Dihydrogen: Preparation
* Hydrogen is the most abundant element in the universe.
* It has three isotopes – protium, deuterium and tritium.
* In spite of its resemblance to both alkali metals and halogens, it occupies a separate place in the periodic table due to its unique behavior.
* Its unique behavior also makes it a commercially important element.
* In the laboratory, it is prepared by the reaction of zinc with dilute hydrochloric acid or with aqueous alkali.
* Commercially, it is usually obtained from petro-chemicals, coal and by the electrolysis of aqueous solutions.
Dihydrogen: Properties and Uses
* Dihydrogen is a colourless, odourless, tasteless, non-poisonous gas.
* The small size of hydrogen atoms is responsible for its unique properties.
* Dihydrogen gas is inert at room temperature.
* Dihydrogen gas reacts with metals and non-metals at high temperatures forming the corresponding hydrides.
* Commercially, it is used as fuel, as a reducing agent and in the manufacture of metal hydrides.
Hydrides
* There are three types of hydrides – ionic or salt-like hydrides, covalent or molecular hydrides and metallic or non-stoichiometric hydrides.
* Ionic or salt-like hydrides are binary compounds of hydrogen with s block elements, which are highly electropositive in nature.
* Covalent or molecular hydrides are binary compounds of hydrogen with more electro-negative elements, such as p block elements.
* Covalent hydrides are further divided into electron-deficient hydrides, electron-precise hydrides and electron-rich hydrides.
* Metallic hydrides are binary compounds of hydrogen with d and f block elements.
* Except for chromium from group 6, the metals of groups 6, 7, 8 and 9 do not form hydrides. The inability of these metals to form hydrides is called the hydride gap of d block.
Water and its Properties
* Water is a tasteless and odourless liquid at standard temperature and pressure.
* Water has strong hydrogen bonding in between molecules, and, therefore exists as associated molecules.
* Due to the presence of extensive hydrogen bonding in between molecules, water has high freezing point, boiling point, heat of vaporisation and heat of fusion. It has high thermal conductivity, surface tension, dipole moment, dielectric constant and highest specific heat.
* The shape of water molecules is V shape or bent.
* The high electro-negativity difference of oxygen and hydrogen in water molecules makes it excellent polar solvent.
* The density of ice is less than the density of water, which makes ice float on water.
Hard and Soft water
* Water is classified as soft and hard, depending on the type of dissolved salts and behavior with soap.
* Hard water can be further classified as temporary and permanent hard water.
* Temporary hardness of water is caused due to the presence of soluble calcium and magnesium hydrogen carbonate.
* Temporary hardness of water can be removed by boiling and by using Clark’s method.
* Permanent hardness of water is due to the presence of salts of magnesium and calcium in the form of chlorides and sulphates.
* Permanent hardness of water can be removed by treating with washing soda, Calgon’s Method,
* Ion-Exchange Method and Synthetic Resins method.
Hydrogen Peroxide
* Hydrogen peroxide is a very pale blue liquid and appears colourless in a dilute solution.
* The hydrogen peroxide molecule is non-planar and has an open book structure.
* In laboratories, hydrogen peroxide is prepared from barium peroxide or acidified sulphate solution.
* Commercially, hydrogen peroxide is prepared by auto-oxidation of 2-ethylanthraquinol.
* Strength of hydrogen peroxide solution is given in terms of the ‘volume of oxygen at NTP that one volume of hydrogen peroxide gives on heating’.
* Hydrogen peroxide has several uses. Its demand has increased as it is used in pollution control treatment of domestic commercial effluents.
Hydrogen as a Fuel
* Hydrogen is an important alternative energy source.
* The use of hydrogen as a fuel provides pollution-free energy as its combustion product is water.
* Heat of combustion per gram of hydrogen is more than twice that of jet fuel.
* Hydrogen is explosive and so is difficult to handle.
* The cost of the production of hydrogen is very high.
