Hydrogen-Physical and Chemical Properties of Hydrogen

Hydrogen, which Cavendish first prepared in 1766 by the action of acids on metals, was named Hydrogen by Lavoisier (hydra = water, Genna’s = maker), meaning water maker. It is the most abundant element in the universe (70% of the universe’s total mass). Ordinary elemental hydrogen exists as a diatomic molecule, called dihydrogen, H2. Hydrogen exists as three isotopes.

It is the only known element whose isotopes have a unique mass ratio of 1:2:3 for Protium, Deuterium, and Tritium. There are three primary sources for the preparation of ordinary hydrogen, which are (i) Water, (ii) Acids, and (iii) Alkalies.

Physical properties of hydrogen

• It is the lightest element with a density = of 0.08987 gL-1.
• It exists as a colourless, odourless, and tasteless gas.
• It has low solubility, about two volumes in 100 volumes of water at 0°C.
• Normal elemental hydrogen exists as a diatomic molecule, called dihydrogen, H2.

Cp / Cv= 𝜸 = 1.40

• It has low melting and boiling points:

Boiling point = -252.5℃ (20.5 K)

Melting point = -259.0℃ (14.0 K)

• The various physical constants for hydrogen are:
  
  

Ionisation potential = 13.54 eV or 313 kcal mol-1

Electron affinity = 17.3 kcal mol-l or 12 kJ mol-l

Electronegativity = 2.1

Critical temperature = -236.9°C

Bond energy, H-H = 436 kJ mol-1

Bond length, H-H = 74 pm

• Hydrogen can be absorbed in large quantities by certain metals like palladium, platinum, iron, cobalt, etc. Palladium can absorb 1000 times its volume of hydrogen. The adsorbed hydrogen is called occluded hydrogen and is more active than ordinary hydrogen. We can use this property to purify hydrogen because only pure hydrogen is adsorbed.

Chemical properties of hydrogen

Combustion

Hydrogen gets easily set on fire; hence it is a flammable or explosive

gas. It burns with a blue flame in the oxygen atmosphere. However, hydrogen does not help in burning.

For example, if a burning splinter is introduced in a jar full of hydrogen, the splinter goes out while the gas burns on the mouth of the pot.

On account of the burning of hydrogen, water is formed.

2H2 + O2 ⟶ 2H2O

Combination reactions 

Hydrogen is not an active element and does not react readily with other elements.

Bond dissociation enthalpy is the energy required to break one mole of the bond to give separated atoms. The H-H bond dissociation enthalpy is 436 kJmol-1, the highest for a single bond between two bits of any element. Its dissociation is only 0.081% at 2000 K and increases to 95.5% at 5000 K.

H2 (g) ⟶ 2H(g) at 5000 K

Hence, most of the reactions of dihydrogen occur at significantly high temperatures.

Under suitable conditions, We can make hydrogen to combine with both metals and non-metals.

(i) With alkali and alkaline earth metals

When heated in the atmosphere, hydrogen combines directly with alkali and alkaline earth metals (except for beryllium). The compounds thus formed are called hydrides. These hydrides are electrovalent compounds, i.e.they conduct electricity in a fused state.

For example, 2Li + H2 = 2LiH;

Ca + H2 = CaH2

2Na + H2 = 2NaH;

(ii) With non-metals

 Under definite conditions, hydrogen combines directly with non-metals like halogens, oxygen, sulphur, nitrogen, etc., forming stable covalent compounds.

With fluorine, hydrogen combines readily even at low temperatures, while the combination with chlorine takes place in sunlight or on heating. Similarly, bromine combines on heating while iodine combines when heated in the presence of a catalyst.

H2 + X2⟶ 2HX

Where X is F, CI, Br, or I

The reactivity of halogens towards dihydrogen decreases

in the order:F2 > Cl2 > Br2 > I2

Hydrogen combines with sulphur on ignition.

H2 + S = H2S

It combines with nitrogen under high pressure and a catalyst at 500°C.

N2 + 3H2 ⇄ 2NH3 ( when Fe is used as a catalyst, at 300 atm )

It combines with carbon at about 1200°C to form methane in small amounts.

C + 2H2⟶ CH4

Reducing nature 

As hydrogen has a great affinity for oxygen, it is suitable for removing oxygen. The oxides of less electropositive metals such as copper, tin, iron, lead, etc., are reduced to the metals when heated in hydrogen.

For example:

PbO + H2 ⟶ Pb + H2o

CuO + H2 ⟶ Cu + H2O

Fe3O4+ 4H2 ⟶ 3Fe + 4H2o

But the oxides of strongly electropositive metals such as alkali and alkaline earth metals are not reduced by dihydrogen.

In suitable catalysts, numerous substances are reduced or hydrogenated by

Hydrogen. The unsaturated fats are changed to saturated fats in finely divided nickel.

Reaction with Carbon monoxide 

Hydrogen combines with CO to form methanol at about 700 K and under high pressure and in the presence of a catalyst (ZnO/Cr2O3).

Hydroformylation of olefins to aldehydes

Alkenes, also called Olefins, combine with carbon monoxide and hydrogen in octacarbonyl dicobalt, CO2(CO)g, as catalysts under high temperature and pressure to form aldehydes which can be further reduced to alcohols.

Conclusion 

The uses of hydrogen, especially as fuel, are determined by its physical and chemical properties. Hydrogen is absorbed in large amounts by metals like palladium, platinum, iron, cobalt, etc. Hence, it can be easily purified. Combustion is an important chemical property that implies that hydrogen does not help in burning. Combination reactions of hydrogen determine if the hydride thus formed can conduct electricity. This property thus further determines the uses of hydrogen as hydrides.

When olefins are combined with carbon monoxide and hydrogen in octacarbonyl dicobalt, CO2(CO)g, they form aldehydes. The thus formed aldehyde can be further reduced to alcohols.