Hydrogen is the first element on the modern periodic table. It has the simplest atomic structure as compared to all other elements. In atomic form, it has one proton and one electron. On the other hand, in elemental form, it exists as a diatomic (H2) molecule called dihydrogen. If Hydrogen loses its one electron, it will become an elementary particle, the proton.
This is what makes Hydrogen unique. Hydrogen is that the most abundant element within the universe. It is not common in the earth’s atmosphere in its free state. However, it is the third most abundant element on the earth’s surface when we consider Hydrogen in a combined state.
What is Dihydrogen?
Dihydrogen, as mentioned above, exists as the diatomic molecule H2 of Hydrogen. It is the primary element in the solar atmosphere and the most abundant in the universe, constituting 70% of the universe’s total mass.
Comparatively, it is much less abundant in the earth’s atmosphere (0.15% by mass) due to its light nature. Nevertheless, it occurs in combined form in several factors like water, plants and animal tissues, proteins, carbohydrates, hydrocarbons, and many other compounds.
Dihydrogen is industrially prepared by the water-gas shift reaction from petrochemicals. It is acquired as a by-product of the electrolysis of brine. Dihydrogen has a very high negative dissociation enthalpy, thus is it rather inactive at room temperature. Yet, it combines with almost all elements under appropriate conditions to form hydrides.
Uses of Dihydrogen
- The largest use of dihydrogen is in the synthesis of ammonia. It is used for the manufacturing of nitric acid and nitrogenous fertilizers.
- Dihydrogen is used for the manufacturing of vanaspati fat by the hydrogenation of polyunsaturated vegetable oils like cotton seeds, soyabean, etc.
- It is also used for manufacturing metal hydrides like LiH, AlH3, CaH2, etc.
- Dihydrogen is used for manufacturing bulk organic chemicals, particularly methanol.
CO (g) + 2H2 (g) ⇢ CH3OH (l) (in presence of Cobalt)
- It is also used for preparing Hydrogen Chloride (HCL). It is used for reducing heavy metal oxides to metals in metallurgical processes.
- Dihydrogen is used as rocket fuel in space research.
- It is used for generating electrical electricity in fuel cells. This has several advantages over conventional fossil fuels and electric power. It is pollutionless and releases larger energy per unit mass of fuel as compared to gasoline and other fuels.
Dihydrogen as a Fuel
Dihydrogen releases a large quantity of heat when it gets combusted. We can compare the energy released by the combustion of various fuels like methane, LPG, Dihydrogen, etc., in the terms of the same amounts in mole, mass and volume.
The Energy Released by Combustion of Various Fuels in Moles, Mass and Volume is mentioned in the below table as:
| The energy released on combustion (in kJ) | Dihydrogen (in a gaseous state) | Dihydrogen (in liquid state) | LPG | CH4 gas | Octane (in liquid state) |
|---|---|---|---|---|---|
| per mole | 286 | 285 | 2220 | 880 | 5511 |
| per gram | 143 | 142 | 50 | 53 | 47 |
| per litre | 12 | 9968 | 25590 | 35 | 34005 |
From the above table, it can be inferred that Dihydrogen releases more energy than petrol (about three times) on a mass for mass basis. Additionally, the combustion of dihydrogen will lead to less pollution than petrol. As dinitrogen is present as an impurity with dihydrogen, the only pollutants will be the oxides of dinitrogen.
This can be minimized by lowering the temperature of the cylinder so that reaction between dinitrogen and dioxygen doesn’t take place. This can be done by injecting a small amount of water into the cylinder. Although, while doing so, we should also consider the mass of the containers that hold the dihydrogen. If we consider the equal amounts of energy that would be produced by the respective fuels, a cylinder of compressed dihydrogen would weigh 30 times more than a tank of petrol.
Moreover, we would have to convert dihydrogen gas to the liquid state by cooling it down to 20K. We would require expensive insulated tanks for this conversion. Tanks of metal alloys like NaNi5, Ti-TiH2, Mg-MgH2, etc are used for storing dihydrogen in small quantities. Therefore, due to these disadvantages and limitations, we have developed alternate techniques to use dihydrogen efficiently.
Hydrogen Economy
Hydrogen Economy is an alternative technique to use dihydrogen efficiently. The basic principle of the Hydrogen Economy is the transporting and storing of energy in liquid form or gaseous dihydrogen. The primary advantage is that energy in this case is transmitted directly in the form of dihydrogen and not in the form of electric power.
Current Advancements
- In October 2005, for the first time in India, a pilot project using dihydrogen as the fuel was launched for running automobiles.
- In the beginning, only 5% of dihydrogen was mixed in CNG for use in four-wheeler automobiles. This percentage is being gradually increased to optimize the usage of dihydrogen as a fuel.
- Nowadays, it is also used in fuel cells for electric power generation. It is expected that economic and safer sources of dihydrogen will be identified in the years to come, for its usage as a common source of energy.
Sample Questions
Question 1: What is dihydrogen?
Answer:
In elemental form, Hydrogen exists as a diatomic (H2) molecule called dihydrogen. It occurs in combined form in several factors like water, plants and animal tissues, proteins, carbohydrates, hydrocarbons, and many other compounds. Dihydrogen is industrially prepared by the water-gas shift reaction from petrochemicals. It is acquired as a by-product of the electrolysis of brine. Dihydrogen has a very high negative dissociation enthalpy, thus is it rather inactive at room temperature. Yet, it combines with almost all elements under appropriate conditions to form hydrides.
Question 2: What is Hydrogen Economy?
Answer:
Hydrogen Economy is an alternative technique to use dihydrogen efficiently. The basic principle of the Hydrogen Economy is the transporting and storing of energy in liquid form or gaseous dihydrogen. The primary advantage is that energy in this case is transmitted directly in the form of dihydrogen and not in the form of electric power.
Question 3: Why is Hydrogen stored in alloys?
Answer:
We need to convert dihydrogen gas to the liquid state by cooling it down to 20K. For this conversion, we require expensive insulated tanks. Tanks of metal alloys like NaNi5, Ti-TiH2, Mg-MgH2, etc are used for storing dihydrogen in small quantities.
Moreover, alloys are used for the following reasons:
- Energy Conversion (as discussed above).
- Hydrogen Separation, recovery and purification.
- Isotope Separation.
- Catalysis.
- Alloy sensors and batteries.
- Storage and transportation of hydrogen.
Question 4: How is hydrogen produced on a large scale?
Answer:
Hydrogen is produced on a large scale by electrolysis of water or by thermochemical reaction cycle.
Question 5: Compare hydrogen and petrol as fuels.
Answer:
Dihydrogen releases more energy than petrol (about three times) on a mass for mass basis. Additionally, the combustion of dihydrogen will lead to less pollution than petrol. As dinitrogen is present as an impurity with dihydrogen, the only pollutants will be the oxides of dinitrogen. We should also consider the mass of the containers that hold the dihydrogen. If we consider the equal amounts of energy that would be produced by the respective fuels, a cylinder of compressed dihydrogen would weigh 30 times more than a tank of petrol.
Question 6: What are the problems that arise with dihydrogen as a fuel?
Answer:
There are several disadvantages and limitations of dihydrogen as a fuel due to which we have developed alternate techniques to use dihydrogen efficiently. One disadvantage is that the storage of dihydrogen becomes very difficult. We need to convert dihydrogen gas to the liquid state by cooling it down to 20K. For this conversion, we require expensive insulated tanks. Tanks of metal alloys like NaNi5, Ti-TiH2, Mg-MgH2, etc are used for storing dihydrogen in small quantities. This is overall not economical and very expensive.