What is the Difference Between KMnO4 and K2Cr2O7

Inorganic compounds are chemical substances that lack carbon-hydrogen bonds. They form the backbone of the Earth’s crust, found in minerals and pure forms.  Examples include water, salts, acids, bases, and compounds such as KMnO₄ and K₂Cr₂O₇.

What is the difference between KMnO₄ and K2Cr₂O₇? KMnO₄ (potassium permanganate) is a stronger oxidizing agent than K₂Cr₂O₇ (potassium dichromate).

Key Areas Covered

1. What is KMnO4 
      – Definition, Features 
2. What is K2Cr2O7
      – Definition, Features 
3. Similarities Between KMnO4 and K2Cr2O7
      – Outline of Common Features
4. Difference Between KMnO4 and K2Cr2O7
      – Comparison of Key Differences
5. FAQ: KMnO4 and K2Cr2O7
      – Answers to Frequently Asked Questions

Key Terms

KMnO_4, K₂Cr₂O_7, Potassium Permanganate, Permanganate of Potash, Potassium Permanganate

What is KMnO₄

KMnO₄, often called potassium permanganate or permanganate of potash, is an inorganic compound. It is a purplish-black crystalline salt. When this compound is dissolved in water, it forms a deep purple solution. It readily accepts electrons from other substances, causing them to oxidize. Hence, it is considered a strong oxidizing agent.

The manganese in KMnO₄ is in its +7 oxidation state. It is a good electron acceptor. Upon heating, KMnO₄ decomposes to potassium manganate (K₂MnO₄), manganese dioxide (MnO₂), and oxygen gas. Moreover, in the presence of a catalyst, this decomposition is accelerated. KMnO₄ also undergoes redox reactions with various substances.

Figure 1: KMnO4 (Potassium Permanganate)

The reducing agent and the reaction condition (whether it is acidic, basic or neutral) determines the products formed after the reaction. This compound is intensively soluble in water. Moreover, KMnO₄ can react violently with concentrated acids, such as sulfuric acid, and organic compounds, leading to fires or explosions.

There are many uses of KMnO₄. In the industry, it is used as a strong oxidizing agent to synthesize various chemicals. In water treatment, it is used to remove iron, hydrogen sulfide, and other impurities that are present in water. KMnO₄ is also used as a disinfectant, which is effective against bacteria, fungi, and viruses. Additionally, in laboratories, it is used in redox titrations and qualitative analysis. Moreover, in the food industry, it is used as a food additive. In agriculture, it acts as a fungicide and a disinfectant. It is also useful as an antidote in emergency treatment. Though KMnO₄ has many benefits, it is advised to handle and deal with care.

What is K₂Cr₂O₇

K₂Cr₂O₇ (potassium dichromate) is an inorganic compound that has a red-orange crystalline outlook. Chemically, it contains two potassium ions and one dichromate ion. It’s a strong oxidizing agent because chromium is in a +6 oxidation state. Chromium in the dichromate ion easily accepts electrons and undergoes reduction while oxidizing the other species. In alkaline solutions, it forms chromates (CrO₄²⁻). In acidic solutions, dichromate ions convert to chromate ions (CrO₄²⁻) and then to chromic ions (Cr³⁺). This is a reversible process that depends on the pH value of the solution. Furthermore, it is a highly toxic and carcinogenic compound, and exposure to it may lead to adverse health effects. Potassium dichromate can oxidize primary alcohols to aldehydes and further to carboxylic acids. Secondary alcohols are oxidized to ketones.

Figure 2: K2Cr2O7 (Potassium Dichromate)

Potassium dichromate is widely used in industry. It is used as an oxidizing agent in organic chemistry. It is also useful in analytical chemistry for redox titrations. K₂Cr₂O₇ is also used to make pigments and dyes due to its colour. In addition, it’s used in etching of metals.

Similarities Between KMnO₄ and K₂Cr₂O₇

1. Both compounds are strong oxidizing agents.
2. They are inorganic compounds.
3. They are soluble in water.
4. Both compounds have an intense characteristic color.
5. Moreover, both are widely used in analytical chemistry.

Difference Between KMnO₄ and K₂Cr₂O₇

Definition 

• KMnO₄, often called potassium permanganate or permanganate of potash, is an inorganic compound that have a deep purple color, while K₂Cr₂O₇ (potassium dichromate) is an inorganic compound that has a red-orange crystalline outlook.

Oxidizing

• KMnO₄ has a stronger oxidizing agent than K₂Cr₂O₇.

Appearance

• KMnO₄ is intense purple color in both solid and solution form, while K₂Cr₂O₇ is red-orange color in solid form and orange in solution.

Stability

• K₂Cr₂O₇ is less stable compared to KMnO₄, especially in acidic solutions.

Use in Medium

• KMnO₄ can be used as an oxidizing agent in acidic, neutral, and alkaline media, while K₂Cr₂O₇ is used in an acidic medium.

Reduced

• Reduction products of KMnO₄ vary depending on the reaction conditions (MnO2, MnO4²-, Mn²⁺) while K₂Cr₂O₇ is reduced to green Cr³⁺ ions in acidic solutions.

Toxicity

• KMnO₄ is less toxic compared to K₂Cr₂O₇.

Conclusion

Both KMnO₄ and K₂Cr₂O₇ are inorganic compounds having slightly different properties and intense colors. KMnO₄ is a stronger oxidizing agent and is more stable and less toxic than K₂Cr₂O₇. This is the basic difference between KMnO₄ and K₂Cr₂O₇. Both are used in redox titrations and have many applications in various industries.

FAQ: KMnO4 and K2Cr2O7

1. Is KMnO4 or K₂Cr₂O₇ stronger?

KMnO₄ is generally a stronger oxidizing agent than K₂Cr₂O₇. This is due to its higher reduction potential. However, this comparison depends on the reaction medium (acidic, basic, or neutral).

2. What is the difference between KMnO₄ and K₂Cr₂O₇ titration?

KMnO₄ (purple) acts as its own indicator, changing color at the endpoint. K₂Cr₂O₇(orange) requires a separate indicator like diphenylamine. KMnO₄ is a stronger oxidant than K₂Cr₂O₇, making it suitable for more vigorous reactions.

3. Why is KMnO₄ the best oxidizing agent?

KMnO₄ is not the best oxidizing agent in all cases. Its strength as an oxidant depends on the reaction conditions (acidic, basic, or neutral). Other factors like the specific reducing agent and desired product also influence the choice of oxidizing agent.

4. What are the properties of KMnO₄ and K₂Cr₂O₇?

KMnO₄ is purple, dissolves in water to form a deep purple solution, and is a strong oxidizing agent. K₂Cr₂O₇ is orange, also water-soluble, and is a strong oxidizing agent but less powerful than KmnO. Both are used in redox titrations.

5. Which hydrocarbon is most easily oxidized by KMnO₄?

Alkenes are the most easily oxidized hydrocarbons by KMnO₄. Moreover, they undergo oxidative cleavage to form carbonyl compounds like aldehydes, ketones, or carboxylic acids, depending on the reaction conditions.

Reference:

1. “Potassium Permanganate.” Wikipedia. Wikipedia Foundation. 
2. “Potassium Dichromate.” Wikipedia. Wikipedia Foundation.

Image Courtesy:

1. “Potassium-permanganate-sample” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
2. “Potassium-dichromate-sample” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia