FSc Notes Part 2 Chemistry Important Chemicals Hydrogen

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Hydrogen

Position of Hydrogen In The Periodic Table

Elements are arranged in the periodic table on the basis of their element configuration. Hydrogen resembles with three groups of periodic table in configuration and properties which are

1. Group IA (Alkali Metals)
2. Group IV (Carbon Family)
3. Group VII A (Halogen)

Hydrogen and Alkali Metals

Similarities

• Both have one electron in valence shell.
• Both are electropositive and form positive ions i.e. H+ and Na+ etc.
• Both are hydrated in aqueous solution as H+, Li+ and Na+ etc.
• Both reacts with non metals to form similar type of compounds as H2O, Na2O and HCl, NaCl etc

Dissimilarities

• Hydrogen in gas whereas IA element are metal.
• H+ is unstable whereas Na+, Li+ are stable.
• Alkali metals don’t form negative ions but Hydrogen i.e. H-.
• Hydrogen forms covalent and ionic compounds whereas alkali metals only form ionic compounds.
• Hydrogen exists as diatomic molecule (H2) where alkali don’t.
• Hydrogen from neutral oxide (H2O) whereas alkali metals form basic oxides (Na2O).

Hydrogen and Halogens (VII A)

Similarities

• Both are gases i.e. H2, F2, Cl2.
• Both require one electron to complete their shell.
• Both forms negative ions as H-, Cl-, Br-.
• Metallic hydrides (NaH) as well as metallic halides (NaCl) are ionic.
• Both exists as diatomic molecules i.e. H2, F2, Cl2.
• Both forms covalent compounds with non-metals i.e. CH4, SiH4, Ccl4.

Dissimilarities

• Hydrogen is colourless whereas halogens are coloured gases.
• Hydrogen has one electron in valence shell (1s(1)), whereas halogens have seven electrons (ns2np2).
• Halogens don’t form positive ions whereas hydrogen forms positive ions H+.
• H- is unstable whereas F-, Cl- are stable.
• Hydrogen is reducing agent where halogens are oxidizing agents.
• Hydrogen forms neutral oxides whereas halogens form acidic oxides.

Hydrogen and Group IV A

Similarities

• Both have half filled valence shell.
• Both forms covalent compounds.
• Both have same electronegativity.
• Both have same ionization potential.
• Both have same electron affinity.
• Liquid Hydrogen and carbon (except graphite) are bad conductors.

Dissimilarities

• Hydrogen is gas whereas IV A elements are metals and metalloids.
• Hydrogen shows mono valency whereas IV shows tetra valency.
• Hydrogen is in s-block whereas IV A elements are in p-block.
• Hydrogen has one electron in valence shell whereas IV A, elements have four.
• Hydrogen does not exhibit allotropy whereas IV A elements exhibit.
• Hydrogen is diatomic whereas IV A are not.

Conclusion

Due to these similarities and dissimilarities, position of hydrogen is not decided. But it is generally placed with group I A elements due to similar electronic configuration.

Industrial Preparation of Hydrogen

1. Electrolysis of Water

When electricity is passed through water in presence of electro hydrogen and oxygen. H2 is collected at cathode and O2 at anode.

2. From Natural Gas

When a mixture of steam and natural gas is heated upto 900ºC in presence of Nickle, then a mixture of hydrogen and carbon monoxide called water gas is produced.

Separation of H2 From Water Gas

Method I

Calculated amount of steam is mixed with water gas and this mixture is passed over iron oxide or cobate oxide at 500ºC. As a result CO is converted into CO2.

Mixture of CO2 and H2 is passed through water under high pressure. As a result CO2 is converted to H2CO3 leaving free hydrogen gas.

H2O + CO2 + H2 —-> H2CO3 + H2 ↑

Method II

It is the liquification method in which water gas is cooled up to -200ºC by means of liquid air. Then CO becomes liquid and free hydrogen gas is separated. But this hydrogen may contain traces of CO which are removed by passing it through the solution of NaOH and pure hydrogen is obtained.

3. By the Action of Steam of Coal

When the steam is passed over red hot coke at about 10000ºC then water gas is produced.

Separation of H2 From Water Gas

Method I

Calculated amount of steam is mixed with water gas and this mixture is passed over iron oxide or cobate oxide at 500ºC. As a result CO is converted into CO2.

Mixture of CO2 and H2 is passed through water under high pressure. As a result CO2 is converted to H2CO3 leaving free hydrogen gas.

H2O + CO2 + H2 —-> H2CO3 + H2↑

Method II

It is the liquification method in which water gas is cooled up to -200ºC by means of liquid air. Then CO becomes liquid and free hydrogen gas is separated. But this hydrogen may contain traces of CO which are removed by passing it through the solution of NaOH and pure hydrogen is obtained.

CO + H2 + NaOH —-> HCOONa + H2↑

4. Steam Methanol Process

When mixture of methanol vapours and steam is heated at about 250ºC then a mixture of hydrogen and carbon dioxide is produced.

CH3OH + H2O —-> CO2 + 3H2

This mixture is passed through water under high pressure. As a result, CO2 is converted to H2CO3 leaving free hydrogen gas.

H2O + CO2 + H2 —-> H2CO3 + H2↑

5. Thermal Decomposition of Hydrocarbon

When a hydrocarbon methane is heated at 800ºC in absence of air or oxygen then it is decomposed into carbon black and hydrogen.

This process is called Pyrolysis or Cracking. Carbon Black is used as pigment for inks, plastics, paints, typewriter, carbon papers. It is used in rubber industry, especially as fillers for tyres.

6. Thermal Decomposition of Ammonia

In this process liquid ammonia from cylinders is vaporized and heated to 1000ºC then passed over an active catalyst.

The mixture of N2 and H2 is cooled to -196ºC when N2 liquefies leaving hydrogen free.

Atomic Hydrogen

Definition

The hydrogen obtained as a result of dissociation of ordinary molecular hydrogen is known as Atomic Hydrogen.

Preparation of Atomic Hydrogen

1. Thermal Decomposition of H2

When ordinary hydrogen is heated at 5000ºC in absence of air in a metallic tube of high melting point then it decomposes into atomic hydrogen.

2. Electrical Dissociation of H2

When high voltage electricity is passed through the molecular hydrogen in a discharge tube at low pressure then atomic hydrogen is obtained.

Properties of Atomic Hydrogen

Usually hydrogen is found in molecular form because atomic hydrogen is very reactive. It has only only electron in outermost shell and requires only one electron to complete its shell. Atomic hydrogen gets their electron from other hydrogen atom and form molecular hydrogen. Therefore, its life period is 0.3 seconds. Some reactions of atomic hydrogen are

P + 3H —-> PH3

O2 + 2H —-> H2O2

S + 2H —-> H2S

Cl2 + 2H —-> 2HCl

CuO + 2H —-> Cu + H2O

AgCl + H —-> Ag + HCl

C2H4 + 2H —-> C2H6

Uses of Atomic Hydrogen

Atomic Hydrogen is used to prepare Atomic Hydrogen Torch which contains a temperature of 4000ºC to 5000ºC. It is used to weld all alloys, Nickle steels. Hydrogen gas is passed through electric are set between tungsten filament. H2 decomposes to form atomic hydrogen which then recombines releasing energy due to which temperature is raised to 5000ºC.

H + H —-> H2 + 104 K. Cal

Difference Between Atomic Hydrogen & Nascent Hydrogen

Atomic Hydrogen

1. Produced as a result of dissociation of molecular hydrogen.
2. It is produced by Thermal Decomposition and electrical dissociation of molecular hydrogen.
3. Produced at high temperature.

Nascent Hydrogen

1. Produces at the time of birth in a chemical reaction.

2. It is produced as a result of displacement reaction, when a compound of hydrogen reacts with highly positive metal.

Zn + 2HCl —-> ZnCl2 + 2[H]

3. Produced at room temperature.

Differences between Atomic Hydrogen and Molecular Hydrogen

Atomic Hydrogen

1. Methods of preparation.
2. Reactions are fast.
3. Reactions don’t require catalyst.
4. Reactions are carried out at room temperature.

Molecular Hydrogen

1. Methods of preparation.
2. Reactions are slow.
3. Reactions require catalyst.
4. Reactions are carried out at high temperature.

Binary Compounds of Hydrogen

Definition

Compounds of hydrogen with other elements are called binary compounds or Hydrides.

Types of Hydrides

There are six binary compounds of hydrogen which are

1. Ionic Hydrides

There are the hydrides of I A and II A (except Be, Mg)

General Formula = M-H+ (I A)

= M+2 H2- (II A)

Ionic hydrides, metal atom form positive ion and hydrogen atom forms negative ion so they are also called True Hydrides.

Preparation

They are prepared by passing hydrogen gas over hot alkali metals or alkaline earth metals.

Properties

They react with water, acids and alcohol to produce hydrogen gas.

Na+H- + H2O —-> NaOH + H2

Na+H- + HCl —-> NaCl + H2

Na+H- + C2H5OH —-> C2H5ONa + H2

Ca+2H2- + 2H2O —-> Ca(OH)2 + 2H2

2. Covalent Hydrides

These are the hydrides of IV A to VII A. They contain covalent bond, therefore they are called Covalent Hydrides. IV A hydrides are neutral, V A hydrides are basic and VI A, VII A hydrides are basic.

Preparation

They are prepared indirectly as

CaC2 + 2H2O —-> Ca(OH)2 + C2H2

Al4C3 + 6H2O —-> 2Al2O3 + 3CH4

Mg3N2 + 6H2O —-> 3Mg(OH)2 + 2NH3

Ca3P2 + 6H2O —-> 3Ca(OH)2 + 2PH3

PCl3 + 3H2O —-> H3PO3 + 3HCl

3. Complex Hydrides

These are the hydrides of III A being themselves unstable combines with IA hydrides to form complex hydrides.

Preparation

LiH + AlH3 —-> LiAlH4

NaH + BH3 —-> NaBH4

Properties

They are soluble in water and give H2

NaAlH4 —-> Al(OH)3 + NaOH + 4H2

4. Metallic Hydrides

These are the hydrides of transition metals. In these hydrides, H2 occupies interestitial spaces (spaces between the atoms) hence, they are also called Interestitial Hydrides.

Properties of these hydrides are those of the pure metals therefore they are also called Metallic Hydrides.

On strongly heating, they release hydrogen in atomic state. So they are used as catalyst in Hydrogenation reaction.

5. Polymeric Hydrides

These are the hydrides of Be and Mg of IIA which polymerize through hydrogen bonding forming polymers hence they are represented by the formula (BeH2)n and (MgH2)n.

Properties

These properties are intermediate between ionic and covalent hydrides.

6. Broder Line Hydride

These are the hydrides of metals of IB, and few metals of IIIA.

Properties

Their properties are intermediate between metallic and covalent hydrides.

Isotopes of Hydrogen

Atoms of same element having same atomic number but different mass number are called Isotopes. They have same number of protons but different number of neutrons. They have same chemical but different physical properties.

Menzel in 1931 reported that neutral H2 consist of two isotopes, Protium and Deutrium. The third is Tritium which can be obtained by the Artificial Atomic Transmutation.

Protium (IHI or H)

• It is ordinary Hydrogen
• It is most abundant, about 99.98%.
• Atomic Number = 1
• Contians 1 electron, 1 proton and no neutron.

Deutrium (IH2 or D)

• It is also called Heavy Hydrogen
• It occurs about 0.015%.
• Atomic Number = 1, Mass Number = 2
• Contains 1 electron, 1 proton and 1 neutron.

Tritium (IH3 or T)

• It is the heaviest isotope of hydrogen.
• It occurs about 4×10(-15)%.
• It is radioactive. Its half life is 12.5 years.
• Atomic Number = 1, Mass Number = 3.
• Contains 1 electron, 1 proton and 2 neutrons.

Behaviour of H+ and H- with Water

H+ and H- are unstable and don’t exist as such in a chemical reaction. H+ exist in water in hydrated from as H904+.

H+ + 4H2O —-> H9O4+

H- ion reacts with water releasing H2 gas.

H- + H2O —-> H2 + OH-

Water Has High Boiling Point

Reason

This is because due to high electronegativity of oxygen atom, water is a highly polar molecule. Due to high polarity strong intermolecular hydrogen bonding develops among water molecule.