HYDROGE

N
Class 11 unit 9
 INTRODUCTION

Hydrogen is the first element in the periodic table.

It has a resemblance to alkali metals, which lose one electron to form unipositive ions.

It also resembles halogens which gain one electron to form non negative ions.

Hydrogen also forms oxides, halides, and sulphides.

It has very high ionization enthalpy and does not possess metallic characteristics under normal conditions.
 POSITION OF
HYDROGEN IN
THE PERIODIC
TABLE
Inspite of the fact that hydrogen resembles to both alkali metals and halogens at some extent its position in periodic table is not
fixed.

Loss of the electron from hydrogen atom results in nucleus (H+) -1.5*103 pm in size which is small compared to normal atomic
and ionic sizes which are of 50 to 200pm.

This is the reason (H+) does not exist freely and is always associated with other atoms or molecules.

Because of its unique nature it is placed separately in the periodic table.

 DIHYDROGEN

It is the most abundant element in the universe.

Consists of 70% of the total mass of the universe.

It is the principal element in solar atmosphere.

In combined form it consists of 15.4% of the earth’s crust and oceans.

It occurs in plant and animal tissues, carbohydrates, proteins, hydrides, including hydrocarbons and many other compounds.
 ISOTOPES OF
HYDROGEN
Hydrogen isotopes : Protium, Deuterium, Tritium.

They differ from each other in respect of presence of neutrons.

In ordinary conditions,
Protium has no neutrons, deuterium has one, tritium has two.
Protium is the predominant form.

Terrestrial Hydrogen contains 0.0156% of deuterium in HD form

Tritium concentration : 1 atom per 10^18 atoms of D

Tritium is radioactive and emits low energy beta particles.

Since, isotopes have same electronic configuration they almost have same chemical properties but they differ in rate of reactions
because of their difference in bond dissociation enthalpy.
 PREPARATION OF
DIHYDROGEN
There are a number of methods for preparing dihydrogen from metals and metal hydrides.
Laboratory Preparation of Dihydrogen:
(i) It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid. Zn + 2H+ → Zn2+ + H2
(ii) It can also be prepared by the reaction of zinc with aqueous alkali. Zn + 2NaOH → Na2ZnO2 + H2

Commercial Production of Dihydrogen:

The commonly used processes are outlined below:
(i) Electrolysis of acidified water using platinum electrodes gives hydrogen.
2H2O -----> 2H2 + O2
(ii) High purity (>99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel
electrodes.
(iii) It is obtained as a byproduct in the manufacture of sodium hydroxide and chlorine by the electrolysis of brine solution.
During electrolysis, the reactions that take place are:
at anode: 2Cl– (aq) → Cl2(g) + 2e–
at cathode: 2H2O (l) + 2e– → H2(g) + 2OH– (aq)
The overall reaction is 2Na+ (aq) + 2Cl– (aq) + 2H2O(l)
↓
Cl2(g) + H2(g) + 2Na+ (aq) + 2OH– (aq)
(iv) Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst yields hydrogen.
CnH2n+2 + nH2O -----> nCO +(2n+1)H2
E.g. CH4 + H2O ----> CO + 3H2O

The mixture of CO and H2 is called water gas. As this mixture of CO and H2 is used for the synthesis of methanol and a number
of hydrocarbons, it is also called synthesis gas or 'syngas'. Nowadays 'syngas' is produced from sewage, saw-dust, scrap wood,
newspapers etc. The process of producing 'syngas' from coal is called 'coal gasification'.
C + H2O -----> CO + H2

The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of
iron chromate as catalyst.
CO+ H2O ---->CO2 + H2
This is called water-gas shift reaction. Carbon dioxide is removed by scrubbing with sodium arsenite solution. Presently ~77%
of the industrial dihydrogen is produced from petrochemicals, 18% from coal, 4% from electrolysis of aqueous solutions and 1%
from other sources.
 PHYSICAL
PROPERTIES OF
DIHYDROGEN
Dihydrogen is a colourless, odourless and tasteless
It is a combustible gas.
It is lighter than air and insoluble in water.
Melting Point: 18.73K
Boiling Point: 23.67K
 CHEMICAL
PROPERTIES OF
DIHYDROGEN
Dihydrogen gas is chemically inert at room temperature due to high h-h bond enthalpy.

The dissociation of dihydrogen into its atoms requires an extremely high temperature i.e. `0.081% around 2000k which increases
to 95.5% at 5000k.

As it’s orbital is incomplete with 1s electronic configuration, it combine with almost all the elements.

It accomplishes reactions by:

Loss of the only electron to give H+.
Gain of an electron to form H-.
Sharing electrons to form a single covalent bond.
The chemistry of dihydrogen can be illustrated by the following reactions:

Reaction with halogens:

It reacts with halogens, X2 to give hydrogen halides,
H2(g) + X2(g) ----> 2HX(g) , where X= F, Cl, Br, I.
While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst.

Reaction with dioxygen:

It reacts with dioxygen to form water. The reaction is highly exothermic.
2H2(g) + O2 (g) ----> 2H2O(l); ∆ H = –285.9 kJ mol–1

Reaction with dinitrogen:

3H2(g) + N2(g) ----> 2NH3(g); ∆ H= −92.6 kJ mol
With dinitrogen it forms ammonia.
This is the method for the manufacture of ammonia by the Haber process.

Reactions with metals:

With many metals it combines at a high temperature to yield the corresponding hydrides
H2(g) +(g) ----> 2MH(s)where M is an alkali metal
Reactions with metal ions and metal oxides:
H2(g) + Pd2+(aq)---->Pd(s) + 2H+(aq)
yH2(g) + MxOy(s)----> xM(s) + yH20(l)
It reduces some metal ions in aqueous solution and oxides of metals (less active than iron) into corresponding metals.

Reactions with non metals:

H2+Br2 ----> 2HBr
The reactivity of halogens towards dihydrogen decreases as, F2> Cl2 > Br2 > I2 as a result, F2 reacts in dark, Cl2 in the
presence of sunlight Br2 reacts only upon heating while the reaction with I2 occurs in the presence of a catalyst.

Reaction with unsaturated hydrocarbons :

H2 reacts with unsaturated hydrocarbons such as ethylene and acetylene to give saturated hydrocarbons.
This reaction is used in the hydrogenation or hardening of oils.
The vegetable oils such as groundnut oil or cottonseed oil are unsaturated in nature as they have at least one double bond.
Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols.
 USES OF
DIHYDROGEN
-The largest single use of dihydrogen is in the synthesis of ammonia which is used in the manufacture of nitric acid and
nitrogenous fertilizers.

-Dihydrogen is used in the manufacture of vanaspati fat by the hydrogenation of polyunsaturated vegetable oils like soyabean,
cotton seeds etc.

-It is used in the manufacture of bulk organic chemicals, particularly methanol.

CO + H2 --(cobalt)--> CH3OH

-It is widely used for the manufacture of metal hydrides.

-It is used for the preparation of hydrogen chloride, a highly useful chemical.

-In metallurgical processes, it is used to reduce heavy metal oxides to metals.

-Atomic hydrogen and oxy-hydrogen torches find use for cutting and welding purposes. Atomic hydrogen atoms (produced by
dissociation of dihydrogen with the help of an electric arc) are allowed to recombine on the surface to be welded to generate the
temperature of 4000 K.

-It is used as a rocket fuel in space research.

-Dihydrogen is used in fuel cells for generating electrical energy. It has many advantages over the conventional fossil fuels and
electric power. It does not produce any pollution and releases greater energy per unit mass of fuel in comparison to gasoline and
other fuels.
 HYDRIDES

Dihydrogen, under certain reaction conditions, combines with almost all elements, except noble gases, to form binary
compounds, called hydrides.
If ‘E’ is the symbol of an element then hydride can be expressed as EHx (e.g., MgH2) or EmHn (e.g., B2H6)

There are three basic types of hydrides:

(i) saline hydride or ionic hydride,
(ii) metallic hydride, and
(iii) covalent hydride which may be distinguished on the basis of type of chemical bond involved
 IONIC OR SALINE
HYDRIDES
These are stoichiometric compounds of dihydrogen formed with most of the s-block elements which are highly electropositive in
character.

However, significant covalent character is found in the lighter metal hydrides such as LiH, BeH2 and MgH2.
In fact BeH2 and MgH2 are polymeric in structure.

The ionic hydrides are crystalline, non-volatile and non conductive in solid state.

However, their melts conduct electricity and on electrolysis liberate dihydrogen gas at anode, which confirms the existence of
H– ion.
2H- --(anode)---> H2 + 2e-

Saline hydrides react violently with water producing dihydrogen gas.

NaH + H2O ----> NaOH + H2

Lithium hydride is rather unreactive at moderate temperatures with O2 or Cl2. It is, therefore, used in the synthesis of other
useful hydrides, e.g.
8LiH + Al2Cl6 ----> 2LiAlH4 + 6LiCl
2LiH + B2H6 -----> 2LiBH4
 COVALENT OR
MOLECULAR
HYDRIDES
These are formed by p block elements and some s block elements primarily due to the reason that the electronegativity
difference between these elements and hydrogen atom is quite small.

General formula is XHn or XH8-n where n is the number of electrons in valence shell.
These hydrides usually consist of discrete covalent molecules which are held together by weak van der Waal forces of attraction.

Hence, they are called covalent or molecular hydrides.

 PROPERTIES

1) Covalent hydrides are usually volatile compounds having low melting and boiling point and do not conduct electricity.

2)Hydrides of group 13 (BH3, AlH3) do not have enough electrons to form normal covalent bond and hence are called electron
deficient hydrides. They exist in polymeric forms such as B ₂H6, B4H10 (AIH3)n.

3) Hydrides of group 14 (CH4, SiH4, SnH4, PbH4) have exact number of electrons to form normal covalent bond and hence are
called electron exact or electron precise hydrides. Their Bond length increases from CH4 to PbH4 as the size of the element
increases from C to Pb.

4) Hydrides of group 15,16 and 17(NH3, PH3, H₂O, H₂S, HF, HCI) have more electrons than required to form normal covalent
bond and hence are called electron rich hydrides.
The excess electrons in these hydrides are present as lone pair of electrons.

Group 15 hydrides have one lone pair, group 16 hydrides have two lone pair, group 17 hydride have 3 lone pair of electrons.
The presence of lone pair of electrons on the highly electronegative oxygen nitrogen and fluorine atoms results in the formation
of intramolecular hydrogen bonding. As a result of Intermolecular hydrogen bond these hydrides exist as associated molecules.
The hydrides of first elements of group 15,16 and 17 have abnormally high boiling point as compared to boiling point of the
hydrides of second element of each group.
The boiling point of the hydrides of the rest of the elements of each group increases as the atomic number or the molecular mass
of the hydride increases down the group.

5) The lighter elements of group 14,15 and 16 form polynuclear hydrides in which two or more atoms of same elements are
linked together this property of self-linking of atoms is called catenation and is maximum for Karbonn than Nitrogen, Oxygen,
and sulphur.
 METALLIC OR NON-
STOICHIOMETRIC
(OR INTERSTITIAL)
HYDRIDES
These are formed by d-group elements like 3, 4,5,10, 11 12, and f block elements on heating with dihydrogen under pressure
form hydrides.

In group 6, chromium alone forms the hydride, CrH.

The metals of group 7, 8 and 9 do not form hydrides. The region of the periodic table from 7 to 9 which does not form hydride is
referred to as hydride gap.

Hydrogen atom being small occupy some in the metallic lattice producing distortion without any change in its type. That is why
these hydrides are called as interstitial hydrides.
 PROPERTIES OF
METALLIC
HYDRIDES
1) They are hard, have a metallic lustre, conduct electricity, and have magnetic properties.

2) The density of these hydrides is lower than those of metals from which they are formed since the crystal lattice expands due to
inculsion of dihydrogen.

3) These hydrides are often nonstoichiometric i.e., in these hydrides law of constant composition does not hold good. The ratio
of hydrogen atoms to the metal atoms in these hydrides is not fixed but varies with the temperature and pressure.
 USES OF
METALLIC
HYDRIDES
Due to interstitial hydride formation these metals adsorb large volume of hydrogen on their surface. This property of adsorption
of a gas by a metal is called as occlusion.

The amount of hydrogen occluded depends upon the nature and physical state of the metal i.e., colloidal palladium > palladium
> platinum> gold> nickel

If red hot Pd is cooled in H₂, it adsorbs about 935 times its own volume of H ₂ gas. This may be used to separate H ₂ or D ₂ from
He other gases. On strong heating, the occluded hydrogen is liberated.
Thus, metallic hydride can be used as hydrogen storage media.
This property has high potential for hydrogen storage and a source of energy.
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