Potassium- Introduction, Occurrence, Physical and Chemical properties of Potassium & Uses

During the sultry and sunny summer days, what is that one drink you would crave while travelling?

Something that is healthy, refreshing, uncontaminated and easily available, right. What could be a better choice than coconut water for this? Now, do you know what this refreshing drink is actually composed of?

Well, it is 94% water and the rest are some vital minerals and sugar (sucrose, glucose, fructose).

The most significant mineral present in coconut water is potassium!

One cup of coconut water consists of 600 mg of potassium. In multiple ways, It is indispensable for both plant and animal life and is thus necessary for us to study it.

TABLE OF CONTENTS

• Definition of Potassium
• History of Potassium
• Occurrence of Potassium
• Isotopes of Potassium
• Physical Properties of Potassium
• Chemical Properties of Potassium
• Chemical Reactions of Potassium
• Uses of Potassium
• Practice Problems
• Frequently Asked Questions-FAQs

Definition of Potassium

Potassium (K) with an atomic number of 19, is an element of Group - I of the periodic table, an alkali metal as well as an integral member of the s-block of the modern periodic table. Its symbol (K) originates from its Latin name ‘Kalium’. Atomic weight of Potassium is 39.0983 u. It's a silver, lustrous shiny white metal. At room temperature, it exists as a soft solid which can be easily cut by a knife.

Other members of its group include Sodium, Lithium, Cesium, Francium, and Rubidium as alkali metals. Potassium is a metal that is extremely reactive and hence does not occur freely in nature. It exists in the form of a compound.

History of Potassium

Sir Humphrey Davy, in 1807, was the first scientist to isolate Potassium from the compound caustic potash (KOH) in its molten state by the process of electrolysis. Potassium was seen to be accumulated at the cathode side. Potassium is the first metal that was successfully isolated by electrolysis. Earlier, it was extremely difficult to distinguish Sodium and Potassium from each other.

Occurrence of Potassium

Potassium is not found in its free state in nature. It is found in shale, igneous rocks, and sediment in some minerals such as orthoclase feldspar and muscovite.

Apart from this, the minerals orthoclase feldspar and muscovite are highly insoluble in water which makes it extremely difficult to obtain the element by refining.

Hence, we use the compounds of Potassium like langbeinite, sylvite, carnallite, and polyhalite that are soluble in water to procure the commercial compounds of Potassium.

We can also use KCI for the production of Potassium. This reaction is termed the sodium reduction of molten potassium chloride (KCI) at a temperature of 870^oC Molten potassium chloride is fed into a packed distilled column along with sodium vapours that are passed through the column. The volatile Potassium will be formed on the top of the distillation tower after condensation.

Isotopes of Potassium

Potassium has three isotopes that occur naturally, K - 39, K - 40 and K - 41.

K - 40 is highly radioactive in nature and can be found in rocks, plants, and animals. This isotope after beta decay breaks down into a stable isotope of calcium and by electron or positron capture it forms an isotope of argon.

Physical Properties of Potassium

• It is among the 10 most abundant elements which can be found on the Earth’s crust. Potassium constitutes about 2.6 % of the Earth’s total mass.
• Atomic number of K is 19. Its electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. It belongs to Group - 1 and 4^th period.
• Its atomic mass is 39.0983 u
• Potassium has a boiling point of 760^oC and a melting point of 63.28^oC
• The specific gravity of Potassium is 0.862 at a temperature of 20^oC
• At room temperature, the density of potassium is 0.89 g cm^-3.
• Potassium shows + 1 oxidation state.
• The van der Waals radius of potassium is 0.235 nm.

Chemical Properties of Potassium

• On Pauling’s scale, the value of electronegativity of Potassium is 0.82.
• The first isolation energy of Potassium is 418.6 kj mol^-1.
• Being a highly reactive alkali metal, potassium reacts vigorously with water and gives off hydrogen gas which can actually catch fire and cause an explosion.
• It can readily react with chlorine, fluorine, sulphur and nitrogen.  
• It gives a lilac/violet coloured flame upon flame test.

Chemical Reactions of Potassium

Reaction of Potassium with Air:

The alkali metals tarnish in dry air due to the formation of their oxides which in turn react with
moisture to form hydroxides.

When Potassium is burned with air, its shiny lustre is lost, and an orange-yellow colour of potassium superoxide (KO₂) is formed as the product. The superoxide O₂ ions are stable only in the presence of large cations such as K, Rb, Cs. KO₂ is yellowish-orange in colour and paramagnetic in nature.

 K(s) + 02(g) --> KO₂(s)

In fact three species of oxides are possible as potassium oxidises in the presence of air.

• Potassium oxide K₂O 
• Potassium peroxide K₂O₂ 
• Potassium superoxide KO₂ 
  8K(s) + 4O²(g) --> 2K₂O(s) + 2KO₂(s) + K₂O₂(s) 

Potassium oxide hydrolysis produces potassium hydroxide only. Potassium peroxide and potassium superoxide hydrolysis give potassium hydroxide and hydrogen peroxide. Oxides, peroxides and superoxides hydrolyse in the following manner respectively:

         K₂O(s) + H₂O(i) + 2KPH(aq)

(Potassium oxide)

        K₂O₂(s) + 2H₂O(i) + 2KOH(aq) + H₂O₂(aq) 

(Potassium peroxide)

         2KO₂(s) + 2H₂O(i) + 2KOH(aq) + H₂O₂(aq)  + O₂(i)

(Potassium superoxide)

Reaction of potassium with water:

Potassium being highly reactive, reacts vigorously with water and gives off hydrogen gas. The product formed is a solution of potassium hydroxide which is also basic in nature. This reaction is highly exothermic and explosive in nature. The rate of this reaction is higher than that of sodium reacting with water but lesser than that of rubidium reacting with water. In the group - I, the reactivity of alkali metals with water increases down the group.

K(s) + 2H₂O(i) + 2KOH(aq) + 2H₂(g): ΔH = -ve

Reaction of potassium with dihydrogen:

Potassium reacts with dihydrogen at about 673 K to form potassium hydride. All the alkali metal hydrides are ionic solids with high melting points.

2K(s) + H₂(g) + 2KH(s)

Reaction of potassium with halogens:

Potassium reacts with halogens to form solid potassium halide salts. The reaction is highly robust. The reactions are as follows:

2K(s) + F2(g) --> 2KF(s)

2K(s) + Ci₂(g) --> 2KFi(s)

2K(s) + FN2g) --> 2KBr(s)

2K(s) + I₂(g) --> 2KI(s)

Reaction of potassium with sulphuric acid:

Potassium readily dissolves in dilute sulphuric acid, which produces a solution of potassium ions along with hydrogen gas. The reaction is as follows:

2K(s) + H₂SO₄(aq) --> 2K₂SO₄(aq) + H₂(g)

Reaction of potassium with potassium peroxide:

Potassium oxide is highly reactive, hence it forms potassium peroxide and potassium superoxide when oxidised. Now, the treatment of potassium peroxide with potassium results in the formation of potassium oxide.

K₂O₂(s) + 2K(s) --> 2K₂O(s)

Uses of Potassium

• Potassium plays an immensely vital role in maintaining the physiological balance of our body internally.
• Potassium compounds are extremely useful due to their ability to form many compounds.
• Potassium chloride (KCI) is found in a number of fertilizers. It can also be used as a substitute for salt as well as in the production of other compounds.
• Potassium hydroxide (KOH) is useful to manufacture detergents, soaps and drain cleaners.
• Potassium carbonate (K₂CO₃) is used for the production of specific types of glasses.
• Potassium nitrate is used to produce pyrotechnics, fertilizers and also match heads.

Practice Problems

Q1. What is the oxidation state of potassium in potassium superoxide?

1. + 1
2. - 1
3. - ½
4. + ½

Answer: (A)

Solution: The formula of potassium superoxide is KO₂ Here the oxidation state of potassium (K) is + 1 and of oxygen (O) is - ½.

Q2. Choose the correct option.

1. The first ionisation energy is greater than the second ionisation energy of potassium.
2. The second ionisation energy is greater than the first ionisation energy of potassium.
3. Potassium when reacted with air produces only potassium oxide.
4. Reaction of potassium with water is an endothermic reaction.

Answer: (B)

Solution: The electronic configuration of potassium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. Ionisation energy is the amount of energy required to remove the outermost electron of an atom in its gaseous state. Now, if you see the configuration, it is rather easy to remove electrons from 4s orbital, as the potassium atom will attain a noble gas configuration (But now it is difficult to remove an electron from K⁺, as it already attained a noble gas configuration. So, the amount of energy required to remove the first electron will always be lesser than removing the second electron from the potassium atom. Hence, the second ionisation energy is greater than the first ionisation energy of potassium.

Three types of oxides are possible when potassium oxidises in the presence of air which is potassium oxide K₂O, potassium peroxide K₂O₂ and potassium superoxide KO₂.

8K(s) + 4O₂(g) --> 2K₂O(s) + 2KO₂(s) + K₂O₂(s)

The reaction of potassium with water is highly exothermic and explosive in nature.

K(s) + 2H₂O(i) --> 2KOH(aq) + 2KH₂(g); ΔH = -ve

Frequently Asked Questions-FAQs

Question 1. Where is potassium metal used?
Answer. It is used in fertilizers, as a salt substitute and to produce other chemicals. Potassium hydroxide (KOH) is used to make soaps, detergents and drain cleaners.

Question 2. Why is potassium stored in kerosene?
Answer. Being highly reactive potassium is stored in kerosene to avoid contact with air. Otherwise, it would react vigorously with air and produce explosion and form oxides, peroxides, superoxides and respective hydroxides too.

Question 3. What are the natural sources of potassium?
Answer. Some natural sources of potassium are green beans, raw onion, coconut water, cucumber, radish, grapes, pineapple, lettuce leaves, eggplant, mushrooms etc.

Question 4. What are the industrial uses of potassium?
Answer. Industrially potassium finds its application in the making of many things like soaps, detergents, gold mining, gunpowder, batteries, dyes and glass production.

Question 5. What is the role of potassium in our body?
Answer. Potassium is an essential electrolyte responsible for the adequate functioning of our cells and tissues. Besides this, potassium helps maintain blood pHlevels, regulates water content in the body, and helps in reducing high blood pressure levels caused by excess sodium intake. It also assists in skeletal and muscular functioning and healthy heartbeat patterns in our body.

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