Hydrogen is the most abundant element or chemical substance that is available in our universe. It makes up 90 per cent of all the atoms. It is the smallest and the lightest of all elements having atomic number 1 and atomic weight as 1.008. Hydrogen is mostly found in the form of a gas and is colourless, odourless but flammable.
Table of Content
- Position of Hydrogen in Periodic Table
- Properties Of Hydrogen
- Isotopes of Hydrogen
- Allotropes of Hydrogen
- Different Forms of Hydrogen
- Methods of Preparation of Hydrogen
- Chemical Reactions of Hydrogen
- Uses Of Hydrogen
Hydrogen even though it is present in large quantity is rarely found in its naturally free state. It is mostly found in chemical compounds where it is bonded to other elements. The most common example that we can take is water where hydrogen is bonded with the oxygen molecule. Interestingly, when hydrogen is burned it usually reacts with oxygen in the atmosphere to form water. This is also the primary reason why it was named “Hydrogen” which has been derived from the Greek word “hydro genes” meaning water creator / former.
Position of Hydrogen in Periodic Table
Hydrogen resembles I A group Alkali metals and VII-A group halogens. It is for this reason that it is placed on top of the periodic table.
Resemblance with I A Group (Alkali Metals)
1. Both the alkali metals and hydrogen have one electron in their valence shell.
H (Z=1) – K1
Li (Z=3) – K2, L1
Na (Z=11) – K2, L8, M1
K (Z= 19) – K2, L8, M8, N1
2. Hydrogen and elements of I A group shows same outer configuration ns1
H (Z=1) – 1s1
Li (Z=3) – 1s2, 2s1
Na (Z=11) – 1s2, 2s2, 2p6, 3s1
K (Z= 19) – 1s2, 2s2, 2p6, 3s2, 3p6, 4s1
3. Hydrogen also forms halides like alkali metals.
I-A: NaCl, KBr.
Hydrogen: HCl, HI.
4. During electrolysis, Hydrogen gas is liberated at Cathode.
5. Oxidation state of Alkali metals and hydrogen is +1.
6. Halides of hydrogen undergo ionization in aqueous solution, similar to alkali metal halids.
HCl(aq)→ H+(aq) + Cl–(aq)
NaCl(aq)→ Na+(aq) + Cl–(aq)
7. Hydrogen may also form non-metal compounds such as alkali metals.
Resemblance Of Hydrogen With Halogens
1. Both Hydrogen and Halogen are non-metals. (iodine is excluded because of its partial metallic character)
2. Hydrogen has a higher electronegativity (2.1).
3. It has high Ionisation potential.
4. It accepts one electron easily and forms hydride ion (H–) like a halide ion (X–).
5. Both Hydrogen and halogen forms compound with metals and non-metals.
Halogen: HCl (H is non-metal)
Hydrogen: H2S ( S is non-metal)
Thus, hydrogen resembles more with halogen than alkali metals. Hence the position of hydrogen is not justified in the periodic table.
Differences From Alkali Metals and Halogens
- It is less electropositive than alkali metals and less electronegative than halogens
- It contains only one proton (but no neutrons) in its nucleus and only one electron in the extranuclear part.
- It forms a neutral oxide.
- The sizes of H and H ions are much smaller than those of alkali metals.
- The hydrogen ion (H+) is unstable in water unlike Na+ and K+ ions.
Properties Of Hydrogen
Physical Properties Of Hydrogen
- Colourless, odourless, neutral gas.
- Less soluble in water.
- Highly inflammable.
- Burns with a blue flame.
- Very low boiling points.
Chemical Properties Of Hydrogen
- Dihydrogen is relatively inert at room temperature because of strong bond enthalpy of H–H bond.
- Atomic hydrogen is produced under a high electric arc.
- Its orbit is incomplete with a single electron.
- Hydrogen combines with almost every element.
Isotopes of Hydrogen
Hydrogen shows three isotopes:
It has zero neutrons
It has one neutrons
It has two neutrons.
Allotropes of Hydrogen
Molecular hydrogen occurs in two isomeric forms.
- Ortho-Hydrogen: In this type two proton nuclear spins are aligned parallel.
- Para-Hydrogen: In this type two proton nuclear spins are aligned antiparallel.
Different Forms of Hydrogen
It is produced when molecular hydrogen is passed through an electric arc (2273k)struck between tungsten electrodes. The reaction is endothermic and the heat is stored up in the atoms.
H2→ 2H- Heat
(Heat at 2273 K in an electric arc)
The liberated hydrogen atoms recombine to form H, with the evolution of a large amount of energy which can be used for welding purposes.
It is the hydrogen at the moment of its generation, i.e. when it is just liberated. Nascent hydrogen is much more reactive and is a much more powerful reducing agent than ordinary hydrogen.
Reason for the activity of nascent hydrogen:
- Atomic state (the atom is more reactive than molecules).
- Chemical energy liberated during reaction.
- High internal pressure due to nascent hydrogen atoms.
Methods of Preparation of Hydrogen
By the Electrolysis of Water
By the Action of Acids on Strong Electro Positive Metals
By the Action of Water on Strong Electro Positive Metals
By the Action of Alkali on Amphoteric Metals like Al, Zn etc
Large Scale Production of Hydrogen
Also Read: Preparation Method And Use Of Hydrogen
Chemical Reactions of Hydrogen
It reacts with dioxygen to form water. The reaction is highly exothermic.
With many metals, hydrogen reacts and combines at a high temperature to yield a corresponding hydride.
It reacts with halogens to give hydrogen halides.
With dinitrogen, it forms ammonia at 73k and 200 atm pressure
3H2(g) + N2(g) → 2NH3 ∆H= -92.6 kJ mol
This is the method for the manufacture of ammonia by the Haber process.
Nuclear Fusion Reactions
It takes place in stars.
Reactions with organic compounds:
It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance. For example,
- Hydrogenation of vegetable oils in the presence of nickel catalyst.
- Hydroformylation of olefins to give aldehyde then into alcohol.
Uses Of Hydrogen
- In the preparation of Ammonia.
- As a fuel in preparation of fuel gases like water gas.
- As a reducing agent in metallurgy
- In Nuclear fusion, a large amount of energy is released in stars.