Unit-9 HYDROGEN complete notes..

HYDROGEN

  Introduction:

 Hydrogen is the first element of periodic table. It has been placed at the top of alkali metal family in group 1 of s-block although it is not a member of the group. It is a typical non-metal and exists as a diatomic molecule (H₂) called dihydrogen in order to distinguish it from atomic hydrogen.

It was discovered by Henery Cavendish in 1766 by the action of dilute H₂SO₄ 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.

Hydrogen is the source of the energy of the stars, including the sun. It undergoes nuclear fusion, which continuously takes place in the sun. This is the source of all energy on earth. Hydrogen may well help us overcome the present energy crisis without polluting our environment. In its reaction with oxygen, it produce large amount of energy and it gives only a non-polluting product water.

Position 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.

Resemblances of Hydrogen with Alkali metals:

1. It is a good reducing agent like alkali metals.

CuO + H₂  →   Cu + H₂O

2. Its electronic Configuration (1s¹) similar to alkali metals (ns¹)

3. It can lose one electron and forms monopositive ion (H⁺) ions like alkali metals (M⁺).

4. It is electropositive in nature like alkali metals.

5. It shows +1 oxidation state like alkali metals.

6. It is liberated at cathode when HCl (aq.) is electrolysed.

7. It forms oxides, sulphides, and halides like alkali metals.

Resemblances of Hydrogen with Halogens:

1. It is a non-metal like Halogens.

2. It exists in nature in diatomic molecules (H₂) like halogens (F₂, Cl₂ etc.).

3. It can gain one electron to form a mononegative ion (H^-) like halogens.

4. It also shows electronegative behaviour like Halogens.

5. Its electronegativity value (2.1) is more closer to halogens.

6. It also shows -1 oxidation state like Halogens.

7. It is liberated at anode when aq. NaH is electrolysed.

NOTE:-

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

Occurrence:

It is estimated that universe contains about 92% hydrogen and 7% helium. All other elements taken together make up the remaining 1%. However, hydrogen, being too light to be held by the earth’s gravitation, is present in our atmosphere only in traces.

Hydrogen occurs in large quantity of water. It is present in all living organisms including plant and animal tissues, carbohydrates, proteins, fats, vitamins, etc.

It is also exist in the form of hydrocarbons like natural gas and petroleum.

Isotopes of Hydrogen:

Hydrogen has three isotopes:

1. protium

2. deuterium

3. tritium

A naturally occurring sample of hydrogen contains about 99.985% protium, 0.015% deuterium (mainly as HD) and only ~10^-16% tritium. The possible molecular forms are H₂, D₂, T₂, HD, HT and DT. Tritium is formed in the upper atmosphere by nuclear reactions caused by cosmic rays.

                          

The lone radioactive isotope of hydrogen is T-a β-emitter with a half-life of 12.33 years.

PROPERTIES OF DIHYDROGEN:

Physical properties of Dihydrogen:

1. It is a colourless, odourless, tasteless gas.

2. It is a diatomic gas.

3. It is highly combustible gas.

4. Its calorific value is very high.

5. It is lighter than air and collected by downward displacement of water because it is insoluble in water.

6. It is neutral towards litmus paper.

Chemical Properties of Dihydrogen:

1. It is less reactive at room temperature because of high bond dissociation energy (436 KJ mol^-1) due to smaller bond length.

2. It forms compounds with almost all elements except noble gases under suitable conditions.

3. Dihydrogen has two nuclear spin isomers called ortho and para-dihydrogen.  atmospheric hydrogen contains 75% ortho hydrogen and 25% para hydrogen.

Preparation of Dihydrogen:

1. By the action of water on metals: 

Metals occurring above hydrogen in the electrochemical series displace hydrogen from water under the conditions depending upon their reactivity.

(a) Action of metals on cold water: 

Elements like sodium, potassium, calcium etc. displace hydrogen from water at room temperature. With alkali metals the reaction is explosive.

2Na + 2H₂O →  2NaOH + H₂ (g)

(b) Action of metals on boiling water: 

Metals like magnesium and aluminium in powder form decompose boiling water, giving hydrogen gas.

Mg + H₂O (boiling) →MgO +H₂

(c) By passing steam over heated metals: 

When steam is passed over some heated metals, such as, Zinc, magnesium and iron, hydrogen gas is obtained.

3Fe + 4H₂O (steam)  →  Fe₃O₄ + 4 H₂ (g)

2. By the Action of water on Metal Hydrides:

Hydrides of alkali and alkaline earth metals react readily with water producing hydrogen.

CaH₂ + 2H₂O  → Ca(OH)₂ + H₂ (g)

3. By the action of Acid on metals: 

Acids contains replaceable hydrogen. Electropositive metals, such as, Zinc, iron, aluminium, magnesium etc. which appear above hydrogen in electrochemical series displace hydrogen from acids. For example,

Zn + dil. H₂SO₄  →  ZnSO₄ + H₂ (g)

Mg + HCl    →  MgCl + H₂ (g)

Note:

Nitric acid makes metals passive by forming a layer of oxide on their surfaces. So, it is not used in such hydrogen displacement reactions.

4. By the action of Alkalies on metals:

Hydrogen can be prepared by the action of boiling caustic soda or caustic potash solutions on metals Zinc, aluminium, tin etc. or on a non-metal like silicon.

Zn + 2NaOH  → NaZnO₂ + H₂

5. By action of methane and steam:

when steam is passed over methne gas dihydrogen is obtained.

CH₄ + H₂O (steam)  →  CO + 3H₂

6. By the action of electrolysis on water:

When water is electrolysed in presence of small amount of acids or bases pure dihydrogen is obtained. Hydrogen so obtained 99.9% pure.

 2H2O  → H2    +   O2

Hydrides:

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

Types of Hydrides:

Hydrides are mainly of three types:

(1) Ionic Hydrides

(2) Molecular or Covalent Hydrides

(3) Metallic or interstitial Hydrides

(1) Ionic Hydrides:

Ionic hydrides are formed by elements of group I, II, (except Be and Mg) by heating them with hydrogen.

These are white colourless crystalline solids having high melting point and boiling point 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.

(2)Molecular or Covalent Hydrides:

Molecular or Covalent hydrides are formed when hydrogen is combined with p-block elements which have higher electronegativity than hydrogen.

These hydrides are further classified into three categories:

(i) Electron rich hydrides

(ii) Electron precise hydrides

(iii) Electron deficient hydrides

(i) Electron rich hydrides:

Electron rich hydrides are those 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, H₂O, HF, HCl, etc.). The excess electrons in these hydrides are present as lone pairs of electrons on the central metal atom. These hydride act as a lewis base as well as a nucleophile.

(ii) Electron precise hydrides:

 Electron precise hydrides are those hydrides which have exact number of electrons needed to form normal covalent bonds. e.g. hydrides of group 14 (CH₄, SiH₄, etc.)

(iii) Electron deficient hydrides:

 Electron deficient hydrides are those hydrides which do not have sufficient number of electrons needed to form normal covalent bonds, e.g., BH₃, AlH₄, etc. These hydrides act as a lewis acid as well as a electrophile.

(3) 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.

Interstitial hydrides are of two types:

(i) Stoichiometric interstitial hydrides:

 These hydrides are formed when the distribution of hydrogen atom is uniform. e.g. TiH, TiH₂, LaH₂, etc.

(ii) Non-Stoichiometric interstitial hydrides:

These hydrides are formed when the distribution of hydrogen atom is not uniform. e.g. TiH_1.73, LaH_2.76 etc.

NOTE: 

(i) non- stoichiometric hydrides and their composition varies with temperature and pressure.

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

Other hydrides:

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:

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.

Chemical Properties of Water:

1. Water is amphoteric in nature.

H₂O(l) + HCl(aq) <= => H₃O⁺(aq) + Cl^-(aq)

H₂O(l) + NH₃(aq) <= => NH₄⁺(aq) + OH^-(aq)

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

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

2F₂(g) + 2H₂O(l) → 4H⁺ + 4F^- + O₂(g)

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.

Heavy Water [D₂O]:

Heavy water 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.

Uses of Heavy Water:

Heavy water is used as:-

1. It is used as a moderator in nuclear reactors to slow down the speed of neutrons.

2. It is used 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 while the water which forms a scum with soap is known as hard water.

Types of Hardness of Water:

(i) Temporary hardness:

Temporary hardness is due to the presence of bicarbonates salt of calcium and magnesium.

Example: Ca(HCO₃)_{2 ,}Mg(HCO₃)₂

(ii) Permanent hardness:

Permanent hardness  is due to the presence of chlorides and sulphates salt of calcium and magnesium.

Example: CaCl_2, CaSO_4, MgCl_2, MgSO₄

  

Removal of Temporary Hardness:

Temporary hardness can be removed by following methods: 

                      

(a) By boiling:

The soluble bicarbonates are converted into insoluble carbonates on boling.

Ca(HCO₃)₂     → CaCO₃ +  H₂O + CO₂

Mg(HCO₃)₂     → MgCO₃ +  H₂O + CO₂

(b) By Clark’s process:

 By adding slaked lime or lime water also converts soluble bicarbonates into insoluble carbonates.

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

Mg(HCO₃)₂  + Ca(OH)₂        → 2MgCO₃ +  2H₂O

Removal of Permanent Hardness:

Permanent Hardness can be removed by following methods:

(i) By adding washing soda:

The calcium or magnesium salts are precipitated as carbonates on adding washing soda.

CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl

CaSO₄ + Na₂CO₃ → CaCO₃ + Na₂SO₄

MgCl₂ + Na₂CO₃ → MgCO₃ + 2NaCl

MgSO₄ + Na₂CO₃ → MgCO₃ + Na₂SO₄

(ii) By Calgon’s process:

Calgon is a trade name of sodium hexa metaphosphate (Na₆P₆O₁₈). When it is added to hard water calcium and magnisium ions from hard water displace sodium ions from the anion of the Calgon.

Na₂(Na₄ P₆O₁₈) + CaCl₂ → Ca(Na₄ P₆O₁₈) + 2NaCl

Na₂(Na₄ P₆O₁₈) + CaSO₄→ Ca(Na₄ P₆O₁₈) + Na₂SO₄

Na₂(Na₄ P₆O₁₈) + MgCl₂ → Mg(Na₄ P₆O₁₈) + 2NaCl

Na₂(Na₄ P₆O₁₈) + MgSO₄ → Mg(Na₄ P₆O₁₈) + Na₂SO₄

(iii) By using ion exchange process (Permutit process):

Permutit is hydrated sodium aluminium silicate Na₂Al₂Si₂O₈.xH₂O. It is generally represented as Na₂Z. Where Z^2- = Al₂Si₂O₈^{2- .} It is also known as Zeolites/permutit. When it is treated with hard water Ca^2+/Mg²⁺ ions goes into the solid phase and an equivalent amount of Na⁺ ions are released to the aqueous phase.

Na₂Z + CaCl₂ → CaZ + 2 NaCl

Na₂Z + CaSO₄ → CaZ + Na₂SO₄

Na₂Z + MgCl₂ → MgZ + 2 NaCl

Na₂Z + MgSO₄ → MgZ + Na₂SO₄

NOTE:-

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

(vi) By the use of synthetic resins method:

Synthetic resins  are large organic molecules having acidic or basic groups. Acid resins contain sulphonic acid group(-SO₃H) and the basic resins contain the basic group [(-NH₃⁺)OH-], 

i.e., substituted ammonium hydroxide group. Acid and basic ion exchange resigns are represented as RSO₃^-H⁺ and RNH₃⁺OH^- respectively.

These are of two types:

(a) Cation exchange resins:

Cation exchange resins are big molecules containing sulphonic acid group (-SO₃H). It is represented by genral formula R’SO₃^-H⁺

R’SO₃^-H⁺ + Ca²⁺ → (RSO₃)₂Ca + 2H⁺

R’SO₃^-H⁺ + Mg²⁺ → (RSO₃)₂Mg + 2H⁺

(b) Anion exchange resins:

Anion exchange resins are also big molecules and can exchange anions. They contain an amino group. These are represented by general formula (R’NH₃⁺OH^-).

R’NH₃⁺OH^- + Cl^- → RNH₃Cl + OH^-

R’NH₃⁺OH^- + SO₄^2- → (RNH₃)₂SO₄ + 2OH^-

NOTE:

Demineralised or Deionised water is obtaind by using this method. 

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. It is a powerful oxidising as well reducing agent in both acidic and alkaline medium. The oxidation state of oxygen in Hydrogen peroxide is -1. It can, therefore, be oxidised to O₂ (Zero O.S.) or reduced to H₂O or OH^- (-2 O.S.). However Hydrogen peroxide is a very powerful oxidising agent and a poor reducing agent.

Structure of Hydrogen Peroxide:

Preparation of Hydrogen Peroxide:

Hydrogen Peroxide can be prepared by following methods:

(i) By sodium peroxide:

When sodium peroxide is treated with ice cold solution of dilute solution (20%) of sulphuric acid hydrogen peroxide is obtained.

Na₂O₂. + H₂SO₄ → Na₂SO₄ + H₂O₂

(ii) By the action of Acids on Metal Peroxides:

Dilute H₂SO₄ or H₃PO₄ acts on metal peroxide to give H₂O₂. Generally hydrated barium peroxide is treated with cold dilute H₂SO₄.

BaO₂.8H₂O + H₂SO₄ → BaSO₄ + H₂O₂ + 8H₂O

(iii) By the Hydrolysis of peroxodisulphate:

When a 50% solution of H₂SO₄ is electrolyzed between Pt electrodes in cold conditions, peroxodisulphuric acid is formed at the anode. The anode solution, on being boiled, gets hydrolysed to form H₂O₂.

2HSO₄^-(aq) → HO₃SOOSO₃H(aq) → 2HSO₄^-(aq) + 2H⁺(aq) + H₂O₂(aq)

(iv) By the auto oxidation of 2- ethylanthraquinol:

When 2- ethylanthraquinol is oxidised with air in presence of H₂/Pd catalyst then 2- ethylanthraquinone and H₂O₂ is obtained.

2- ethylanthraquinol + O₂  (H₂/Pd)→ 2- ethylanthraquinone + H₂O₂

Physical properties of Hydrogen Peroxide:

(i) Hydrogen peroxide is a colourless, odourless syrupy liquid with a blueish tinge.

(ii) It has bitter taste.

(iii) It is soluble in water, ether and alcohol.

(iv) Its density is 1.469g/mL

(v) Its melting point is 272.4 K and boiling point is 423.2 K

(vi) It is diamagnetic in nature.

Chemical Properties of Hydrogen Peroxide:

(i) Stability:

Hydrogen peroxide thermodynamically unstable. It decomposes slowly on standing into water or oxygen in the presence of heat, light, dust or catalyst etc.

2H₂O₂ → 2H₂O + O₂

(ii) Acidic Nature:-

Hydrogen peroxide is a weak acid, Its dissociation constant for the equilibrium, is 1.5 x 10^-12 at 20˚ C

H₂O₂ ↔ H⁺ + HO₂^-

With alkalises it gives corresponding Peroxides.

H₂O₂ + 2NaOH → Na₂O₂ +2H₂O

(iii) Oxidising property:

Hydrogen peroxide act as an Oxidant in Acidic as well as alkaline medium.

(a) Oxidising action in acidic medium:

In acidic medium, H₂O₂ acts as an oxidant as follows:

H₂O₂ + 2H⁺ +2e^- → 2H₂O

(b) Oxidising action in basic (alkaline) medium:

In alkaline medium, H₂O₂ acts as an oxidant as follows:

H₂O₂ +2e^- → 2OH^-

(iv) Reducing property:

Hydrogen peroxide also acts as a Reductant in Acidic as well as alkaline medium.

(a) Reducing action in acidic medium:

In acidic medium, H₂O₂ acts as a reductant as follows:

H₂O₂ → O₂ + 2H⁺ +2e^-

(b) Reducing action in basic (alkaline) medium:

In alkaline medium, H₂O₂ acts as a reductant as follows:

H₂O₂ + 2OH^- → 2H₂O +2e^- + O₂

(v) Bleaching action:

H₂O₂ bleaches coloured flower’s, fabrics, Hair, etc. by oxidising the colouring matter to a colourless product.

[Coloured substance + H₂O₂ → Colourless substance + H₂O+ O˚]

Uses of Hydrogen Peroxide:

(i) It is used as a mild bleaching agent for bleach wool, silk, cotton, hair, teeth etc.

(ii) It is used as an antiseptic in surgery and surgical treatments.

(iii) It is used as an oxidising agent in laboratory.

(iv) It is used as oxidant for a fuel like hydrazine in the propulsion in rockets and torpedoes.

(v) It is used as a antichlor agent in textile industries for removal of excess chlorine after bleaching.

By:
Anjani Kumar Singh.