Hydrogen and its compounds »NCERT Notes

Hydrogen and its compounds: Introduction

Hydrogen is the lightest element in the periodic table with Atomic number Hydrogen 1and the symbol H. The standard atomic weight of Hydrogen element 1.008,. It is the most abundant chemical substance in the Universe, approximately 75% of the whole baryonic mass is composed of Hydrogen. Non-remnant stars and sun in the plasma state are principally composed of hydrogen. The 03 common isotopes of hydrogen exist, namely protium –₁H¹(rarely used name, symbol 1H), has a proton, and has no neutrons, Deuterium-₁H² has one proton and one Neutrons, Tritium-₁H³ (radioactive isotope) has one proton and 2 Neutrons.

Hydrogen and its isotopes

Name	Symbol	Atomic Number	Mass Number	Relative abundance	Nature
Protium or Hydrogen	₁H¹or H	1	1	99.985 %	Non-radioactive
Deuterium	₁H²or D	1	2	0.015%	Non-radioactive
Tritium	₁H³or T	1	3	7 x 10^-16%	Non-radioactive

Hydrogen and its isotopes

(1) Position of hydrogen in the periodic table:

Hydrogen is the first element in the periodic table. Hydrogen is placed in no specific group due to its property of gaining electron(When H is formed) and also losing electron (When H is formed)

(i) Hydrogen is placed in group I (Alkali metals)

(a) It has one electron in its (Outer) shell- 1s like other alkali metals which have ns configuration.
(b) It forms a monovalent H ion like Li, Na.
(c) Its valency is also 1.
(d) Its oxide (H,O) is stable as Li O, Na2O.
(e) It is a good reducing agent (In atomic as well as molecular state) like Na, Li…

(ii) Hydrogen also resembles halogens (Group VII A) as,

(a) It is also diatomic (H₂) like F₂, CI.
(b) It also forms anion H like F, CI… by gain of one electron
(c) H has stable inert gas (He) configuration as CH₄ C₂H₆ like halogens CCl₄, SF₂,Cl₂, etc.
(d) H is one electron short of duplet (Stable configuration) like F, CI… which are also one electron deficient than F 2s² 2p⁵; CI -3s²3p⁵

(e) IE of H (1312 kJ mol^-1) is of the same order as that of halogens.

(iii) IE of H is very high in comparison with alkali metals.

Also size of H is very small compared to that of alkali metal ion. H forms stable hydride only with strongly electropositive metals due to smaller value of its electron affinity

(iv) In view of the anomalous behaviour of hydrogen, it is difficult to assign any definite position to it in the periodic table. Hence it is customary to place it in group 1 (Along with alkali metals) as well as in group VII (Along with halogens).

(2) Discovery and occurrence:

It was discovered by Henry Cavendish in 1766. Its name hydrogen was proposed by Lavoisier. Hydrogen is the 9th most abundant element in the Earth’s crust. Hydrogen exists in a diatomic state but in a triatomic state, it is called Hyzone. The systematic name of water is oxidane.

(3) Preparation of Dihydrogen :

Dihydrogenate can be prepared by the following methods,

(i) By action of water with metals

(a) Active metals like Na, K react at room temperature

2M+2H₂O, -———> 2MOH + H2 Metal – Na, K etc

(b) Less active metals like Ca, Zn, Mg, Al liberate hydrogen only on heating

2Al+3H₂0 —> Al₂O₃ +3H₂

3Fe+4H₂O(steam) —> Fe₃O₄ +4H₂

(ii) By the action of water on alkali and alkaline earth metals Hydrides

NaH+H₂O —à NaOH + H₂

CaH₂+2H₂0—à Ca(OH)₂ +2H₂

(iii) By reaction of metals like Zn, Sn, Al with alkalies (NaOH or KOH

Zn+2NaOH—–>Na₂ZnO₂+ H₂

Al+ NaOH —–> 2NaAlO₂ +2H₂

(iii) Commercial production of dihydrogen

(a) Bosch process : In this method, water gas is mixed with twice of its volume of steam and passed over heated catalyst Fe₂O₃in the presence of a promoter Cr,O₃ or ThO₂ at 773 Kwhen CO₂ and H2, are obtained. CO₂is removed by dissolving it in water under pressure (20-25 atm) and H₂, left undissolved is collected

C + H₂0—–> CO + H₂ {Water gas}

H₂+CO+H₂O—–> CO₂+2H₂

About 18% of the world’s production of H2 is obtained from coal.

(b) Lane’s process : By passing steam over sponge iron at 773-1050 K

3Fe + 4H₂0 —–>Fe₃O₄+4H₂

The ferrosoferric oxide (Fe₃O₄) so produced is reduced back to iron with water. this reaction is known as Vivification reaction.

Fe₃O₄ +4H₂O—–> 3Fe+ 4H₂O

Fe₃O₄+4CO —–> 3Fe + 4CO₂

(c) By electrolysis of water Electrolysis of acidified water using platinum electrodes is used for the bulk preparation hydrogen.

(d) From hydrocarbons Hydrocarbons (alkanes) react with steam at high temperature to produce carbon monoxide and Hydrogen, e.g.,

CH₄(g) + H₂O(g) —–> CO(g) + 3H₂(g)

The mixture of CO and H₂ so obtained can be converted into hydrogen as in Bosch process. About 77% of the world’s production of H₂, is obtained from hydrocarbons.

(e) It is also produced as a by-product of the brine electrolysis process for the manufacture of Cl₂ and NaOH.

(iv) Laboratory preparation of Hydrogen: in laboratory hydrogen gas can be produced by the reaction of dil sulphuric acid and granulated zinc metal

(4) Physical properties of dihydrogen :

It is a colourless tasteless and odourless gas. It is slightly soluble in water. It is high combustible. The Physical constants of atomic hydrogen are,

Atomic radius (pm) – 37

Ionic radius of H^– ion (pm) – 210

Ionization energy (kJ mol^-l) +1312

Electron affinity (kJ mol^-l) -72.8

Electronegativity – 2.1

(5) Chemical properties of dihydrogen

Dihydrogen is quite stable and dissociates into hydrogen atoms only when heat above 2000 K, H2 2000KH+H. Its bond dissociation energy

is very high, H2 H+H; AH 435.9kJ mol Due to its high bond dissociation energy, it is not very reactive. However, it combines with many elements or compounds.

(i) Action with metals To form corresponding hydrides.

Heat 2Na+ H₂—–> 2NaH;

Heat Ca+ H2 —–>CaH₂

With transition metals (elements of d block) such as Pd, Ni Pt etc. dihydrogen forms interstitial hydrides in which the smallest molecules of hydrogen occupy the interstitial sites in the crystal lattices of these hydrides. As a result of formation of interstitial hydrides, these metals adsorb large volume of hydrogen on their surface. This property of adsorption of a gas by a metal is called occlusion. The occluded hydrogen can; be liberated from the metals by strong heating.

(ii) Reaction with Non-metals

2H₂ + O₂ —–>+ 2H₂O

N₂+3H₂ —–>2NH₃

H₂+F₂ —–> 2HF

H₂+Cl₂ —–> 2HCl

H₂+l₂ —–> 2HI

H₂+Br₂ —–> 2HBr

The reactivity of halogen towards hydrogen decreases as, F₂ > Cl₂> Br₂ > I₂,

Separation of Hydrogen Isotopes

There are mainly three types of techniques are used for isotope separation:

1. Technique, which are based directly on the atomic weight of the isotope.

2. The technique, which is based on the small differences in chemical reaction rates produced by different atomic weights.

3. The technique, which is based on properties not directly connected to atomic weight, such as nuclear resonances.

ELECTROLYTIC MIGRATION THROUGH A PALLADIUM MEMBRANE:

This procedure depends on the speculation that if a voltage drop is maintained on a layer of fluidized palladium, the hydrogen particles will be released in the palladium in the negative surface and will spread through the film to the positive surface.

There they will consolidate with oxygen particles and return the electrolyte as water.In the event that the present thickness is adequately low, an implacable state will be reached in which hydrogen is not advanced on the negative surface and not

In addition, it is sensible to expect that if hydrogen isotopes are available in the arrangement, fractionation will occur in the palladium layer and the lighter isotope will be released especially was assessed that the shedding factor would be equivalent or somewhat more noticeable than that achieved by ordinary electrolysis.

New Technique of Isotope Separation: Cage molecules act as molecular sieves for hydrogen

Scientists have developed a new hybrid material at the University of Liverpool, which will develop carbon-free nuclear fusion energy. The separation of the three hydrogen isotopes (hydrogen, deuterium, and tritium) is of key importance for fusion energy technology, but current technologies are energy-intensive and inefficient.

Nanoporous materials have the potential to separate hydrogen isotopes by a process known as kinetic quantum sieving (KQS), but low levels of performance currently prohibit scale up. Deuterium, also called heavy hydrogen, has several commercial and scientific uses, including nuclear energy, NMR spectroscopy, and pharmacology. These applications need high purity deuterium, which is expensive due to their low natural abundance. Deuterium enrichment from raw materials that contain hydrogen, such as seawater, is an important industrial process, but it is expensive and requires a lot of energy.

Porous organic cages are an emerging porous material, first reported by Professor Andrew Cooper’s group at the University of Liverpool in 2009, which has been previously used for separation of xylene isomers, noble gases, and chiral molecules. However, purifying deuterium from hydrogen/deuterium gas mixtures in this way is difficult because both isotopes have the same size and shape under normal conditions. By combining small-pore and large-pore cages together in a single solid, the group has now produced a material with high-quality separation performance that combines excellent deuterium/hydrogen selectivity with a high deuterium uptake.

The research was led by Professor Andrew Cooper FRS, whose team at the Materials Innovation Factory designed and synthesized the new cage systems. A separate team led by Dr. Michael Hirscher at the Max Planck Institute for Intelligent Systems tested the separation performance using cryogenic thermal desorption spectroscopy.

Professor Cooper said: “The separation of hydrogen isotopes are some of the hardest molecular separations known today. The “Holy Grail’ for hydrogen/deuterium separation is to introduce precisely the right pore size to achieve high selectivity without compromising the gas uptake too much.”