[Chemistry Class Notes] Hydrogen Pdf for Exam

It is the first element in the periodic table. It shows similarity with alkali metals and halogens as well. Although in recent researches and experiments, scientists have found hydrogen in its metallic form. It has one electron in its outermost shell, like other alkali metals, and forms a monovalent ion H+ but in terms of ionization energy, it resembles halogens. 

Occurrence of Hydrogen 

Named from the Greek words hydro, which means water, and the genes for forming, hydrogen is the most abundant element in the universe. It is found in its molecular form as dihydrogen. It comprises 70% of the total mass of the universe. It is the principal element of the solar system. The large planets such as Jupiter and Saturn are mainly composed of hydrogen. In the combined form it constitutes 15.4% of the earth’s crust and the oceans. 

Discovery of Hydrogen 

Hydrogen was discovered by English scientist Henry Cavendish in 1766. He gave it the name ‘inflammable air’. French chemist Antoine Lavoisier named it hydrogen in 1783. 

Isotopes of Hydrogen 

All three isotopes of hydrogen have the same electronic configuration. That’s why they show almost the same chemical properties. but they differ largely in their physical properties due to differences in their mass numbers. Out of all three isotopes, protium is the most common isotope of hydrogen which is abundantly found in nature. 

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Preparation of Dihydrogen 

Dihydrogen can be prepared by following methods –

Laboratory methods to prepare dihydrogen – Following two methods are used in the preparation of dihydrogen –

Zn + 2H+ ? Zn2+ + H2

• The reaction of zinc with aqueous alkali – Dihydrogen can also be prepared by reaction of zinc and aqueous alkali in laboratories. After the reaction, we get sodium zincate as a product. The reaction is given below –

Zn + 2NaOH ? Na2ZnO2 + H2

Commercial methods to prepare dihydrogen – Commonly used methods for commercial production of dihydrogen are listed below –

2H2O ? 2H2 + O2

• It can be prepared by the electrolysis of aqueous barium hydroxide solution using nickel electrodes. Thus, obtained dihydrogen is highly pure in nature. 

• When sodium hydroxide and chlorine are manufactured then dihydrogen is produced as a byproduct in the reaction. 

• It can also be produced by the reaction of steam on hydrocarbons or coke at high temperatures in the presence of a catalyst. Reactions are given below –

CnH2n+2 + nH2O  1270K,Ni → nCO + (2n+1)H2

• Bosch process – In this method, water gas (CO + H2) is mixed with twice its volume of steam and passed over heated catalyst Fe2O3 in the presence of a promoter Cr2O3 or ThO2 at 773 K when CO2 and H2 are obtained. CO2 is removed by dissolving it in water under pressure (20 – 25 atm) and H2 left undissolved is collected. About 18% of the world’s production of H2 is obtained from coal.

C + H2O ? CO + H2

H2 + CO + H2O ? CO2 + 2H2

3Fe + 4H2O ? Fe3O4 + 4H2

Fe3O4 so produced is reduced back iron with water and this reaction is known as vivification reaction. The equation is given below –

Fe3O4 + 4H2 ? 3Fe + 4H2O 

Fe3O4 + 4CO ? 3Fe + 4CO2

Properties of Dihydrogen 

Physical Properties of Dihydrogen are Listed Below –

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

• Its melting point is 13.96K. 

• Its boiling point is 20.39K. 

• Its density is 0.09 g/L. 

• Its enthalpy of fusion is 0.117 kJ/mol.

Chemical Properties of Dihydrogen are Listed Below

Dihydrogen is quite stable and dissociates into hydrogen atoms only when heated above 2000 K, H2 ? H + H. Its bond dissociation energy is very high. For dihydrogen ∆H = 435.9kJ/mol. Due to its high bond dissociation energy, it is not very reactive. However, it combines with many elements or compounds. It shows the following reactions –

Reaction with Metals – It reacts with metals and forms corresponding hydrides. For example, it reacts with sodium and forms sodium hydride. The reaction is given below –

2Na + H₂ ? 2NaH

Reaction with Non-Metals – It reacts with non–metals, and forms respective products. Few of its reactions with various non – metals are given below –

2H2 + O2 ? 2H2O

N2 + 3H2 ? 2NH3

H2 + Cl2 ? 2HCl 

Reaction with unsaturated hydrocarbons – It reacts with unsaturated hydrocarbons such as ethylene and acetylene to give saturated hydrocarbons. The reaction is given below –

H2C=CH2 + H2  Ni, 473K →  CH3-CH3

HCCH + 2H2  Ni, 473K →  CH3-CH3

This reaction is used in the hydrogenation or hardening of oils. Vegetable oils such as groundnut oil or cotton-seed oil are unsaturated in nature because they contain at least one double bond in their molecules. Dihydrogen is passed through the oils at about 473 K in the presence of a catalyst to form solid fats. The vegetable ghee such as Dalda, Rath, etc. is usually prepared by this process.

Vegetable oil + H2 Ni, 473K →  Fat 

Uses of Dihydrogen 

• It is used as a reducing agent. 

• It is used in the hydrogenation of vegetable oils. 

• It is used as rocket fuel in the form of liquid hydrogen. 

• It is used in the manufacturing of synthetic petrol. 

• It is used in Haber’s process of preparation of ammonia. 

• It is used in the preparation of HCl. The reaction is given below –

H2 + Cl2 ? 2HCl

H2 + CO + H2  ZnO.CrO3→  CH3OH

• It is used in the production of metal hydrides. 

• It is used in the manufacture of vanaspati fats. 

• It is used in the metallurgical processes of heavy metals. 

• Atomic hydrogen and oxy-hydrogen torches find use for cutting and welding purposes. 

• It is used as rocket fuel in space research. 

• It is used in fuel cells for generating electrical energy. 

Hydrogen Peroxide 

It is an important compound of hydrogen and oxygen which is mainly used in pollution control treatment of domestic and industrial effluents. It was discovered by French Chemist Thenard. 

Preparation of hydrogen peroxide – It can be prepared by following methods –

Acidifying barium peroxide and removing excess water by evaporation under reduced pressure gives hydrogen peroxide. The reaction is given below –

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

Peroxodisulphate obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide. The reaction is given below –

2HSO4–  Electrolysis →  HO3SO OSO3H  Hydrolysis →  2HSO4– + 2H+ + H2O2

Industrially, it is prepared by the auto-oxidation of 2-alkyllanthraquinols. 

2-ethyl anthraquinol  O2 ↔  H2O2 + oxidized product.

Properties of Hydrogen Peroxide – It’s Physical And Chemical Properties Are Listed Below –

Physical Properties –

1. It is a colorless compound. 

2. It is odorless.

3. It is miscible in water. 

4. Pure hydrogen peroxide is a pale blue syrupy liquid. 

5. It freezes at – 0.5°C and has a density of 1.4 in a pure state. 

6. Hydrogen peroxide is diamagnetic. 

7. It is more highly associated with hydrogen bonding than water. 

8. Although it is a better polar solvent than H₂O. However, it can’t be used as such because of its strong autooxidation ability. 

9. Dipole moment of H2O2 is 2.1 D. 

Chemical Properties 

1. It acts as an oxidizing as well as a reducing agent in both acidic and alkaline media. 

2FeSO4 + H2SO4 + H2O2 ? Fe2(SO4)₃ + 2H2O 

(in acidic medium as oxidizing agent)

2. Pure hydrogen peroxide is an unstable liquid and decomposes into water and oxygen either at room temperature or on heating. The reaction is given below –

2H2O2  ? 2H2O  + O2   ∆H= -196 kJ

Uses of Hydrogen Peroxide 

Few uses of hydrogen peroxide are listed below –

• It acts as a bleaching agent due to the release of nascent oxygen. It is used to bleach delicate materials like silk, wool, leather, etc. It is used as a hair bleach as well. 

• It is used in the manufacturing of many chemicals such as sodium perborate and percarbonate etc. 

• It is used in the synthesis of hydroquinone, tartaric acid, etc. 

• It is used in environmental chemistry. for example, in pollution control treatment of domestic and industrial effluent. 

Structure of Water 

Water is the oxide of hydrogen. It is an important component of animal and vegetable matter. Water constitutes about 65% of our body. It is the principal constituent of the earth’s surface. Its structure is given below –

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Due to the presence of lone pairs, the geometry of water is distorted, and the H-O-H bond angle is 104.5°, which is less than the normal tetrahedral angle (109.5°). The geometry of the molecule is regarded as angular or bent. In water, each O-H bond is polar because of the high electronegativity of oxygen (3.5) in comparison to that of hydrogen (2.1). The resultant dipole moment of the water molecule is 1.84D. In ice, each oxygen atom is tetrahedrally surrounded by four hydrogen atoms (two by covalent bonds and two by hydrogen bonds). The resulting structure of ice is the open structure having a number of vacant spaces. Therefore, the density of ice is less than that of water and ice floats over water. 

Hard and Soft Water

Soft water – Water that produces lather with soap solution readily is called soft water. e.g. distilled water, rainwater, and demineralized water. 

Hard water – Water that does not produce lather with soap solution readily is called hard water. e.g. seawater, river water, well water, and tap water.

Type of Hardness of Water 

The hardness of water is of two types –

• Temporary hardness 

• Permanent hardness 

Temporary Hardness – This is due to the presence of bicarbonates of calcium and magnesium. It is also called carbonate hardness. 

Permanent Hardness – This is due to the presence of chlorides and sulphates of calcium and magnesium. It is also called non-carbonate hardness.  

Heavy Water 

Chemically heavy water is deuterium oxide D2O. It was discovered by American scientist and Nobel Laureate Harold Urey. It is obtained as a by-product in some industries where H2 is produced by the electrolysis of water. Heavy water (D2O) is used –

• as a moderator and coolant in nuclear reactors 

• in the study of the mechanism of chemical reactions 

• as a starting material for the preparation of a number of deuterium compounds. For example – SO3 + D2O ? D2SO4

• preparation of heavy water – It is prepared by the exhaustive electrolysis of water. 

Comparison between Properties of Water and Heavy Water 

This ends our coverage on the topic “Hydrogen”. We hope you enjoyed learning and were able to grasp the concepts. You can get separate articles as well on various subtopics of this unit such as permanent hardness and temporary hardness, hydrogen peroxide, etc. on the website. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to the website or download Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests, and much more.