How the medium determines reaction products
In oxidation-reduction (redox) reactions, in addition to the direct oxidizing and reducing agents, the medium is always involved. It can be provided by acids ($H^+$ ions), alkalis ($OH^-$), or the solvent itself, most often water ($H_2O$).
Depending on the acidity level (pH), the same reactants can transform into completely different substances. Furthermore, balancing reaction equations becomes more complex because the ions of the medium actively incorporate into the final molecules, forming salts, water, or precipitates. A change in the oxidation state of atoms directly affects their acid-base properties: higher oxidation states enhance acceptor (acidic) properties, while lower oxidation states enhance donor (basic) properties.
Behavior of potassium permanganate
Potassium permanganate ($KMnO_4$) is a classic example of how an oxidizing agent changes its properties depending on the pH.
- In an acidic medium ($pH < 7$), upon the addition of sulfuric acid, the solution is completely decolorized. Manganese changes its oxidation state from $+7$ to $+2$, forming $MnSO_4$ salt.
- In a neutral medium ($pH = 7$), i.e., in water, a brown precipitate of manganese(IV) oxide ($MnO_2$) is formed. The oxidation state changes from $+7$ to $+4$.
- In an alkaline medium ($pH > 7$), in the presence of potassium hydroxide, the solution turns green. Manganese is reduced to $+6$, turning into potassium manganate ($K_2MnO_4$).
Oxidizing properties of dichromate
A solution of potassium dichromate ($K_2Cr_2O_7$) has a characteristic orange color. If mixed with a colorless reducing agent (e.g., potassium iodide) in a neutral medium, no visible reaction occurs—the solution remains orange.
However, as soon as an acid is added (e.g., $H_2SO_4$), the liquid instantly turns red-brown. This indicates that the dichromate has exhibited strong oxidizing properties: chromium is reduced from $+6$ to $+3$, and free iodine ($I_2$) is liberated from the iodide, which provides the coloration.
How to choose the required medium
The main rule when planning a redox reaction states: the reaction is promoted by a medium capable of neutralizing the products formed under neutral conditions.
For example, if the reduction of the dichromate ion ($ ext{Cr}_2 ext{O}_7^{2-}$) were to occur in water, hydroxide ions ($ ext{OH}^-$) would be released as a byproduct. To prevent the reaction from slowing down and to shift the equilibrium toward the products, these alkaline ions must be bound. This is precisely why the reaction is carried out in an acidic medium by adding a proton source ($ ext{H}^+$).
Briefly: answers to topical questions
What products are formed during the reaction of potassium permanganate with hydrogen peroxide in an acidic medium?
- Manganese sulfate ($MnSO_4$) is the manganese reduction product.
- Oxygen ($O_2$) is the hydrogen peroxide oxidation product.
In the reaction $KMnO_4 + H_2O_2 + H_2SO_4 \rightarrow MnSO_4 + O_2\uparrow + \dots$, manganese changes its oxidation state from $+7$ to $+2$, and the oxygen in hydrogen peroxide from $-1$ to $0$.
The phrase "Acid is colorless, water is brown, and alkali turns green" helps remember the reduction products of KMnO4.
Class and exam questions
What is a disproportionation reaction?
This is a process in which the same element acts as both an oxidizing and a reducing agent, transitioning from one oxidation state to both a higher and a lower one. For example, chlorine behaves this way in hot alkali.
Why is water added to reaction equations?
Water acts either as a reactant creating a neutral medium or as a neutralization product formed during the interaction of hydrogen and hydroxide ions.
How do an element's properties change with a change in its oxidation state?
When an atom's oxidation state increases, its acidic (acceptor) properties intensify, while a decrease enhances its basic (donor) properties.
What else to study on this topic
- Mechanism of oxygen abstraction from the permanganate ion
- Conproportionation reactions
- Effect of oxidation state on acid-base properties
- Algorithm for balancing complex redox equations
- Predicting redox products based on the nature of the medium
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