Hydrogen is an odourless, colourless, tasteless, and flammable gaseous substance, which is one of the simplest elements on earth. A Hydrogen atom holds a nucleus containing a proton, a single unit of positive electrical charge; and an electron, a single unit of negative electrical charge, orbiting about the nucleus. Hydrogen gas is a loose aggregation of diatomic hydrogen molecules, H2, under ordinary conditions. The earliest known essential hydrogen chemical property is that it reacts with oxygen to form water, H₂O (hydrogen oxide).
Physical and Chemical Properties of Hydrogen
Let us look at some of the important physical and chemical properties of hydrogen.
Hydrogen has an atomic number of 1
It has an atomic weight of 1.0080
The ionization potential of hydrogen is given as 13.595 electron volts
Its electron affinity is 0.7542 electron volts
Its nuclear spin is ½
The nuclear magnetic moment (nuclear magnetons) of hydrogen is 2.7927
Its electronegativity (Pauling) is 2.1
The hydrogen contains a nuclear quadrupole moment as 0.
Hydrogen is completely transparent to infrared light, visible light, and ultraviolet light, to wavelengths below 1800 Å. It has a molecular weight lower than any other gas, and its molecules hold a velocity higher than any other gas at a given temperature, and it diffuses faster than that of any other gas. Consequently, kinetic energy can be distributed faster through hydrogen compared to any other gas, it has. It has the greatest heat conductivity. A hydrogen molecule is defined as the simplest possible molecule. It also consists of two electrons and two protons held together by electrostatic forces. Similar to atomic hydrogen, the assemblage exists in a number of energy levels.
Ortho-hydrogen and Para-hydrogen
There are two types of molecular hydrogen, that are ortho and para. These differ in the magnetic interactions of the protons because of the spinning motions of the protons. In ortho-hydrogen molecules, the spins of both protons are aligned in the same direction. It means they are parallel.
Whereas in para-hydrogen molecules, the spins are aligned in the opposite directions, and therefore, they lie antiparallel. The relationship of the spin alignments defines the atom's magnetic properties. In general, the transformations of one type into another (it means, conversions between the ortho and para molecules) do not take place, and the ortho-hydrogen and para-hydrogen are regarded as two hydrogen's distinct modifications. However, these two forms may interconvert under some particular conditions. Equilibrium between these two forms is established in many ways. One is by the introduction of catalysts (like activated charcoal or different paramagnetic substances); the other method is to heat it to a high temperature or to apply an electrical discharge to the gas.The para-hydrogen concentration in a mixture that has achieved equilibrium between these two forms depends on the temperature as represented by the below figure:
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Importantly, the pure para-hydrogen is formed by bringing the mixture into contact with charcoal at liquid hydrogen temperature, and this converts the entire ortho-hydrogen into the para-hydrogen. On the other hand, the ortho-hydrogen cannot be directly prepared from the mixture since the concentration of para-hydrogen is never below 25 per cent.
The two forms of hydrogen have slightly varied physical properties. The melting point of para-hydrogen is given as 0.10°, which is lower than that of a 3:1 mixture of ortho and para-hydrogen. At a temperature of −252.77°C, the pressure exerted by the vapour over the liquid para-hydrogen is given as 1.035 atmospheres (one atmosphere is given as the pressure of the atmosphere at a sea-level under standard conditions, which is equal to up to 14.69 pounds per square inch), compared to 1.000 atmospheres for the vapour pressure of the ortho–para mixture in the ratio 3:1. As a result of various vapour pressures of para-hydrogen and ortho-hydrogen, the forms of hydrogen are separated by low-temperature gas chromatography, which is an analytical process that separates various molecular and atomic species based on their differing volatilities.
Reactivity of Hydrogen
One hydrogen molecule dissociates into two atoms (H₂ → 2H) when the energy is either equal to or greater than the dissociation energy (it means, the amount of energy needed to break the bond that holds together the atoms present in the molecule) is supplied. The dissociation energy of the molecular hydrogen is given as 104,000 calories per mole, which can be written as 104 kcal/mole. For example, sufficient energy can be obtained, when the gas is brought into contact either with a white-hot tungsten filament or when an electric discharge is established in the gas.