Hydrogen Economy
As you are aware, the reserves of fossil fuels are limited. Therefore, attempts are being made to use alternate materials as sources of energy. One such material is dihydrogen.
Because of the wide use of hydrogen in the processing industries and for the hydrogenation of various oils and fats in the food industry, hydrogen has become much better understood during the past several decades.
However, it was only after the late 1960s that hydrogen could be thought of as a major energy source.
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However, to date, it has been used only to a limited extent due to its non-availability in the free state of nature. Also, putting pure hydrogen vehicles on the road would create another major problem—the safe storage of hydrogen.
Thus, to use hydrogen as a major fuel two problems need to be overcome—those related to production and safe storage. Though water is a potential source of dihydrogen, its use for the production of hydrogen is limited due to the cost involved in electrolysis.
The hydrogen economy involves the transportation and storage of energy in the liquid or gaseous form of hydrogen.
Compressing the gas needs energy and compressed hydrogen contains far less energy than the same volume of gasoline.
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The hydrogen economy aims to solve the problems associated with the transfer of energy in the form of hydrogen. One solution is the storage of hydrogen in a solid, sodium borohydride.
This compound is made from borax. After releasing hydrogen, sodium borohydride gives back borax, which can be recycled.
One more technology already in use involves the — production of large quantities of hydrogen using electricity and the storage of hydrogen in insulated — cryogenic tanks. This is used in space programs.
Once the storage problem is solved, hydrogen stations and pipelines have to develop. Small quantities of hydrogen can be stored using some metal alloys like Ti—TiH2 and Mg—MgH2.
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The present concept of a hydrogen fuel economy involves a primary energy source such as a nuclear reactor, a geothermal source or a solar-powered source with hydrogen being produced as a portable energy carrier.
Thermal energy would be used to generate electricity that would then be used to electrolyse water for the production of hydrogen and oxygen.
The hydrogen would be distributed by pipelines to distant points of use with storage provided underground in gaseous or liquid form.
Hydrogen Advantages
Using hydrogen as a fuel eliminates major problems created by using fossil fuels. The advantages include:
- Elimination of pollutants like NO2, CO, CO2 and SO2. The only by-product is water.
- If hydrogen is produced from the electrolysis of water then no greenhouse gases are added to the environment.
- Hydrogen can release more energy than petrol and other fuels like methane and butane.
- Hydrogen can be produced anywhere with electricity and water. There has been a major breakthrough in fuel-cell technology and fuel cells for regeneration of electric power are a success commercially.
- A fuel cell is a device that converts the chemical energy of a fuel directly into electricity. In batteries, the electrodes are the source of the active ingredients, in fuel cells the gas or liquid fuel (often hydrogen) is supplied continuously to one electrode and oxygen or air to the other from an external source. So as long as the fuel and the oxidant are supplied, the fuel cell will continue to function.
- The hydrogen economy will also help nations which depend on the Middle East for its oil reserves.
Active Forms Of Hydrogen
You already know that the diatomic hydrogen molecule is almost inert due to its high bond enthalpy. However, under appropriate conditions, hydrogen exists in two active forms.
Nascent Hydrogen
It is a reactive form of hydrogen produced in situ, which means it is generated and used in the reaction mixture itself.
Some elements and compounds which do not readily react with ordinary molecular hydrogen can be easily reduced by nascent hydrogen.
For example, when a current of ordinary molecular hydrogen is passed through acidified potassium permanganate there is no reaction.
However, if a piece of granulated zinc is added to the solution, the latter loses its pink colour.
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This is due to the reduction of potassium permanganate by nascent hydrogen, which is produced by the action of the acid on zinc.
⇒ \(\underset{\text { pink }}{2 \mathrm{KMnO}_4}+3 \mathrm{H}_2 \mathrm{SO}_4+\underset{\substack{\text { nascent } \\ \text { hydrogen }}}{10 \mathrm{H}} \longrightarrow \underset{\text { colourless }}{\mathrm{K}_2 \mathrm{SO}_4}+2 \mathrm{MnSO}_4+8 \mathrm{H}_2 \mathrm{O}\)
Studies have revealed that nascent hydrogen is probably the hydrogen molecule in the excited state, which is used up before it returns to the ground state.
Atomic Hydrogen
Atomic hydrogen is the hydrogen obtained by passing the ordinary molecular hydrogen through an electric arc at low pressure.
On coming in contact with the surface of a metal, atomic hydrogen forms molecular hydrogen with the liberation of a large amount of heat.
The heat produced is so much that the temperature rises to about 4200 K. This reaction is made use of in the atomic hydrogen torch, used for welding metals.
Hydrides
Binary compounds of elements with hydrogen are known as hydrides. The kind of hydride an element forms depends on its electronegativity. Hydrides can be of the following types.
- Ionic, or saltlike
- Covalent, or molecular
- Metallic, or interstitial
Ionic (Saltlike) Hydrides
Most of the alkali and alkaline earth metals form nonvolatile ionic, or saltlike hydrides. The hydrides of alkali metals have rock-salt structures.
The thermal stability of the hydrides decreases as the size of the metal cation increases from Li+ to Cs+ (as lattice energy decreases with an increase in size).
Ca, Sr and Ba form ionic hydrides at high temperatures. The order of stability of these hydrides is CaH2 > SrH2 > BaH2.
Ionic hydrides are solids with high melting points. However, hydrides of beryllium (BeHz) and magnesium (MgH2) have covalent polymeric structures.
In the molten or fused state, they can be electrolysed, giving hydrogen to the anode. This confirms the presence of H- ions.
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Ionic hydrides react explosively with water to liberate dihydrogen gas.
⇒ \(\mathrm{LiH}+\mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{LiOH}+\mathrm{H}_2\)
They are powerful reducing agents as illustrated by the following examples.
⇒ \(\begin{gathered}
\mathrm{NaH}+2 \mathrm{CO} \longrightarrow \mathrm{HCOONa}+\mathrm{C} \\
2 \mathrm{CaH}_2+\mathrm{PbSO}_4 \longrightarrow \mathrm{PbS}+2 \mathrm{Ca}(\mathrm{OH})_2
\end{gathered}\)
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Complex metal hydrides like LiAlH4 and NaBH4, which are extensively used as reducing agents in inorganic { as well as organic synthesis, can be obtained from NaH and LiH.
⇒ \(\begin{gathered}
4 \mathrm{LiH}+\mathrm{AlCl}_3 \longrightarrow \mathrm{LiAlH}_4+3 \mathrm{LiCl} \\
4 \mathrm{NaH}+\mathrm{B}\left(\mathrm{OCH}_3\right)_3 \longrightarrow \mathrm{NaBH}_4+3 \mathrm{NaOCH}_3
\end{gathered}\)
Covalent (molecular) Hydrides
They are hydrides of p-block elements and have the formula XH„ or XH8_„ where n is the group in the periodic table to which X belongs.
The molecules of these hydrides are held together only by weak van der Waals forces and are thus usually volatile.
They are obtained as follows.
1. Reaction of dihydrogen with certain nonmetals under appropriate conditions.
⇒ \(3 \mathrm{H}_2+\mathrm{N}_2 \underset{\mathrm{Fe}_2 \mathrm{O}_3}{\stackrel{700 \mathrm{~K}, 200 \mathrm{~atm}}{\longrightarrow}} 2 \mathrm{NH}_3 \text { (Haber process) }\)
⇒ \(\mathrm{H}_2+\mathrm{Cl}_2 \longrightarrow 2 \mathrm{HCl}\)
The method is used to prepare pure hydrogen chloride.
2. Reaction of a halide with LiAlH4 in a dry solvent
⇒ \(\mathrm{SiCl}_4+\mathrm{LiAlH}_4 \longrightarrow \mathrm{SiH}_4+\mathrm{AlCl}_3+\mathrm{LiCl}\)
⇒ \(4 \mathrm{BCl}_3+3 \mathrm{LiAlH}_4 \longrightarrow 2 \mathrm{~B}_2 \mathrm{H}_6+3 \mathrm{AlCl}_3+3 \mathrm{LiCl}\)
The names of the covalent, or molecular, hydrides are derived from the name of the element and the suffix ane. For instance, phosphane for PH3, oxidane for H2O and azane for NH3.
However, common names like phosphine, water and ammonia are retained and used more often than their systematic names.
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Covalent, or molecular, hydrides can be electron-deficient (e.g., diborane, B2H6), electron-precise (e.g., methane, CH4) and electron-rich (e.g., ammonia, NH3). All elements of Group 13 form electron-deficient hydrides whereas those of Group 15-17 form electron-rich hydrides.
The electron-rich hydrides have lone pair(s) of electrons. Owing to the presence of lone pairs in the electronegative atoms, these hydrides usually form hydrogen bonds.
The presence of hydrogen bonding in compounds like water, ammonia and hydrogen fluoride leads to a change in their physical properties.
Metallic (Nonstoichiometric) Hydrides
Such hydrides are obtained by the reaction of dihydrogen with many elements in the d block, and the lanthanides and actinides.
However, the elements in the middle of the d block do not form hydrides—this part of the block is referred to as the hydride gap.
The elements of Groups 7, 8 and 9 do not form hydrides and only chromium from Group 6 forms a hydride.
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Metallic hydrides are generally prepared by heating the corresponding metal along with hydrogen.
However, at higher temperatures, the hydrides decompose, and this phenomenon can be used to prepare very pure hydrogen.
When finely powdered metallic hydrides are heated, finely divided metals are obtained, and they can be used as catalysts in various reactions.
Metallic hydrides were originally called interstitial hydrides because it was thought that hydrogen occupied varying numbers of interstitial positions in the metal lattice, causing distortion.
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However recent experimental findings reveal that except for a few hydrides (e.g., that of Ni), the other hydrides have a lattice different from that of the parent metal.
Unlike ionic hydrides, these are nonstoichiometric compounds. (Nonstoichiometric compounds are those chemical compounds in which the elements do not combine in simple ratios.) For example, LaHi87, YbH 255, TiHlg ,ZrHJ19, VH16, NbHO7 and PdHO7
