Hydrogen

The student will be able to explain: Position of Hydrogen in periodic table Isotopes of hydrogen Methods of preparation, properties and uses of dihydrogen.

What do you understand by word “Hydrogen” ? Introduction What do you understand by word “Hydrogen” ? Hydrogen in Greek means Water producer Hydrogen has the simplest atomic structure among all the elements around us in Nature. In atomic form it consists of only one proton and one electron. However, in elemental form it exists as a diatomic (H2) molecule and is called dihydrogen.

Position of Hydrogen in Periodic Table Positioned by resemblance to alkali metals “Compromise” Position Different Periodic Tables use different positions for H Positioned by resemblance to halogens

Lightest element known having atomic number 1. Dihydrogen The position of hydrogen is anomalous in periodic table Alkali metals Halogens In modern periodic table it is located in first group.

Resemblance with alkali metals 1. Electronic configuration 2. Electropositive character H+, Na+, K+ etc. 3. Oxidation state: +1

Resemblance with halogens 1. Electronic configuration: Both contain one electron less than the nearest noble gas configuration 2. Non-metallic character: Typical non-metals. 3. Atomicity: Diatomic molecules.

4.Formation of similar types of compounds i. Halides: CCl4, SiCl4, GeCl4 ii. Hydrides: CH4, SiH4, GeH4 5. Oxidation state: –1

Occurrence Most abundant element in the Universe (70% of the total mass of the universe) The giant planets Jupiter and Saturn consist mostly of hydrogen. 3rd most abundant element in the Earth's crust, found in minerals, oceans and all living things

Isotopes of hydrogen

Atomic and Physical Properties of Hydrogen

Laboratory preparation of Dihydrogen Zn + dil 2HCl ZnCl2 + H2 It can also be prepared by the reaction of zinc with aqueous alkali. Zn + 2NaOH Na2ZnO2 + H2

Can we use concentrated sulphuric acid and pure zinc in the preparation of dihydrogen? (a) Conc. H2SO4 cannot be used because it acts as oxidizing agent also and gets reduced to SO2. (b) Pure Zn is not used because it is non-porous and reaction will be slow. The impurities in Zn help in constitute of electrochemical couple and speed up reaction.

Methods for commercial production of dihydrogen 1. Electrolysis of water 2. By partial oxidation of hydrocarbons 3. Water gas shift reaction

By Lane process (1) Oxidation stage (2) Reduction stage

Bosch process CO2 dissolves + H2 is collected

Physical Properties Dihydrogen is a colourless, odourless, tasteless, combustible gas. It is lighter than air and insoluble in water. Its other physical properties.

Chemical properties (i) Reaction with halogens 2H2 (g) + X2 (g) 2HX (X= F, Cl, Br, I) (ii) Combustion:- It burns with pale blue flame 2H2 (g) + O2 (g) 2H2O(l) (iii) Reaction with Nitrogen 3H2 (g) + N2 (g) 2NH3 (g) (iv) Reaction with Oxygen

Chemical properties (v) Reaction with metals Reactive metals like Na, K, Ca, Li Metals like Pt, Pd, Ni form interstitial hydrides by absorbing large volume of hydrogen. Such hydrogen is called ‘occluded hydrogen’ (vi) Reaction with metal oxides Hydrogen reduces oxides of less active metals to corresponding metal

Reactions with organic compounds It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance. For example: In metallurgical processes, it is used to reduce heavy metal oxides to metals. In metallurgical processes, it is used to reduce heavy metal oxides to metals. It is used as a rocket fuel in space research.

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..

Ortho and para hydrogens At room temp. 75% 25% stability less stable more stable Different physical properties Similar chemical properties.

Atomic hydrogen Highly reactive. Half life period is 0.3 sec. 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 dehydrogen.