Hydrogen

Isotopes of hydrogen

Atoms of the same element having different having different mass numbers are called isotopes.The difference in mass number arises because the nucleus contains a different number of neutrons.Naturally occurring hydrogen contains three isotopes.

i)Protium $$[ _1^{1}H]$$

ii)Deuterium $$[ _1^{2}H]$$

iii)Tritium $$[ _1^{3}H]$$

Each of the three isotopes contains one proton and 0,1 and 2 neutrons respectively in the nucleus.Protium is the most abundant form of hydrogen.Naturally occurring hydrogen contains 99.986% of the protium isotopes,0.014% of the deuterium isotopes and 7×10⁻¹⁶% of the tritium isotope,so the properties of the hydrogen are essentially those of the lightest isotope.

Isotope Effects

Since the electronic configuration of all the three isotopes of hydrogen is the same (1s¹),their chemical properties are same.However,these isotopes show a quantitative diffrerence in their equilibrium constants and rates of reactions.For example:

i)H₂ is more rapidly adsorbed on the surfaces than D₂.

ii)H₂ reacts with Cl₂ 13 times faster than D₂ does.

iii)A bond to a protium atom can be broken as much as 18 times faster than that to a deuterium atom.

iv)The addition of H₂ to ethylenic compounds takes place two times faster than with the deuterium at the same temperature.

These differences in properties which are due to the difference in masses of the deuterium and protium.Such differences in the properties which are due to the differences in the masses of the isotopes are called isotope effects.

Isotope effect is the most pronounced in the case of hydrogen because hydroden is light, the percentage difference in mass between protium , deuterium and tritium is greater than between the isotopes if any other elements.Thus the isotopes of hydrogen show much greater difference in physical properties than are found between the isotopes of other elements.

Deuterium

It is called heavy hydrogen.It is prepared by using heavy water,D₂O.

$$2D_2O+2Na\rightarrow 2NaOD+D_2$$

$$D_2O+Mg\rightarrow MgO+D_2$$

Principle of Isolation Of Deuterium

Protium water H₂O dissociates to about three times faster than heavy water D₂O does.The equilibrium constant for the dissociation of H₂O is 1.0×10^-14 and the equilibrium constant for the dissociation of D₂O is 3.0×10^-14.

$$H_2O\rightleftharpoons H^++OH^-$$

$$D_2O\rightleftharpoons D^++OD^-$$

Protium bonds are broken more rapidly than deuterium bonds.Thus when water is electrolyzed,H₂ is liberated much faster than D₂, and the remaining water thus becomes enriched in heavy water D₂O.If the process is continued until a small volume remains,then almost pure D₂O is obtained.About 29000 litres of water must be electrolyzed to give one litre of D₂O that is 99% pure.This is the normal way of separating deuterium.Deuterium can br prepared from the heavy water D₂O.

Preparation of Deuterium Compounds

Heavy water D₂O undergoes all of the reactions of ordinary water and is useful in the preparation of other deuterium compounds.Deuterium compounds are commonly prepared by exchange reactions where under suitable conditions deuterium is exchanged for hydrogen in compounds.Hence D₂ reacts with H₂ at high temperature to give HD,and it also exchanges with NH₃ and CH₄ to give NH₂D,NHD₂,ND₃ and CH₃D_CD₄.It is usually to prepare deuteriated compounds using D₂O rather than D₂.The D₂O may be used directly in the preparation instead of H₂O.The three reactions for the preparation of deuterium compounds by exchange reactions using D₂O are given below.

$$NaOH+D_2O \rightarrow NaOD+HDO$$

$$NH_4Cl+D_2O \rightarrow NH_3DCl+HDO$$

$$Mg_3N_2+3D_2O \rightarrow 2ND_3+3MgO$$

The direct reactions for the preparation of deuterium compounds by using D₂O are given below:

$$SO_3+D_2O \rightarrow D_2SO_4$$

$$P_4O_{10}+6D_2O \rightarrow 4D_2PO_4$$

Uses of heavy Hydroden(deuterium)

i)Deuterium and its compounds are used as a tracer in the study of the mechanism of various chemical and metabolic reactions taking place in the body.

ii)It has been used in the preparation of deuterium compounds which are very useful,example:D₂O is used as a moderator in nuclear reactors to slow down the speed of fast moving neutrons.

iii)It has been used as a source of deuterons which are capable of initiating several nuclear transformations through the formation of compound nucleus at relatively low energies.Example:

(D,α) reaction.

$$ _8O^{12}+ _1^{2}D \rightarrow [ _9F^{18}] \rightarrow _7N^{14}+ _2He^{4}$$

(D,n) reaction

$$ _6C^{12}+ _1D^{2} \rightarrow [ _7N^{14}] \rightarrow _7N^{13}+ _0n^{1}$$₂

iv)It is used in the preparation of hydroden bond.

v)Deuteriated compounds are used as solvents in proton NMR spectroscopy.

Tritium(T)

Because of the unfavourable neutron to proton ratio,tritium is a radioactive isotope and decays by β-emission.It emits low energy β-particles and is changed into a lighter isotope of helium,$$[ _2He^{3}]$$

$$_1^{3}T \rightarrow _2^{3}He+ _0n^{1}$$

It has relatively short half-life period of 12.26 years.Thus any T present when the earth was formed has decayed already, and the small amount present now has been formed recently by reaction induced by cosmic rays in the upper atmosphere.

$$_7^{14}N+ _0n^{1} \rightarrow _6^{12}C+ _1^{3}T$$

$$_7^{14}N+ _1^{1}H \rightarrow _1^{3}T+other fragments$$

$$_1D^{2}+ _1D^{2} \rightarrow _1^{3}T+ _1^{3}T$$

Preparation of Tritium

Tritium was first made by bombarding compounds such as D₃PO₄,ND₄Cl,(ND₄)₂SO₄ etc.,with high energy deuterons D⁺.

$$_1D^{2}+ _1n^{2} \rightarrow _1^{3}T+ _1^{1}H$$

Tritium only occurs to the extent of one part T₂ to 7×10¹⁷ parts H₂.Tritium is now obtained in a large scale by iradiating lithiumwith slow neutrons in a nuclear reactor.

$$_3^{6}Li+ _o^{1}n \rightarrow _2^{4}He+ _1^{3}T$$

The T₂ gas is usually stored by making UT₃,which on heating to 400°C releases T₂.

Uses of tritium

i)Tritium is used to make thermonuclear devices,and for research into nuclear fusion reactions as a means of producing energy.

ii)It is widely used as a radioactive tracer in chemistry,medicine and biology because it is relatively cheap and easy to work with.It only emits low energy β radiation,with no γ radiation.The β radiation is stopped by 0.6 cm of air,so no shielding is required.It is non-toxic except if labelled compounds are swallowed.

Preparation of Tritiated Compounds

Tritiated compounds are made from T₂ gas.T₂O is prepared as follows:-

$$T_2+CuO \rightarrow T_2O+Cu$$

$$2T_2+O_2 \xrightarrow[]{pd catalyst}2T_2O$$

Many tritiated compounds can be made by storing the compounds undre T₂ gas for a few weeks,when exchange of H and T occurs.Many tritiated compounds can be prepared by catalytic exchange in solution using either T₂ gas dissolved in water or T₂O.

$$NH_4Cl+T_2O(or HTO) \leftrightharpoons NH_3TCl$$

References

Poudel, Bhoj Raj Poudel and Dr.Megh Raj. A Text Book Of Inorganic Chemistry. Kathmandu: National Book Centre, 2012.

Sons, John Wiley and. J.D Lee Concise Inorganic Chemistry. 2007.