Physical and Chemical Properties Of Hydrogen

Introduction to hydrogen

Hydrogen was first discovered in 1766 by Henry Cavendish while the name was proposed by Lavoisier. Hydrogen is the most copious substance existing in a variety of forms. It is extremely light, colourless, odourless and tasteless in the gaseous state.  Hydrogen aggregates about 75% of the total baryonic mass. Structurally, it is one of the simplest atoms, having only 1 proton and 1 electron while there are no neutrons in it. Hydrogen is found in many substances which we use regularly, in fact, it is the main ingredient of water (H2O), fats, petroleum products, table sugar (C6H12O6), ammonia (NH3), and hydrogen peroxide (H2O2). The atomic number of hydrogen is 1, atomic symbol-H and the atomic weight is 1.0079.



Basic attributes-of hydrogen 

  1. Electronic Configuration: 1s1

  2. Oxidation States: 1, -1

  3. Atomic Radius: 78 pm

  4. Melting Point: -259.34°C

  5. Boiling Point: -252.87° C

  6. Elemental Classification: Non-Metal

  7. At Room Temperature, hydrogen is colourless & Odourless.



Position of Hydrogen in the periodic table

  Hydrogen usually appears at the top of Group IA in the periodic table because of the following reasons:

  1. It has one electron in its outermost shell. 

  2. It forms a monovalent ion to become stable, H+

  3. Just like other alkali metals, it has a valency-1,

  4. The oxide of hydrogen such as water molecule is stable as sodium oxide.

  5. Just like sodium, hydrogen is a good reducing agent in both atomic and molecular states.



Hydrogen can also be placed at the top of Group 7A because:

  1. Hydrogen is a diatomic molecule

  2. It forms anion by gaining one electron.

  3. Hydrogen ion has a stable, inert gas configuration as XH4. X2H6 (like halogens).

  4. The ionization energy of hydrogen is same as that of other halogens.


It is important to note that, the ionization energy of hydrogen is higher than that of the corresponding Group 1A. The hydrogen ion has a smaller size than the alkali metal ion and only forms stable hydride with strong electropositive metals because of it`s minimal tendency to gain an electron(s).  So, hydrogen is unique in this regard, behaving like both a halogen and an alkali metal.



Occurrence of hydrogen

Hydrogen is ranked as the most plentiful element in the universe. Free hydrogen is very rare on the earth but it exists as a compound. Water is the most abundant hydrogen-containing substance on Earth. 



Methods of  Preparation of hydrogen

1. Bosch process: In the Bosch process, steam is passed over red-hot iron filings. The iron combines with oxygen in the steam, and hydrogen is liberated. Tin, cobalt, and nickel can also be used in place of iron. The reaction can be written as 3Fe (s) + 4H2O (g)  →  Fe2O3 (s) + 4H2 (g).


2. Preparation by the action of water on active and inactive Metals: Active metals such as sodium and potassium react with water at room temperature to liberate hydrogen gas. This can be derived by    K(s) + H2O (l)   →   KOH (aq) + H2 (g). Less reactive metals such as calcium, aluminium, magnesium, and zinc also react with water to liberate hydrogen gas, it can be written as  Mg(s) + H2O (l)   →   MgO (s) + H2 (g).


3.Preparation by the action of water on metal hydrides: Hydrogen gas can be prepared by allowing water on either an alkaline metal hydride or an alkaline metal hydride. If  Sodium hydride reacts with water it helps to liberate hydrogen gas and it is illustrated as NaH (s) + H2O (l)   →   NaOH (aq) + H2 (g).  Calcium hydride, on the other hand, reacts with water to form calcium hydroxide and hydrogen gas that can be illustrated as CaH(s) + H2O (l)   →   Ca (OH) 2(aq) + H2 (g)


4. Preparation by the action of Acids on metals: Hydrogen gas can be produced by the reaction between all active metals that are above hydrogen gas in the reactivity series along with the dilute acids such as hydrochloric acid and sulfuric acid. The reaction can be written as  Zn(s) + H2SO4 (aq) →   ZnSO4 (aq) + H2 (g).


5. Preparation of hydrogen from the electrolysis of acidified water: Pure hydrogen gas can be prepared from the following methods.

  1. It can be prepared by the action of pure dilute sulfuric acid on polished magnesium ribbons. It is indicated as Mg(s) + H2SO4 (aq) →   MgSO4 (aq) + H2 (g)

  2. Hydrogen can also be prepared by Electrolysis of warm barium hydroxide using nickel electrodes 

  3. The reaction between aqueous potassium hydroxide and aluminium metal will produce hydrogen. Pure hydrogen gas can be obtained by reacting aqueous potassium hydroxide and aluminium metal as illustrated as Al(s) + 2KOH (aq) →   2KAlO2 (aq) + H2 (g).

  4. Hydrogen gas can be prepared commercially from the Bosch process. In this process, steam is passed over red-hot iron filings. The iron combines with oxygen in the steam, and hydrogen is liberated. Tin, cobalt, and nickel can be used in place of iron.It is written as  3Fe (s) + 4H2O (g)  →  Fe2O3 (s) + 4H2 (g).


Furthermore, hydrogen can also be produced commercially by the reaction between steam and methane, CH4 a process known as steam reforming. Here, methane reacts with the steam at temperatures above 1000ºC with a finely divided nickel catalyst. The carbon monoxide produced is removed by cooling and compressing the mixture of gases. The CO liquefies out leaving hydrogen gas behind. CH4 (g) + H2O (l)  →  CO (g) + 3H2 (g).



Physical properties of hydrogen gas

 Hydrogen is a colourless, odourless and tasteless gas. It is slightly soluble in water and therefore can be collected over water.



Electron affinity


Ionization energy

1312 kJmol-

Atomic radius

37 pm

Ionic radius

210 pm





Quick Question: Explain why hydrogen occurs in a diatomic form rather than a monoatomic form under normal conditions?

Answer: The ionization energy of the hydrogen atom is very high, 1312kJmole-1. For this reason, it is not easy to remove an electron in the hydrogen atom. As a result, its tendency to exist in the monoatomic form is rather low. Instead, hydrogen forms a covalent bond with hydrogen and exists as a diatomic molecule.



Chemical properties of dihydrogen

Dihydrogen can only dissociate into hydrogen atoms at temperatures above 2000k because it is stable. The bond dissociation energy in hydrogen is as high as 435.9 kJmol-. where the hydrogen is not very reactive because of its high bond dissociation.


1.The reaction of hydrogen with metals: Hydrogen reacts with metals to form corresponding metal hydrides. For instance, hydrogen reacts with sodium to form sodium hydride. This can be written as , Na(s) + H2 (g) →  2NaH. Hydrogen reacts with d-block elements such as palladium, nickel, and platinum to form interstitial hydrides. The dihydrogen molecules in the interstitial hydrides occupy the interstitial position in the lattices. This makes these metals adsorb much volume of hydrogen on their surface. Occlusion refers to the process in which the metal adsorbs the gas on its surface. Strong heating of these metals can lead to the liberation of the occluded hydrogen.


2. The reaction of hydrogen with non-metals:  Here are some of the reactions between hydrogen and the non-metals

N2(g) + 3H2(g) →  2NH3(g)

O2(g) + 2H2(g) →  2H2O(l)

Br2 (g) + H2 (g) →  2HBr (aq)

I2 (g) + H2 (g) →  2HI (aq).  The reactivity of halogens with dihydrogen decreases down the group. Therefore, fluorine can react with dihydrogen in darkness; chlorine gas reacts in the presence of sunlight, bromine reacts when it is heated while the reaction between dihydrogen and iodine only occurs in the presence of a catalyst.






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