www.gradeup.co
1
www.gradeup.co
Preparation and Properties of KMnO4 and K2Cr2O7
In this article we are providing the keynotes to cover the two important compounds from the topic:
Properties of D and F block elements in just 10 points. In the previous article we discussed about
the Physical Properties of D-Block elements (click on it to read).
Revise the two most important compounds of transition elements (K2Cr2O7) and (KMnO4) in just 10
points each:
(A) Potassium dichromate (K2Cr2O7):
1. It is an inorganic chemical reagent and is used as an oxidizing agent in various health and industryrelated problems (mainly used to oxidize alcohols)
2. It is acutely and chronically harmful to health.
3. It is a crystalline ionic solid with a very bright, red-orange color and is moderately soluble in cold water
but freely soluble in hot water.
4. It has a triclinic geometry with a tetrahedral coordination geometry.
5. Preparation: It is prepared from chromite ore (FeCr2O4). This preparation involves many steps:
i. Preparation of Sodium Chromate
Chromite ore is finely powdered and is heated with sodium carbonate in the presence of air in a
reverberatory furnace. This reaction produces sodium chromate.
ii. Conversion of sodium chromate into sodium dichromate:
The sodium chromate formed in the above reaction is treated with concentrated sulphuric acid to form
sodium dichromate.
2
www.gradeup.co
Sodium sulphate on cooling separates as Na2SO4.10H2O leaving sodium dichromate.
iii. Conversion of Sodium dichromate into potassium dichromate
The sodium dichromate formed above is treated with KCl to form potassium dichromate.
6. Action of heat: On heating, potassium dichromate decomposes and forms potassium chromate and
chromic oxide:
4K2Cr2O7 (on heating) → 4K2CrO4 + 2Cr2O3 + 3O2
7. Action of alkalis:
On reaction with alkali, it is converted into chromate which on acidifying gives back dichromate.
K2Cr2O7 + 2KOH → 2K2CrO4 + H2O
2K2Cr2O7 + H2SO4 → K2Cr2O7 + K2SO4 + H2O
In dichromate solution the Cr2O72– ions are in equilibrium with Cr2O72– ions at pH = 4.
Cr2O72– + H2O ⇒ 2CrO42– + 2Hl
(orange red)
(yellow)
8. Action of conc. sulphuric acid solution:
Cold conditions: K2Cr2O7 + 2H2SO4 ———→ 2CrO3 + 2KHSO4 + H2O
Hot conditions : 2K2Cr2O7 + 8H2SO4 ———→ 2K2SO4 + 2Cr2(SO4)3 + 8H2O + 3O2
9. Oxidising properties: It is a powerful oxidising agent. In the presence of dil. H2SO4 it furnishes 3
atoms of available oxygen.
K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O + 3O
Some of the oxidizing properties of K2Cr2O7 are:
It liberates I2 from KI
It oxidises ferrous salts to ferric salts
It oxidises S-2 to S
It oxidises nitrites to nitrates
K2Cr2O7 + 7H2SO4 + 6Kl → 4K2SO4 + Cr2(SO4)3 + 3l2 + 7H2O
K2Cr2O7+ 7H2SO4 + 6FeSO4 → K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 2H2O
K2Cr2O7 + 4H2SO4 + 3H2S → K2SO4 + Cr2(SO4)3 + 7H2O + 3S
K2Cr2O7+ 4H2SO4 + 3NaNO2 → K2SO4 + Cr2(SO4)3 + 3NaNO3 + 4H2O
3
www.gradeup.co
It oxidises SO2 to SO42–
K2Cr2O7 + H2SO4 + 3SO2 → K2SO4 + Cr2(SO4)3 + 3H2O
10. Uses of Potassium dichromate:
i. It is used in volumetric analysis for the estimation of Fe2+ and I-.
ii. In chrome tanning in leather industry.
iii. In photography and in hardening gelatin film.
(B) Potassium Permanganate (KMnO4):
1. Potassium permanganate is an inorganic chemical and medication. As a medication, it is used for
cleaning wounds.
2. It is also a strong oxidising agent. It dissolves in water to give intensely pink or purple solutions.
3. It is on the "WHO Model List of Essential Medicines", the most important medications needed in a
basic health system.
4. Its geometry is orthorhombic.
5. Dilute solutions of KMnO4 convert alkenes into diols (glycols)
6. Preparation: It is prepared from the mineral pyrolusite (MnO2). The preparation involves following
steps:
i. Conversion of MnO2 to potassium manganate
4
www.gradeup.co
The powdered pyrolusite is fused with potassium hydroxide in presence of air to form potassium
manganate, which is a green colored compound.
ii. Oxidation of potassium manganate to potassium permanganate
Oxidation can be done in two methods. They are (a) Chemical oxidation and (b) Electrolytic oxidation.
(a) Chemical Oxidation
In this method, potassium manganate is treated with chlorine or ozone to form potassium permanganate.
(b) Electrolytic Condition
In this method, potassium manganate solution is electrolyzed.
In this process, manganate ions are converted to permanganate.
After oxidation, the green color solution changes to a purple color solution which is concentrated by
evaporation and forms crystals on cooling.
7. Action of heat: On heating, KMnO4 decomposes to potassium manganate and oxygen.
8. Action of sulphuric acid:
With cold conc. H2SO4 it gives Mn2O7 which on warming decomposes to MnO2.
2MnO2 + 2H2SO4 → Mn2O7 + 2KHSO4 + 2H2O
2Mn2O7 4MnO2 + 3O2
5
www.gradeup.co
With hot Conc. H2SO4 O2 is evolved
4KMnO4 + 6H2SO4 → 2K2SO4 + 4MnSO4 + 6H2O + 5O2
9. Oxidising properties
The actual oxidizing action depends upon the medium i.e. acidic, basic or neutral:
1) In neutral solution, it acts as moderate oxidizing
agent.
2)In strong alkaline solution, it is converted into
2KMnO4 + H2O → 2KOH + 2MnO2 + 3O
2KMnO4 + 2KOH → 2K2MnO4 + H2O + O
2KMnO4 + H2O + Kl → 2MnO2 + 2KOH + KlO3
+7
3) In acidic medium, Mn is converted into Mn
+2
2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O +
5O
10. Uses of Potassium dichromate:
i. It is used in volumetric analysis for the estimation of ferrous salts, oxalates, iodides and H2O2.
ii. It is used as oxidizing agent in the laboratory as well as in industry.
iii. It is also used as disinfectant and germicide.
iv. Potassium permanganate is used extensively in the water treatment industry.
Comment below on how you liked the article and share it with your friends.
Sharing is Caring :)
Click on the given links to attempt the quiz on d-block elements.
Quiz on D-block Elements
All the Best!
Team Gradeup
Download Gradeup, the best NEET Preparation App
Attempt subject wise questions in our new practice section and complete previous year papers of NEET and
Other Medical Entrance Exams
6
www.gradeup.co
7