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Chapter 9 : Hydrogen

Topics covered in this snack-sized chapter:

9.2 Dihydrogen, H2 arrow_upward

  • Hydrogen, the most abundant element in the universe and the third most abundant on the surface of the globe, is being visualised as the major future source of energy.

  • 9.2.1 Occurrence

  • Dihydrogen is the most abundant element in the universe (70% of the total mass of the universe) and is the principal element in the solar atmosphere.
  • The giant planets Jupiter and Saturn consist mostly of hydrogen.
  • In the combined form besides in water, it occurs in plant and animal tissues, carbohydrates, proteins, hydrides including hydrocarbons and many other compounds.

  • 9.2.2 Isotopes of Hydrogen

  • Hydrogen has three isotopes:
    • Protium, 1 1 H – It has no neutron.
    • Deuterium, 2 1 H or D - It has 1 neutron.
    • Tritium, 3 1 H or T - It has 2 neutrons.

    9.3 Preparation of Dihydrogen, H2 arrow_upward

  • There are a number of methods for preparing dihydrogen from metals and metal hydrides.

  • 9.3.1 Laboratory Preparation of Dihydrogen

  • It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid.
  • It can also be prepared by the reaction of zinc with aqueous alkali.

    9.3.2 Commercial Production of Dihydrogen

  • Electrolysis of acidified water using platinum electrodes gives hydrogen.
  • High purity (>99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.
  • 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'.

  • 9.4 Properties of Dihydrogen arrow_upward

  • Some properties of dihydrogen are:
    • Physical properties
    • Chemical properties

    9.4.1 Physical Properties

  • Dihydrogen is a colorless, odorless, tasteless, combustible gas.
  • It is lighter than air and insoluble in water.

  • 9.4.2 Chemical Properties

  • The chemical behaviour of dihydrogen (and for that matter any molecule) is determined, to a large extent, by bond dissociation enthalpy.
  • Reaction with halogens:
  • It reacts with halogens, X2 to give hydrogen halides, HX.
  • 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.
  • Reaction with dinitrogen:
  • With dinitrogen it forms ammonia.

  • 9.4.3 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 soybean, cotton seeds etc.
  • It is used in the manufacture of bulk organic chemicals, particularly methanol.
  • It is widely used for the manufacture of metal hydrides (Section 9.5)

  • 9.5 Hydrides arrow_upward

  • 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 Em Hn (e.g., B2 H6 ).
  • The hydrides are classified into three categories:
    • Ionic or saline or salt-like hydrides
    • Covalent or molecular hydrides
    • Metallic or non-stoichiometric hydrides

    9.5.1 Ionic or Saline Hydrides

  • These are stoichiometric compounds of dihydrogen formed with most of the s-block elements which are highly electropositive in character.
  • The ionic hydrides are crystalline, non-volatile and nonconducting in solid state. However, their melts conduct electricity and on electrolysis liberate dihydrogen gas at anode, which confirms the existence of H ion.
  • Saline hydrides react violently with water producing dihydrogen gas.
  • 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.,

  • 9.5.2 Covalent or Molecular Hydride

  • Dihydrogen forms molecular compounds with most of the p-block elements.
  • Examples: CH4 , NH3 , H2 O and HF.
  • Molecular hydrides are classified into:
    • Electron-deficient
    • Electron-precise,
    • Electron-rich hydrides.
  • Electron-deficient hydride has too few electrons. For example: Diborane (B2 H6 )
  • Electron-precise compounds have the required number of electrons. For example: CH4
  • Electron-rich hydrides have excess electrons which are present as lone pairs.

  • 9.5.3 Metallic or Non-stoichiometric (or Interstitial) Hydrides

  • These are the hydrides of transtion elements (except elements of group I-B and II-B).
  • In transition elements there are small empty spaces among the atoms.

  • 9.6 Water arrow_upward

  • A major part of all living organisms is made up of water. Human body has about 65% and some plants have as much as 95% water.

  • 9.6.1 Physical Properties of Water

  • It is a colorless and tasteless liquid.
  • The unusual properties of water in the condensed phase (liquid and solid states) are due to the presence of extensive hydrogen bonding between water molecules.
  • This leads to high freezing point, high boiling point, high heat of vaporisation and high heat of fusion in comparison to H2 S and H2 Se.

  • 9.6.2 Structure of Water

  • In the gas phase water is a bent molecule with a bond angle of 104.5°, and O–H bond length of 95.7 pm.
  • In the liquid phase water molecules are associated together by hydrogen bonds.
  • The crystalline form of water is ice. At atmospheric pressure ice crystallizes in the hexagonal form, but at very low temperatures it condenses to cubic form.
  • Density of ice is less than that of water.

  • 9.6.3 Structure of Ice

  • Ice has a highly ordered three dimensional hydrogen bonded structure as shown in the figure:
  • Each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm.
  • Hydrogen bonding gives ice a rather open type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.

  • 9.6.4 Chemical Properties of Water

  • Water reacts with a large number of substances. Some of the important reactions are:
  • Amphoteric Nature:
  • It has the ability to act as an acid as well as a base i.e., it behaves as an amphoteric substance. In the  sense it acts as an acid with NH3 and a base with H2 S.
  • Redox Reactions Involving Water:
  • Water can be easily reduced to dihydrogen by highly electropositive metals.
  • Thus, it is a great source of dihydrogen.
  • Water is oxidised to O2 during photosynthesis.
  • Hydrolysis Reaction:
  • Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds.
  • However, certain covalent and some ionic compounds are hydrolysed in water.

  • 9.6.5 Hard and Soft Water

  • Presence of calcium and magnesium salts in the form of hydrogen carbonate, chloride and sulphate in water makes water ‘hard’.
  • Hard water does not give lather with soap.
  • Water free from soluble salts of calcium and magnesium is called Soft water. It gives lather with soap easily.
  • Hard water forms scum/precipitate with soap.
  • Soap containing sodium stearate (C17 H35 COONa) reacts with hard water to precipitate out Ca/Mg stearate.
  • The hardness of water is of two types:
    • Temporary hardness, and
    • Permanent hardness

    9.6.6 Temporary Hardness

  • Temporary hardness is due to the presence of magnesium and calcium hydrogen-carbonates. It can be removed by:
    • Boiling: During boiling, the soluble Mg(HCO3 )2 is converted into insoluble Mg(OH)2 and Ca(HCO3 )2 is changed to insoluble CaCO3 . These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.

    • Clark’s method: In this method calculated amount of lime is added to hard water. It precipitates out calcium carbonate and magnesium hydroxide which can be filtered off.

    9.6.7 Permanent Hardness

  • It is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water. It can be removed by the following methods:
    • Treatment with washing soda (sodium carbonate): Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.

    • Calgon’s method: Sodium hexametaphosphate (Na6 P6 O18 ), commercially called ‘calgon’, when added to hard water, the following reactions take place.

    9.7 Hydrogen Peroxide (H2 O2 ) arrow_upward

  • Hydrogen peroxide is an important chemical used in pollution control treatment of domestic and industrial effluents.

  • 9.7.1 Preparation

  • It can be prepared by the following methods:
  • Acidifying barium peroxide and removing excess water by evaporation under reduced pressure gives hydrogen peroxide.
  • Industrially it is prepared by the auto oxidation of 2-alklylanthraquinols.

  • 9.7.2 Physical Properties

  • In the pure state H2 O2 is an almost colourless (very pale blue) liquid.

  • 9.7.3 Structure

  • Hydrogen peroxide has a non-planar structure. The molecular dimensions in the gas phase and solid phase are shown in the figure given below:

  • 9.7.4 Chemical Properties

  • It acts as an oxidising as well as reducing agent in both acidic and alkaline media.
  • Oxidising action in acidic medium:
  • Reducing action in acidic medium

  • 9.7.5 Storage

  • H2 O2 decomposes slowly on exposure to light.
  • It is stored in wax-lined glass or plastic vessels in dark. Urea can be added as a stabiliser.

  • 9.7.6 Uses

  • It is used as hair bleach and as a mild disinfectant.
  • It is used to manufacture chemicals like sodium perborate and per-carbonate, which are used in high quality detergents.

  • 9.8 Heavy Water, D2 O arrow_upward

  • It is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms.
  • It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries.
  • It is used for the preparation of other deuterium compounds, for example:

  • 9.9 Dihydrogen as a Fuel arrow_upward

  • Hydrogen can be used as a source of energy.
  • It involves the production of large quantities of hydrogen electrically and its storage in liquid form in vacuum insulated cryogenic tanks and used in space programmes.
  • It produces as a automobile fuel because it is pollution free.

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