chemistry

Potassium manganate

Potassium manganate is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Potassium manganate rather than just read about it. In short: Potassium manganate is the inorganic compound with the formula K2MnO4. This green-colored salt is an intermediate in the industrial synthesis of potassium permanganate (KMnO4), a common chemical.

Key takeaways

Potassium manganate belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Potassium manganate to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Potassium manganate from memory before moving on to harder problems.

Reference excerpt

Potassium manganate is the inorganic compound with the formula K2MnO4. This green-colored salt is an intermediate in the industrial synthesis of potassium permanganate (KMnO4), a common chemical. Occasionally, potassium manganate and potassium permanganate are confused, but each compound's properties are distinct.

Structure and bonding K2MnO4 is a salt, consisting of K+ cations and MnO2−4 anions. X-ray crystallography shows that the anion is tetrahedral, with Mn-O distances of 1.66 Å, ca. 0.03 Å longer than the Mn-O distances in KMnO4. It is isostructural with potassium sulfate. The compound is paramagnetic, owing to the presence of one unpaired electron on the Mn(VI) center.

Synthesis The industrial route entails treatment of MnO2 with air and potassium hydroxide:

2 MnO2 + 4 KOH + O2 → 2 K2MnO4 + 2 H2O The transformation gives a green-colored melt. Alternatively, instead of using air, potassium nitrate can be used as the oxidizer:

2 KOH + KNO3 + MnO2 → K2MnO4 + H2O + KNO2 One can test an unknown substance for the presence of manganese by heating the sample in strong KOH in air. The production of a green coloration indicates the presence of Mn. This green color results from an intense absorption at 610 nm. In the laboratory, K2MnO4 can be synthesized by heating a solution of KMnO4 in concentrated KOH solution followed by cooling to give green crystals:

4 KMnO4 + 4 KOH → 4 K2MnO4 + O2 + 2 H2O This reaction illustrates the relatively rare role of hydroxide as a reducing agent. The concentration of K2MnO4 in such solutions can be checked by measuring their absorbance at 610 nm. The one-electron reduction of permanganate to manganate can also be effected using iodide as the reducing agent:

2 KMnO4 + 2 KI → 2 K2MnO4 + I2 The conversion is signaled by the color change from purple, characteristic of permanganate, to the green color of manganate. This reaction also shows that manganate(VII) can serve as an electron acceptor in addition to its usual role as an oxygen-transfer reagent. Barium manganate, BaMnO4, is generated by the reduction of KMnO4 with iodide in the presence of barium chloride. Just like BaSO4, BaMnO4 exhibits low solubility in virtually all solvents. An easy method for preparing potassium manganate in the laboratory involves heating crystals or powder of pure potassium permanganate. Potassium permanganate will decompose into potassium manganate, manganese dioxide and oxygen gas:

2 KMnO4 → K2MnO4 + MnO2 + O2 This reaction is a laboratory method to prepare oxygen, but produces samples of potassium manganate contaminated with MnO2. The former is soluble and the latter is not.

Reactions

Manganate salts readily disproportionate to permanganate ion and manganese dioxide:

3 K2MnO4 + 2 H2O → 2 KMnO4 + MnO2 + 4 KOH The colorful nature of the disproportionation has led the manganate/manganate(VII) pair to be referred to as a chemical chameleon. This disproportionation reaction, which becomes rapid when [OH−] < 1M, follows bimolecular kinetics.[1]

See also Category:Manganates for a list.

Sources

Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.

External links National Pollutant Inventory - Manganese and compounds Fact Sheet

Illustrations

Worked examples

Example 1 — a first encounter with Potassium manganate

Start with the simplest possible case. Write down what Potassium manganate claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Potassium manganate before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Potassium manganate ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Potassium manganate

In research: Potassium manganate appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Potassium manganate in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Potassium manganate is common in secondary-school and first-year university syllabi. It links to neighbouring topics Manganates, Potassium compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Potassium manganate outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Potassium manganate in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Potassium manganate means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Potassium manganate out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Potassium manganate in simple terms?

Potassium manganate is the inorganic compound with the formula K2MnO4. This green-colored salt is an intermediate in the industrial synthesis of potassium permanganate (KMnO4), a common chemical.

Why does Potassium manganate matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Potassium manganate?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Potassium manganate.