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Hydrogen PDF Notes, Important Questions and Formulas



  • Hydrogen is the first element of the periodic table.
  • In the elemental form, hydrogen exists as a diatomic molecule H2 and is called dihydrogen.
  • It resembles Group 1 elements because it has an electronic configuration (1s1) similar to alkali metals and can lose one electron to form unipositive ions.
  • Hydrogen also resembles elements of group 17 (halogens). Like halogen, it requires just one electron to acquire the configuration of the nearest noble gas, i.e. helium.
  • It is the lightest elements known because it has an atomic mass of 1.0079.

Isotopes of Hydrogen

Hydrogen has three isotopes-protium, deuterium and tritium.


Number of Protons (p)

Number of neutrons (n)

Number of electrons (e)

Mass number (m)

Protium ( )





Deuterium ( or T)





Tritium ( or T)





Preparation of Dihydrogen

Dihydrogen is prepared by several methods and metal hydrides.

  • Laboratory Preparation of Dihydrogen
    Hydrogen is prepared in the laboratory by the action or dilute hydrochloric acid or dilute sulphuric granulated zinc.
  • Reaction of granulated zinc with dilute hydrochloric acid
    Zn + 2H+ → Zn2++H2
  • Reaction of zinc with aqueous alkali
  • Pure hydrogen gas is also produced by water on sodium hydride

  • The mixture of CO + H2 is called water gas. This mixture is used for the synthesis of methanol and several hydrocarbons called synthesis gas or syngas.

  • Syngas is produced from sewage, sawdust, scrap wood and newspapers. The process of producing syngas from coal is called coal gasification.

    begin mathsize 12px style straight C left parenthesis straight s right parenthesis plus straight H subscript 2 straight O rightwards arrow with 1270 straight K on top CO left parenthesis straight g right parenthesis plus straight H subscript 2 left parenthesis straight g right parenthesis end style
  • The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as a catalyst. This reaction is called the water-gas shift reaction.
    begin mathsize 12px style CO left parenthesis straight g right parenthesis plus straight H subscript 2 straight O left parenthesis straight g right parenthesis rightwards arrow from Catalyste to 673 of CO subscript 2 left parenthesis straight g right parenthesis plus straight H subscript 2 left parenthesis straight g right parenthesis end style

Properties of Dihydrogen

  • Physical properties of Dihydrogen
    • Dihydrogen is a colourless, odourless and tasteless gas.
    • It is highly combustible, lighter than air and insoluble in water.
  • Chemical Properties of Dihydrogen
    • Dihydrogen is not active because of high bond dissociation enthalpy.
    • It is relatively inert at room temperature because of high H-H bond enthalpy.
    • It does not combine with almost all the elements because its orbitals is incomplete with 1s1
    • Many reactions of hydrogen are due to
      1. Loss of an electron from H+ ion.
      2. Gain of an electron to form H+
      3. Sharing electrons to form a single covalent bond


  • The binary compounds of hydrogen with other elements are called hydrides.
  • If ‘E’ is the symbol of an element, then hydride can be expressed as EHx

  • Ionic Or saline Hydrides
  • Ionic hydrides or saline hydrides are binary compounds of hydrogen with s-block elements which are highly electropositive.
  • In the solid state, the ionic hydrides are crystalline, non-volatile and non-conducting. However, their melts conduct electricity and liberate dihydrogen gas at the anode on electrolysis, confirming the existence of H- ions.

    begin mathsize 12px style 2 straight H to the power of minus left parenthesis melt right parenthesis rightwards arrow with anode on top straight H subscript 2 left parenthesis straight g right parenthesis plus 2 straight e to the power of minus end style
  • Saline hydrides react violently with water producing dihydrogen gas.
    NaH(s) +H2O (aq) → NaOH (aq) + H2 (g)

  • Covalent or Molecular Hydrides
  • Covalent or molecular hydrides are binary compounds of hydrogen with elements of comparatively high electronegative such as p-block elements.
  • Covalent hybrids are classified as electron-rich, electron-deficient or electron-precise hydrides.
  • Electron-rich hydrides have excess electron than required to form normal covalent bonds. Excess electrons are present as lone pairs.
    Example: Hydrides of Group 15, 16 and 17
  • Electron deficient hydrides do not have sufficient number of electrons to form normal covalent bonds.
    Example: Hydrides of Group 13
  • Metallic or Non-Stoichiometric (or Interstitial) Hydrides
    • Metallic hydrides or interstitial hydrides are the compounds of hydrogen with transition metals of Groups 3, 4 and 5 of the d-block elements and Cr metal of group 6 and f-block elements.
    • Since the composition of metallic hydrides does not correspond to simple whole number ratios, they are also called non-stoichiometric hydrides.
      TiH1.8-2, LaH2.87, YbH2.55, ZrH1.3-1.75, VH0.56, NiH0.6-0.7
  • Water
  • Structure of water
    • The molecular formula of water is H2O.
    • Each hydrogen atom is bonded to the oxygen atom with a covalent bond.
    • Molecular mass of water= 18 a.m.u.
  • The oxygen atom acquires a partial negative charge (denoted as delta minus) and the hydrogen atom acquires a partial positive charge (denoted as delta positive).
  • The oxygen atom has a lone pair of electrons. Because of repulsion between these lone pair of electrons, the geometry or shape of the water molecule is distorted, tetrahedral or angular.
  • Hydrogen to oxygen to hydrogen bond angle in the water molecule is 104.50
  • Structure of Ice
  • The crystalline form of water is ice.

  • At atmospheric pressure, ice crystallises in the hexagonal form, but at very low temperatures, it condenses to the cubic form.
  • The density of water is more than that of ice. Water has maximum density (1 g/cm3) at 4℃

  • Chemical Properties of water

    Amphoteric Nature

    It can act as an acid and a base, and such behavior is called amphoteric nature.

    According to Bronsted, it acts as an acid with NH3 and a base with H2S.

    H2O(l)+NH3(aq)→OH-(aq)+Nbegin mathsize 12px style straight H subscript 4 superscript plus end style (aq)


    Redox Reactions Involving Water

    Water can be easily reduced to dihydrogen by highly electropositive metals.



    Water is oxidized to O2 during photosynthesis.




    Hydrolysis Reaction

    Because of high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds.




    Hydrates Formation

    From aqueous solutions, many salts can be crystallised as hydrated salts.

    1. Coordinated Water

  • Hard and soft water
    • Water free form soluble salts of calcium and magnesium is called soft water. It gives lather with soap.
    • Water containing soluble salts of calcium and magnesium in the form of hydrogen carbonate, chlorides and sulphates is called hard water. It does not give lather with soap.
    • Temporary hardness in water is due to soluble salts of hydrogen carbonates of magnesium and calcium.
    • Methods to remove temporary hardness.
      • Boiling: During boiling, soluble magnesium hydrogen carbonate [Mg (HCO3)2] is changed to magnesium hydroxide. Calcium hydrogen carbonate on heating gets converted to calcium carbonate.
        Being insoluble, magnesium hydroxide and calcium carbonate are filtered off.


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  • Clark’s method: A calculated amount of calcium hydroxide is added to a given amount of water. It Precipitates calcium as calcium carbonate and magnesium as magnesium hydroxide which can be filtered off.

    Ca (HCO3)+ Ca (OH) 2→2CaCO3 ↓2H2O

    Mg (HCO3)+ 2Ca (OH) 2→2CaCO3 ↓ +Mg (OH) 2↓ +2H2O

  • Permanent hardness in water is due to soluble salts of chlorides and sulphates of calcium and magnesium
  • Methods to remove permanent hardness.
    • Treatment with washing soda: Washing soda reacts with hard water forming insoluble metal carbonates which can be filtered and removed.
      MCl2+Na2CO3→MCO3↓ +2NaCl
      MSO4+Na2CO3→MCO3↓ +Na2SO4
  • Calgon’s method: Calcium and magnesium ions are rendered ineffective by the addition of sodium hexametaphosphate which is commercially known as Calgon.
    M2++Na4P→ [Na2MP6O18]2-+2Na+
    The complex anion keeps Mg2+ and Ca2+ ions in the solution. 
  • Ion-exchange method: This method is also called as zeolite/permutit process. In this method, ions responsible for hardness of water exchanged by certain less damaging ions present in chemical compounds such as sodium silicate (NaAlSiO4),which can also be written as NaZ (zeolites).
    When this is added in hard water, exchange reactions take place.
    2NaZ(s) + M2-(aq) → MZ2(s) + 2Na(aq)

                                                 (M=Mg, Ca)

    Zeolite is said to be exhausted when all the sodium in it is used up. It is regenerated for further use by treating it with aqueous sodium chloride solution.
    MZ2(s) + 2NaCl (aq) → 2NaZ(s) + MCl2 (aq)

    Synthetic resins method: These are insoluble polymeric solid having a giant hydrocarbon network congaing reactive acidic or basic groups. They perform functions similar to zeolites, but they are superior to zeolites because they can remove all type of ions in water.
    2RNa(s) + M2-(aq) → R2M(s) + 2Na+(aq) 
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