It was discovered by Henry Cavendish in 1766 by the action of dilute H₂O₄ on iron. It was named ‘inflammable air’, Lavoisier gave it the name hydrogen (Creek: Hydra = water, gennas = producer]. It occurs in free state as well as in combined state.

Position of Hydrogen in the Periodic Table

Hydrogen resembles with alkali metals (group I) as well as halogens (group 17), At the same time, it differs from both in certain characteristics. That is why hydrogen if; called “rogue element”.

However. it has been placed in group 1 on the basis of its configuration 1s¹, which is the basis of modern classification of elements.

Isotopes of Hydrogen

Hydrogen exists in the form of three sotopes :

Dihdrogen [H₂]

Methods of Preparation

(a) Lab pletbods

Metals which have reduction potential lesser than H, can liberate H₂ from acids.

Pure zinc is not used because it reacts slowly. The presence of some impurities increases the rate of reaction due to the
formation of electrochemical couples Cone sulphuric acid is also not used because it oxidises, H₂formed into H₂O.

Zn + 2H₂SO₄(conc.) → ZnSO₄ + SO₂ + 2H₂O

(ii) It can also be prepared by the reaction of zinc with aqueous alkali.

b) Commercial production of dihydrogen

(i) By the electrolysis of acidified water

ii) From water gas (Bosch process)

Carbon dioxide is removed by dissolving it in water under pressure (20-25 atm) and hydrogen left behind is collected.

(iii) From steam (Lane’s process) Super heated steam is passed over iron filings heated to about 1023-1073 K when hydrogen is
formed.

(iv) Highly pure (> 99.95%)dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel
electrodes.

(v) From hydrocarbons by partial oxidation

vi) It is also obtained as a by-product in the manufacture of NaOH and chlorine hy the electrolysis of brine solution.

During electrolysis, the reactions that take place are

Physical Properties of Dihydrogen

Dihydrogen is a colourless, odourless, tasteless, combustible gas. It is lighter than air and insoluble in water. It is neutral to litmus.

Chemical Properties of Dlhydrogen

(i) Reactivity The relative inertness of dihydrogen at room temperature is because of Its high enthalpy of H-H bond i.e.. high bond dissociation energy. So its reactions take place under specific conditions only (at high temperature).

(ii) Action with non-metals

(iii) Reaction with metals Here H₂ acts as oxidising agent.

(iv) Reducing action of dihydrogen

(v) Reactions with metal ions and metal oxides

(vi) Reaction with organic compounds

Uses of Dihydrogen

1. It is used in the manufacture of CH₃OH.
2. It produces temperature of 2850°C and oxy-atomic hydrogen flame produces a temperature of 4000°C, so it is used in oxy-hydrogen flame.
3. The largest single use of H₂ is in the synthesis of NH₃ which is used in the manufacture ofHNO₃ and fertilizers.
4. Liquid hydrogen (LH₂) is used as rocket fuel.
5. H₂ is used as a reducing agent in extraction of metals.
6. H₂ is used in fuel cell for generating electrical energy.
7. Hydrogen is used in the manufacture of synthetic petrol.

(By heating H₂ with coal and heavy oils under very high pressure in the presence of catalyst.)

Different Forms of Hydrogen

Atomic Hydrogen

It is very reactive and its half-life period is 0.33 s.

Nascent Hydrogen

Freshly prepared hydrogen is known as nascent hydrogen and is more reactive than ordinary hydrogen. It causes the reduction of certain compounds which IS not possible with ordinary hydrogen. It can never be isolated.

Activity of nascent H depends upon the reaction by which it is obtained.

Adsorbed Hydrogen

Adsorption of hydrogen at the metal surface is called occlusion. This hydrogen brings out many chemical changes such as reduction and hydrogenation. Occlusion decreases with rise in temperature.

Ortho and Para Hydrogen

When in hydrogen molecule, the nuclear spins are in the same direction, it is known as ortho hydrogen. On the other hand when the nuclear spins arc in tho opposite direction. it is known as para hydrogen. At room temperature hydrogen consists of 75% ortho and 25% para hydrogen.

Hydrogen Economy

Hydrogen economy is the use of liquid hydrogen as an alternate source of energy. The technology involves the production, transportation and storage of energy in the form of liquid hydrogen. Large scale production of hydrogen can be done by electrolysis of water or by thermochemical reaction cycle. Storage of hydrogen in liquid form can be done in vacuum insulated cryogenic tanks or in a metal or in all alloy like iron-titanium alloy as interstitial hydride. Hydrogen fuel has many advantage over conventional fuels in that it is non-polluting and it liberates large amount of energy on combustion.

Pbotohydrogen is used to obtain renewable energy from sunlight by using microscopic organism such as bacteria or algae.

Hydrides

The compounds of hydrogen with metals and non-metals are called hydrides.

Ionic Hydrides

These are formed by elements of group I, II, (except Be and Mg) by heating them in hydrogen.

These are white colourless solids (crystalline) having high m.p. and b.p. easily decomposed by water, CO₂ or SO₂.

CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂

CaH₂ + 2CO₂ → (HCOO)₂Ca

They are strong reducing agents. Alkali metal hydrides are used for making LiAlH₄, NaBH₄, etc and for removing last traces of water from organic compounds.

Molecular or Covalent Hydrides

These are formed by elements of p-block having higher electronegativity than hydrogen.

1. Electron deficient hydrides These are the hydrides which do not have sufficient number of electrons needed to form normal covalent bonds, e.g., BH₃, AlH₃, etc.
2. Electron precise hydrides These are the hydrides which have exact number of electrons needed to form normal covalent bonds. e.g. hydrides of group 14 (CH₄, SiH₄, etc.)
3. Electron rich hydrides These are the hydrides which have greater number of electrons than required to form normal covalent bonds. e.g., hydrides of group 15, 16, 17, (NH₃, PH₃ ,H₂S, HF, HCl, etc). The excess electrons in these hydrides are present as lone pairs of electrons.

Metallic or Interstitial Hydrides

The transition metals and rare earth metals combine with hydrogen to from interstitial hydrides. They exhibit metallic properties and are powerful reducing agents. They are non-stoicluometric hydrides and their composition varies with temperature and pressure for e.g., LaH_2.76, TiH_1.73.

Metals of group 7, 8 and 9 do not form hydrides and this region of the Periodic Table is called hydride gap.

Polymeric Hydrides and Complex Hydrides

Polymeric hydrides are formed by’ elements having electronegativity in the range 1.4 to 2.0, e.g., (BeH₂)_n, (AlH₃)_n, etc. In complex hydrides H^– acts as ligand and is attached to central metal atom, e.g., LiAlH₄, LiBH₄, etc.

Water

Water is the most abundant and widely distributed on the earth. It occurs in all the three physical states. H₂O is a covalent molecule in which oxygen is sp³ hybridised. It has bent structure.

Physical Properties of Water

1. Water is a colourless, odourless, tasteless liquid. It has abnormally high b.p., f.p., heat of vaporisation due to hydrogea bonding.
2. Pure water is not a good conductor so it is made conductor by adding small amount of acid or alkali.
3. Density of ice (which is mass per unit volume) is lesser than that of water and it floats over water.
4. Water has maximum density at 4°0.
5. Water is a highly polar solvent with high dielectric constant 78.39. It interacts with polar or ionic substances effectively with the release of considerable amount of energy due to ion dipole interaction. The dissolution of covalent compounds like urea, glucose and C₂H₅OH, etc is due to the tendency of these molecules to form hydrogen bond with water.

Chemical Properties of Water

1. Water is amphoteric in nature.

2. In redox reactions,water reacts with metals and non- metals both.

3. In hydrated salts, water may remain in five types such as coordinated water, hydrogen bonded water, lattice water, clathrate water and zeolite water.

4. A number of compounds such as calcium hydride, calcium phosphide. etc ., undergo hydrolysis with water.

Purification of Water

It involves two processes

1. Removal of suspended impurities
2. Destroying the bacteria.

Suspended particles are removed by coagulation with alum followed by filtration.

Exposure to sunlight, boiling, chlorination (treatment with liquid Cl₂ or bleaching powder), ozonisation and addition of CuSO₅ are some processes which are employed to destroy bacteria.

Heavy Water [D₂O]

It was discovered by Urey in 1932. It can be prepared by exhaustive electrolysis of ordinary water using nickel electrodes. It is colourless, odourless, tasteless liquid.

Chemical Reactions of Heavy Water

Usesof Heavy Water

It is used

1. in nuclear reactors to slow down the speed of neutrons and called moderator.
2. as a tracer compound to study the mechanisms of many reactions.

Soft and Hard Water

The water which produces large amount of lather with soap is known as soft water and which forms a scum with soap is known as hard water.

Typesof Hardness of Water

• Temporary hardness It is due to the presence of bicarbonates of calcium and magnesium.
• Permanent hardness Tt is due to the presence of chlorides and sulphates of calcium and magnesium.

Removal of Temporary Hardness

It can be achieved:

(a) By boiling The soluble bicarbonates are converted into insoluble carbonates.

(b) By Clark’s process By adding lime water or milk of lime.

Removal of Permanent Hardness

(i) By adding washing soda The calcium or magnesium salts are precipitated as carbonates.

(ii) By adding caustic soda The temporary and permanent hardness can be removed by adding caustic soda.

(iii) By adding sodium phosphate (Na₃PO₄) The phosphates of calcium and magnesium are precipitated.

Similarly, magnesium also precipitate out in the form of magnesium phosphate, Mg₃(PO₄)₂.

(iv) Calgon’s process Calgon is sodium hexa metaphosphate (Na₆P₆O₁₈). This calgon when added to hard water form soluble complex.

Similarly. Mg²⁺ can also precipitate as Na₂[Mg₂(PO₃)₆] and water becomes free from Ca²⁺ and Mg2+ Ions.

(v) Permutit process Permutit is hydrated sodium aluminium silicate Na₂Al₂Si₂O₈.xH₂O. It exchanges its sodium ions for divalent ions Ca²⁺ and Mg^2+..

Permutit when fully exhausted can be regenerated by treating with 10% solution of sodium chloride. It is most efficient method to gel water with zero degree of hardness.

(vi) By synthetic resins

These are of two types:

(a) Cation exchange resins are big molecules containing sulphonic acid group (-SO₃H). It is first changed into sodium salt with the general formula RNa. The hard water is passed through it so Ca²⁺ and M²⁺ are exchanged and removed.

The resins like permutit can be regenerated with a solution of NaCl.

(b) Anion exchange resins are also big molecules and can exchange anions. They contain an amino group.

The water is first passed through cation resins and then through anion tesin and pure distilled water is obtained.

Measurement of Degree of Hardness

Degree of hardness is defined as the number of parts of calcium carbonate or equivalent to various calcium and magnesium salts present in one million parts of water by mass. It is expressed in ppm.

Degree of hardness (in ppm) = (wt. of CaCO₃ (g)/ wt. of hard water (g)) x 10⁶

The molecular wt. of Ca(HCO₃)₂, Mg(HCO₃)₂, CaCl₂, MgCl₂, CaSO₄ and MgSO₄ is 162, 146, 111, 95, 136 and 120 respectively. The mol. wt. of CaCO₃ is 100.

Thus, 162 g Ca(HCO₃)₂, 146 g Mg(HCO₃)₂, 111 gCaCl₂, 95 g MgCl₂ 136 g CaSO₄ and 120 g MgSO₄ are equivalent to 100 g CaCO₃.

Hydrogen Peroxide [H₂O₂]

H₂O₂ was discovered by J.L. Thenard in 1818. It is an important compound used in pollution control treatment of domestic and industrial effluents.

Methods of Preparation

Strength of Hydrogen Peroxide

The most common method to express the strength of H₂O₂ is in terms of the volume (in mL) of oxygen liberated at NTP by decomposition or 1 mL of that sample of H Z 0 2. A solution of H₂O₂labelled as ’10 volume’ actually means “1 mL of such a solution of H₂O₂ on decomposition by heat produces 10 mL of oxygen at NTP”.

(i) Strength of H₂O₂ in terms of normality

(68 x X/22.4) = 17 x N ⇒ X = 5.6 x N

where, X is volume strength of H₂O₂.

(ii) % strength = (17/56) x volume strength

(iii) X = 11.2 x molarity.

Storage of Hydrogen Peroxide (H₂O₂)

It is stored in the presence of traces of alcohol, acetanilide or sodium pyrophosphate which slow down the rate of decomposition of hydrogen peroxide.

Chemical Properties of H₂O₂

1. Acidic nature It is weakly acidic in nature and pure hydrogen peroxide turns blue litmus red.
2. Oxidising agent It acts as a strong oxidising agent in acidic as well as in basic medium.

e.g., oxidising action of HzOz is

(iii) Reducing agent

(a) In acidic medium

(b) In basic medium

(iv) Bleaching properties Its bleaching action is due to oxidation by atomic oxygen and permanent.

H₂O₂ rarr; H₂O + [O]

dye + [O] → dye is oxidised and bleached