Hydrogen and Its Compounds

Hydrogen, the most abundant element in the universe and the third most abundant on the globe's surface, is being visualized as a significant future energy source.

Hydrogen boasts the simplest atomic structure in nature, comprising just one proton and one electron. However, in its elemental form, it exists as a diatomic (H₂) molecule known as dihydrogen. Remarkably, hydrogen forms more compounds than any other element.

1.0 Position Of Hydrogen In The Periodic Table

The position of hydrogen is not fixed in the periodic table due to the following reasons :

• Hydrogen shares similarities with group 1 elements, also known as alkali metals.
• Hydrogen exhibits characteristics akin to group 17 elements, commonly called halogens.
• However, hydrogen has distinct properties apart from alkali metals and halogens.

2.0Dihydrogen: Occurrence and Preparation 

Occurrence:

• Highly abundant in the universe, constituting 70% of its total mass.
• Present in minimal amounts (0.15% by mass) in Earth's atmosphere.
• Found in various compounds such as water, carbohydrates, proteins, and hydrocarbons.

Preparation:

Laboratory Preparation: Dihydrogen is typically prepared by

• Zinc reacts with dilute hydrochloric acid or aqueous alkali

Zn + 2HCl  →ZnCl₂+H₂

Zn + 2NaOH → Na₂ZnO₂ + H₂

Commercial Production of Dihydrogen

• Electrolysis of acidified water using platinum electrodes

 2H₂O (l)      →     2H₂ (g)   +  O₂ (g)

• Electrolysis of warm aqueous barium hydroxide solution between nickel electrodes.
• Byproduct during the electrolysis of brine solution.

 2Na⁺ (aq) + 2Cl^– (aq) + 2H₂O(l) → Cl₂(g) + H₂(g) + 2Na⁺ (aq) + 2OH^– (aq)

• Steam reforming of hydrocarbons or coke at high temperatures with a nickel catalyst

 CH₄ (g) + H₂​O (g)     → CO (g) +  3H₂ (g)

This resulting mixture of carbon monoxide and hydrogen, known as synthesis gas or syngas, synthesizes methanol and various other hydrocarbons. The method of generating syngas from coal is termed coal gasification.

• Carbon monoxide from syngas mixtures can be reacted with steam, catalyzed by iron chromate at 673 K, to enhance the production of dihydrogen.
•   CO (g) + H₂O (g)  ⟶    CO₂ (g) + H₂ (g)

This process is referred to as the water-gas shift reaction. Carbon dioxide is eliminated by scrubbing with a sodium arsenite solution.

3.0Hydrogen:Isotopes, Physical And Chemical Properties

Isotopes 

Hydrogen has three isotopes:

• Protium (₁H): This is the most abundant isotope, making up approximately 99.98% of naturally occurring hydrogen. It consists of a single proton and an electron.
• Deuterium (₂H or D): Deuterium is a stable isotope of hydrogen and is twice as heavy as protium. It contains one proton, one neutron, and an electron.
• Tritium (₃H or T): Tritium is a radioactive isotope of hydrogen. It contains one proton, two neutrons, and an electron 

While isotopes of hydrogen share similar chemical properties, there are quantitative differences between them due to variances in atomic mass and stability.

Physical Properties

Hydrogen is an odorless, colorless, and tasteless gas. It is combustible, lighter than air, and does not dissolve in water.

Chemical Properties 

• Like any molecule, dihydrogen's behavior is largely influenced by its bond dissociation enthalpy. 
• The H–H bond has the highest dissociation enthalpy among single bonds between two atoms of any element.
• It undergoes reactions by

(i) donating its sole electron to become H⁺

(ii) accepting an electron to become H^–, and

(iii) sharing electrons to create a single covalent bond.

Reaction with halogens: 

Dihydrogen reacts with halogens (X₂) to produce hydrogen halides (HX), such as HF, HCl, HBr, and HI.

• H₂(g)  +  X₂(g)  → 2HX(g)  ❴ F,Cl,Br,I ❵

While the reaction with fluorine happens spontaneously, even in the absence of light, the reaction with iodine requires a catalyst.

Reaction with dioxygen: 

It combines with dioxygen to produce water. This reaction is highly exothermic.

• 2H₂(g)   +    O₂(g)    →  2H₂O(g)  (ΔH = -ve)

Reaction with dinitrogen: With dinitrogen it forms ammonia. (at 673 k and 200 atm, in the presence of Fe as a catalyst)

•  N₂(g) +  3H₂ (g)  →   2NH₃(g)     

Reactions with metals: At elevated temperatures, it reacts with numerous metals to form their respective hydrides

•  H₂(g) +2M(g) → 2MH(s); where M is an alkali metal

4.0Hydrides

Under specific reaction conditions, dihydrogen readily forms binary compounds, known as hydrides, with nearly all elements except noble gases. 

• Ionic Hydrides: Composed of hydrogen ions and metals, they conduct electricity when melted. Commonly used as reducing agents and fuel sources. Example: Sodium hydride (NaH).
• Metallic Hydrides: Formed between hydrogen and metals, known as interstitial hydrides. They absorb hydrogen, crucial for catalytic reactions and hydrogen storage. Example: Palladium (Pd) and platinum (Pt) can store large volumes of hydrogen.
• Covalent Hydrides: Created when nonmetals or metalloids bond with hydrogen. They can be volatile or non-volatile and are categorized based on electron distribution. Example: Boron hydride (B2H6).

These hydrides play vital roles in various chemical reactions, from hydrogen storage to catalytic processes.

5.0Water: Its Structure,Physical Properties And Chemical Properties 

Water Structure 

In its gaseous phase, the water molecule exhibits a bent shape with a bond angle measuring 104.5 degrees and an O–H bond length of 95.7 picometers, as shown in Figure (a).It possesses a high degree of polarity (Figure (b). Its orbital overlap picture is shown in Figure (c). In the liquid phase, water molecules are linked together through 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. Therefore, an ice cube floats on water.

Physical Properties 

• Pure water is colorless, tasteless and odorless. 
• It freezes at 0°C and boils at 100°C.
• Its maximum Density (298K)/g cm^{– 3} is 1.00 at 4°C.
• It is a polar molecule with a V-shaped structure. The bond angle is 104.5°.
• It has a high dielectric constant. The polar character of water makes it an excellent solvent for polar and ionic substances.
• It is a poor conductor of electricity.
• Boiling point of water is high due to the presence of hydrogen bonding.

Chemical Properties 

Water, with its simple molecular structure of two hydrogen atoms covalently bonded to one oxygen atom (H₂O), exhibits several important chemical properties: Some of the important reactions are given below

Amphoteric Nature: Acts as both acid and base.

With NH₃:     H₂O + NH₃ → NH₄⁺ + OH⁻

With H₂S:     H₂O + H₂S → H₃O⁺ + HS⁻

Ionization: Undergoes autoprotolysis, forming OH⁻ and H₃O^+.

2H₂O ⇌ H₃O⁺ + OH⁻

Redox Reactions: In redox reactions, like reacting with sodium to produce NaOH and H₂.

2H₂O + 2Na → 2NaOH + H₂

Hydrolysis: Facilitates hydrolysis, dissolving ionic compounds and breaking down covalent compounds.

Example: SiCl₄ + 2H₂O → SiO₂ + 4HCl

6.0Hard and Soft Water

Water is classified as either soft or hard based on its mineral content and  ability to form lather with soap.

Water Hardness:

• Caused by calcium and magnesium salts like chloride, sulfate, carbonate, or bicarbonate ions.
• Carbonate and bicarbonate salts precipitate upon boiling.
• Hard water forms scum instead of lather with soap, hindering cleansing.

Hardness Removal:

• Boiling can precipitate carbonate and bicarbonate salts.
• Adding washing or slaked lime, Calgon, or sodium metaphosphate aids in hardness removal.
• In the Calgon process, sodium ions displace calcium and magnesium ions, forming a complex.

Soft Water:

• Contains low calcium and magnesium concentrations.
• Chemically softened water has high sodium content.
• It may taste salty when consumed.

7.0Hydrogen Peroxide (H₂O₂)

Hydrogen peroxide is a crucial chemical utilized for pollution control and treating both domestic and industrial effluents. 

8.0Heavy Water: D₂O

Heavy water, also known as  D₂O, finds wide application as a moderator in nuclear reactors and for investigating reaction mechanisms through exchange reactions. It can be obtained through thorough electrolysis of water or as a by-product in certain fertilizer industries.

9.0Dihydrogen As A Fuel

Dihydrogen is widely used in various applications, including electrochemical cells, which offer greater energy output compared to diesel and gasoline. This has led to the concept of a Hydrogen Economy, aiming to leverage low-carbon energy sources. With its high efficiency and relatively low cost, hydrogen serves as an economical fuel option for commercial use.

Its remarkable calorific value and heat output make hydrogen highly desirable. Its high heating value results in a higher ignition temperature and prolonged flame sustaining time. Due to its abundance, hydrogen can be harnessed as a cost-effective fuel. Fuel cells operate akin to batteries, generating electricity through electrochemical reactions.