Hydrogen comes from Greek meaning “water producer” (“hydro” =water and “gennao”=to make). First isolated and identified as an element by Cavendish in 1766, hydrogen was believed to be many different things. Cavendish himself thought that it was “inflammable air from metals”, owing to its production by the action of acids on metals. Before that, Robert Boyle and Paracelsus both used reactions of iron and acids to produce hydrogen gas and Antoine Lavoisier gave hydrogen its name because it produced water when ignited in air. Others thought it was pure phlogiston because of its flammability. Hydrogen is among the ten most abundant elements on the planet, but very little is found in elemental form due to its low density and reactivity. Much of the terrestrial hydrogen is locked up in water molecules and organic compounds like hydrocarbons.
Naturally Occurring Hydrogen
Hydrogen is the fuel for reactions of the Sun and other stars (fusion reactions). Hydrogen is the lightest and most abundant element in the universe. About 70%- 75% of the universe is composed of hydrogen by mass. All stars are essentially large masses of hydrogen gas that produce enormous amounts of energy through the fusion of hydrogen atoms at their dense cores. In smaller stars, hydrogen atoms collided and fused to form helium and other light elements like nitrogen and carbon(essential for life). In the larger stars, fusion produces the lighter and heavier elements like calcium, oxygen, and silicon.
On Earth, hydrogen is mostly found in association with oxygen; its most abundant form being water (H2O). Hydrogen is only .9% by mass and 15% by volume abundant on the earth, despite water covering about 70% of the planet. Because hydrogen is so light, there is only 0.5 ppm (parts per million) in the atmosphere, which is a good thing considering it is EXTREMELY flammable.
Other Sources of Hydrogen
Hydrogen gas can be prepared by reacting a dilute strong acid like hydrochloric acids with an active metal. The metal becomes oxides, while the H+ (from the acid) is reduced to hydrogen gas. This method is only practical for producing small amounts of hydrogen in the lab, but is much too costly for industrial production:
Zn (s) +2H + (aq) →Zn 2+ (aq) +H 2(g)
The purest form of H2(g) can come from electrolysis of H2O(l), the most common hydrogen compound on this plant. This method is also not commercially viable because it requires a significant amount of energy (ΔH=572kJ ):
2H 2 O (l) →2H 2(g) +O 2(g)
\(H_2O\) is the most abundant form of hydrogen on the planet, so it seems logical to try to extract hydrogen from water without electrolysis of water. To do so, we must reduce the hydrogen with +1 oxidation state to hydrogen with 0 oxidation state (in hydrogen gas). Three commonly used reducing agents are carbon (in coke or coal), carbon monoxide, and methane. These react with water vapor form H2(g):
C (s) +2H 2 O (g) →CO(g)+H 2(g)
CO (g) +2H 2 O (g) →CO2+H 2(g)
Reforming of Methane:
CH 4(g) +H 2 O (g) →CO(g)+3H 2(g)
These three methods are most industrially feasible (cost effective) methods of producing H2(g).
There are two important isotopes of hydrogen. Deuterium (2H) has an abundance of 0.015% of terrestrial hydrogen and the nucleus of the isotope contains one neutron.
Three Hydrogen Isotopes
Protium (1H) is the most common isotope, consisting of 99.98% of naturally occurring hydrogen. It is a nucleus containing a single proton.
Deuterium (2H) is another an isotope containing a proton and neutron, consisting of only 0.0156% of the naturally occurring hydrogen. Commonly indicated with symbol D and sometimes called heavy hydrogen, deuterium is separated by the fractional distillation of liquid hydrogen but it can also be produced by the prolonged electrolysis of ordinary water. Approximately 100,000 gallons of water will produce a single gallon of D2O, “heavy water”. This special kind of water has a higher density, melting point, and boiling point than regular water and used as a moderator in some fission power reactors. Deuterium fuel is used in experimental fusion reactors. Replacing protium with deuterium has important uses for exploring reaction mechanisms via the kinetic isotope effect.
Tritium (3H) contains two neutrons in its nucleus and is radioactive with a 12.3-year half-life, which is continuously formed in the upper atmosphere due to cosmic rays. It is can also be made in a lab from Lithium-6 in a nuclear reactor. Tritium is also used in hydrogen bombs. It is very rare (about 1 in every 1,018 atoms) and is formed in the environment by cosmic ray bombardment. Most tritium is manufactured by bombarding Li with neutrons. Tritium is used in thermonuclear weapons and experimental fusion reactors
Hydrogen is prepared in the laboratory:
- by the action of a dilute strong acid on metals, such as zinc:
Zn + H2SO4 → ZnSO4 + H2;
- by reaction amphoteryc metals with a strong base, such as sodium hydroxide:
2Al + 6NaOH + 6H2O → 2Na3[Al(OH)6]3 + 3H2,
Zn + 2NaOH + 2H2O → Na2[Zn(OH)4] + H2;
- by electrolysis of water:
Industrially, hydrogen is prepared from water and hydrocarbons. Until recently the water-gas reaction was an important way of hydrogen preparing. The water-gas reaction is an industrial process in which steam is passed over red-hot coke giving a gaseous mixture of carbon monoxide and hydrogen:
C + H2O(g)→ CO + H2.
Now two general processes are used, both starting with natural gas or petroleum hydrocarbons. The steam-reforming process is an industrial preparation of hydrogen and carbon monoxide mixtures by the reaction of steam and hydrocarbons at high temperature and pressure over a nickel catalyst. For example,
CH4 + H2O(g) → CO + 3H2
C2H8 + 3H2O(g) → 3CO + 7H2 .
The second process involves the partial oxidation of hydrocarbons. Natural gas, for example, is mixed with a limited supply of oxygen and burned at elevated pressures:
2CH4 + O2→ 2CO + 4H2.
As natural gas and petroleum become more expensive, the water-gas reaction may become widely used again.
The gaseous mixture of carbon monoxide and hydrogen from these reactions (called synthesis gas) is used in the preparation of methanol. Hydrogen is obtained from this mixture free of carbon monoxide by means of the water-gas shift reaction,an industrial process in which carbon monoxide reacts with steam in the presence of a catalyst producing carbon dioxide and hydrogen:
CO + H2O → CO2 + H2.
Carbon dioxide is removed by dissolving the gas in a basic solution to give carbonate ion.
- Hydrogen reacts with many nonmetals. In these reactions it derives hydrogen-cation, H+. Hydrogen combines with nitrogen in the presence of a catalyst forming ammonia:
3H2 + N2→ 2NH3;
with sulfur forming hydrogen sulfide:
H2 + S → H2S;
with chlorine forming hydrogen chloride:
H2 + Cl2→ 2HCl;
and with oxygen forming water:
2H2 + O2→ 2H2O.
The reaction of oxygen and hydrogen takes place at room temperature in the presence of a catalyst such as finely divided platinum only. When hydrogen is mixed with the air or oxygen and ignited, the mixture explodes.
- Hydrogen also combines with some metals. Because of the small electron affinity of the hydrogen atom (73 kJ/rnol, compared with 349 kJ/mol for Cl), ionic compounds containing the hydride ion, H–, are formed with metals of the lowest ionization energy only: the alkali metals and the alkaline earth metals. Thus, sodium and calcium react with hydrogen gas at moderate temperatures giving the hydrides:
2Na + H2→ 2NaH;
Ca + H2→ CaH2.
These ionic hydrides conduct electricity when molten, indicating the presence of ions. Hydrogen liberates at the electrode connected to the positive terminal of the battery, according to the electrode reaction:
2H–→ H2 + 2e–.
Ionic hydrides react with water, giving hydrogen:
NaH + H2O → NaOH + H2;
CaH2 + 2H2O→ Ca(OH)2 + 2H2.
According to the last reaction, calcium hydride is a convenient, portable source of small quantities of hydrogen.
- It acts as a reducing agent on metallic oxides, such as copper oxide, removing the oxygen and leaving the metal in a free state:
CuO + H2→ Cu + H2O.
- Hydrogen reacts with unsaturated organic compounds forming corresponding saturated compounds:
CH2=CH2 + H2→ CH3–CH3 .
General Test for Hydrogen
1) Hydrogen gas, H2(g) has no colour or smell.
2) Hydrogen gas has no effect on moistlitmus paper
or moist universal indicator paper – it is neutral.
3) Hydrogen gas burns with a characteristic ‘pop’.
Specific Test for Hydrogen Gas?
Hydrogen gas is recognised by the ‘pop’ when it burns.
The ‘pop’ is the sound of a small explosion.
Hydrogen gas is highly flammable!
hydrogen + oxygen water (hydrogen oxide).
2H2(g) + O2(g) 2H2O(l)
This highly exothermic reaction is an example of oxidation.
Collection of Hydrogen
Hydrogen gas is less dense than air and can be collected
by upward delivery, over water or by using a gas syringe
Uses & Application
The vast majority of hydrogen produced industrially today is made either from treatment of methane gas with steam or in the production of “water gas” from the reaction of coal with steam. Most of this hydrogen is used in the Haber process to manufacture ammonia.
Hydrogen is also used for hydrogenation vegetable oils, turning them into margarine and shortening, and some is used for liquid rocket fuel. Liquid hydrogen (combined with liquid oxygen) is a major component of rocket fuel (as mentioned above combination of hydrogen and oxygen relapses a huge amount of energy). Because hydrogen is a good reducing agent, it is used to produce metals like iron, copper, nickel, and cobalt from their ores.
Because one cubic feet of hydrogen can lift about 0.07 lbs, hydrogen lifted airships or Zeppelins became very common in the early 1900s.However, the use of hydrogen for this purpose was largely discontinued around World War II after the explosion of The Hindenburg; this prompted greater use of inert helium, rather than flammable hydrogen for air travel.
Recently, due to the fear of fossil fuels running out, extensive research is being done on hydrogen as a source of energy.Because of their moderately high energy densities liquid hydrogen and compressed hydrogen gas are possible fuels for the future.A huge advantage in using them is that their combustion only produces water (it burns “clean”). However, it is very costly, and not economically feasible with current technology.
Combustion of fuel produces energy that can be converted into electrical energy when energy in the steam turns a turbine to drive a generator. However, this is not very efficient because a great deal of energy is lost as heat. The production of electricity using voltaic cell can yield more electricity (a form of usable energy). Voltaic cells that transform chemical energy in fuels (like H2 and CH4) are called fuel cells. These are not self-contained and so are not considered batteries. The hydrogen cell is a type of fuel cell involving the reaction between H2(g) with O2(g) to form liquid water; this cell is twice as efficient as the best internal combustion engine. In the cell (in basic conditions), the oxygen is reduced at the cathode, while the hydrogen is oxidized at the anode.
Reduction: O2(g)+2H2O(l)+4e– → 4OH–(aq)
Oxidation: H2(g) + 2OH–(aq) → 2H2O(l) + 2e-
Overall: 2H2(g) + O2(g) → 2H2O(l)
E°cell= Reduction- Oxidation= EO2/OH– – EH2O/H2 = 0.401V – (-0.828V) = +1.23
However, this technology is far from being used in everyday life due to its great costs.
Image of A Hydrogen Fuel Cell.
1.Using relevant chemical equations,state the three general methods of preparing Hydrogen in the Air.
- Mention 3 chemical properties of hydrogen giving balanced chemical equation.
- What are the general test for Hydrogen?
- What are the specific test for Hydrogen?
- How is Hydrogen collected in the Laboratory?
(post your answers in the forum for review)
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