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Friday, 17 February 2017

Hydrogen IV

Uses of Hydrogen:- it used
1. As a reducing agent.
2. In the manufacture of vanaspati fat, ammonia, metal hydrides, methanol, fertilizers such as urea etc.
3. In the manufacture of synthetic petrol.
4. In the atomic hydrogen torch and oxy hydrogen torches for cutting and welding. Dihydrogen is dissociated with the help of an electric arc and the hydrogen atoms produced are allowed to recombine on the surface to be welded. High temperature of about 4000 k is generated.
5. In the fuel cell for generating electrical energy.
Ortho and parahydrogens:- A molecules of dihydrogen
They show different physical properties. 
For example :
(i) The thermal conductivity of para hydrogen is about 50 % greater than that of ortho hydrogen.
(ii) The melting point of para hydrogen is 0.15 k below that of hydrogen containing 75% ortho hydrogen.
They show similar chemical properties.

Atomic hydrogen:-
Because of high H—H bond enthalpy, atomic hydrogen is produced only at high temp in an electric arc or under ultraviolet radiation.
Highly reactive:-
Half life period is 0.3 sec and therefore, it immediately gets converted into the molecular form liberating a large amount of energy which is used for cutting and welding purposes.

Nascent hydrogen:- 
The hydrogen produced in contact with the substance to be reduced is known as ‘nascent hydrogen’. It is very reactive form of hydrogen better reducing agent than ordinary dihydrogen.

Under certain conditions H2 combines with almost all the elements ,except noble gases to form compounds called hydrides.
There are three types of hydrides ,they are
(i) Ionic or saline hydrides
(ii) Covalent or molecular hydrides
(iii) Metallic or non-stoichiometric hydrides

(i) Ionic or saline hydrides:-
These are the compounds of H2 formed with most of the s-block elements which are highly electro positive.

(ii) Covalent or molecular hydrides:-
These are the compounds of hydrogen formed with most of the p-block elements

[a] Electron deficient:- The hydrides which do not have sufficient number of electrons to form normal covalent bonds is called electron deficient hydride. For example, hydride of group 13(BH3, AlH3, etc.) They are known as Lewis acids i.e., electron acceptors. To make up their deficiency they generally exist in polymeric forms such as B2H6, Al2H6, etc.

[b] Electron precise:- The hydrides which have sufficient number of electrons required for forming covalent bonds is called electron precise hydride. For example, hydrides of group 14(CH4, SiH4, GeH4, SnH4, PbH4 etc.) they have tetrahedral geometry.

[c] Electron rich hydrides:- The hydrides which have excess electrons as required to form normal covalent bonds is called electron rich hydride. For example, hydrides of group 15 to 17 (NH3, PH3, H2O, H2S, H2Se, H2Te, HF etc.)

(iii) Metallic or non-stoichiometric hydrides:-

  • These are formed by many d-block and f-block elements
  • These hydrides conducts heat and electricity though not efficient.

Water:- Water! It is the major part of all living organisms.water is also known as the river of life.
Human body has about 65%and some plants haveasmuch as 95%water.

In a gas phase water is bent molecule with a bond angle of 104.5 and O-H bond length of 95.7pm It is highly polar molecule.

Structure of ice:- Ice has a highly ordered 3D hydrogen bonded structure. Each oxygen atom is surrounded tetrahedrally by four other four other oxygen atoms at a distance of 276 pm

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