Forms of Hydrogen and their uses

Introduction

Hydrogen is a colourless, odourless, tasteless, flammable gas. Hydrogen is indicated by H  and its atom has a nucleus consisting of a proton with one unit of positive electrical charge and an electron bearing 1 unit of negative electrical charge. Under the normal room temperature, hydrogen gas is a loose aggregation of hydrogen molecules, each consisting of a pair of atoms, a diatomic molecule( H2). The earliest known important chemical property of hydrogen is that it burns with oxygen to form water, H2O. The name hydrogen is derived from Greek terminologies meaning “maker of water”. In addition to the 3 isotopes, hydrogen can exist in a variety of forms. Some of the most common forms are Atomic hydrogen, Nascent hydrogen, Hydride forms, Para and ortho-hydrogen, and dihydrogen. This article is designed to explain the different forms of hydrogen and their uses.

1. Introduction

2. Atomic hydrogen

3. Nascent hydrogen( newly born hydrogen)

4. Para and ortho-hydrogen 

5. Hydrides

     5.1 Ionic hydrides

     5.2 Interstitial hydrides

     5.3 Covalent hydrides

6. Uses of Dihydrogen

 

Atomic hydrogen

The atomic hydrogen is mainly obtained through the dissociation of hydrogen molecules. It is highly reactive and can only be stable for less than one second. In the process of manufacturing atomic hydrogen, the gas is passed through an electric arc that is placed in between the 2 tungsten rods under atmospheric pressure. The purpose of the electric arcs is to maintain the temperature at 4000◦C-4500◦C. The molecules of dihydrogen gas then pass through the electric arc.

 

Nascent hydrogen( newly born hydrogen)

Nascent hydrogen is formed as a result of a mixture of a number of reactants with which hydrogen has to react.  Some scientists call it newly born hydrogen. Nascent hydrogen can be distinguished from the ordinary hydrogen by comparing their reactivity. Nascent hydrogen is more reactive than normal hydrogen gas.

 

Para and ortho-hydrogen 

There are two atoms( H2) present in a dihydrogen molecule. The nuclei of both the atoms in the individual molecule of dihydrogen are spinning. Ortho hydrogen is the molecule in which the spin of both nuclei is in the same direction while para-hydrogen refers to the hydrogen molecule in which the spins of both the nuclei are in the 2 opposite directions. The ratio of the parahydrogen and orthohydrogen can only be 1:1 if and only if the temperatures of air are liquefying. Furthermore, at STP( Standard Temperature and Pressure), the parahydrogen molecule is more stable than the orthohydrogen.  At RTP( Room Temperature and Pressure), the ratio of the parahydrogen to ortho hydrogen is always 1:3 and this ratio is maintained even if the temperature is increased.

 

Hydrides

 The hydrides of hydrogen are grouped into three main categories as explained below.

1. Ionic hydrides

Ionic hydrides are also known as saline hydrides. They are mainly formed from most of the s-block metals. Ionic hydrides are non-volatile and non-conducting crystalline solids. An exception is beryllium and magnesium hydrides as they have covalent polymeric structures. Indeed, these ionic hydrides have a rock-salt-like structure. Beryllium hydride, magnesium hydride, and lithium hydride have significant covalent character. i,e LiH> NaH > RbH > CsH. Hydrogen gas is liberated at the anode when the ionic hydrides are electrolyzed in a molten alkali halide solution. The reaction between ionic hydrides and water is explosive and should not be carried out in the laboratory. The formula for this is written as NaH (s) + H2O (l)     NaOH (aq) + H2 (g). Alkali metal hydrides are mainly used in cleansing water. They are also used in the manufacture of LiAlH4 and NaBH4.

2. Interstitial hydrides

Also known as metallic hydrides. Interstitial hydrides include elements of group 3,4,5(d-block elements) and the f-block elements. Chromium is the only element of group 6 that forms the hydride. On the other hand, group 7,8 and 9 metals don’t form hydrides. In the periodic table, there is a region between group 7 and group 9 which is referred to as the hydride gap. Examples of metal hydrides from group 3 to 5 includes; scandium hydride, yttrium hydride, hafnium hydride, zirconium hydride, titanium hydride, lanthanum hydride, vanadium hydride, and tantalum hydride. We should note that the f-block metals form hydrides of limiting the composition of MH2 and MH3. These hydrides are non-stoichiometric. The advantage of using these metal hydrides is that they are good conductors of electricity when in solid-state and they can also be used to store hydrogen.

3.Covalent hydrides

Covalent hydrides are also known as molecular hydrides. The covalent hydrides are formed when hydrogen reacts with p-block elements such as boron, carbon, nitrogen, oxygen, fluorine, silicon, phosphorus, chlorine, gallium, germanium, arsenic, antimony, bromine, indium, tin, tellurium, iodine, thallium, lead and astatine. Water, methane, and ammonia are the simple hydrides. As we know, stability in the periodic table increases with an increase in electronegativity. Ie. CH4< NH3< H2O < HF. The covalent hydrides are further sub-divided into 4  sub-categories, namely;

  1. Electron-rich molecular hydrides: This refers to the hydrides that contain one or more unpaired electrons around the central more electronegative element.

  2. Electron precise molecular hydrides: This refers to the hydrides in which the bond length increases down the group. A good example is the methane molecule.

  3. Electron-deficient molecular hydrides: This refers to the hydrides which contain the reduced number of electrons than the expected number in the Lewis structure. An example is diborane.

  4. Systematic molecular hydride: This refers to hydrides whose systematic names are derived from the name of the elements and the suffix-ane, ie. Phosphane, PH3.

 

Uses of Dihydrogen

  • Hydrogen gas is used as a primary raw material in the manufacture of ammonia. The equation for this process can be written as

             N2 (g) + 3H2(g)  2NH3(g)

  • Large volumes of hydrogen are used in the conversion of vegetable oils. This process is known as hydrogenation. In this process, oils retreated with hydrogen at high temperatures and pressure in the presence of a catalyst (nickel/platinum).

  • Liquid hydrogen is most useful as a rocket fuel because it is lightweight and yield high chemical energy.

  • Hydrogen is used in the balloons because of its low density.

  • Hydrogen is also used as a reducing agent in some of the chemical processes.

  • Hydrogen is used in the welding industry where the temperature can be as high as 2500◦C .

Quick question: Briefly explain how the atomic hydrogen oxy-hydrogen torch function for cutting and welding purposes?

Answer: Atomic hydrogen is mainly obtained by the dissociation of dihydrogen with the help of an electric arc. The huge amount of energy is released during the reaction, and it is used to generate a temperature of 4000K, which is ideal for welding and cutting metals. Hence, atomic hydrogen/oxy-hydrogen torches are used for these purposes.



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