution of water molecules and chlorine ions between the inner and outer coordination spheres of the complex, which affects their color (green or purple) and chemical reactivity with silver nitrate.
What else to study on this topic
- Calculation problems on the change in mass of a metal plate (or wire) when metals are displaced from salt solutions.
- Oxidative alkaline fusion of chromium(III) oxide to form chromates.
- Aluminothermic reduction of chromates.
- Oxidation reactions of iron(II) compounds by nitrogen(IV) oxide and nitric acid.
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