Hydrogen

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Hydrogen is an element with the symbol H and the atomic number 1. It is a colorless, light gas at room temperature, and with most hydrogen atoms consisting of a single proton and electron, they are the simplest possible atoms in the universe. Some atoms may have one or two neutrons in their nucleus, forming the isotopes deuterium and tritium.

Properties

Physical properties

Hydrogen is a very light gas, with an atomic mass of 1.00797 and a density of 0.08988 g/L. Balloons filled with hydrogen will readily rise. Producing liquid hydrogen is completely infeasible to the amateur, but it has one of the highest energy densities of all fuels per mass. Hydrogen normally exists as a diatomic gas, which has two spin isomers: orthohydrogen and parahydrogen.

Chemical properties

Hydrogen will form compounds with many different elements. Combustion with hydrogen produces water and a large amount of heat, light and sound. With fluorine, the corresponding redox reaction occurs explosively when the gases contact each other, forming hydrogen fluoride. Hydrogen and chlorine will react explosively on exposure to ultraviolet light to form hydrogen chloride, which can be dissolved in water to form hydrochloric acid. Bromine and iodine need sufficient activation energy to form their respective hydrogen halides. Some metals will react with hydrogen to form hydrides. Others alloy with the gas - palladium is notable for being able to absorb 900 times its weight in hydrogen.

With the proper catalyst (palladium, platinum, or Raney nickel) alkene and alkyne functionality can be converted to alkane functionality. With Lindlar's catalyst, a poisoned variant, alkynes can be reduced only to alkenes.

Availability

Hydrogen is available as compressed gas in cylinders, though it's availability varies.

An important thing to remember is that hydrogen must NEVER be stored in other common gas cylinders, such as propane tanks. The presence of oxygen in the cylinder poses a risk when liquifying the gas inside. Hydrogen liquifies at 800 atm, a pressure difficult to achieve by an amateur chemist. Lastly, hydrogen will also cause embrittlement in many types of steels, such as high-strength and low-alloy steels, as well as titanium and nickel alloys.

Preparation

Hydrogen gas can be liberated by dissolving any sufficiently electropositive metal in a non-oxidizing acid, such as hydrochloric acid or dilute sulfuric acid. Normally, aluminium, magnesium or zinc are used for this process. The acid can also be replaced with a strong base, such as sodium hydroxide, but this will only work with amphoteric metals that can form hydroxometalate salts with sodium, such as aluminium and zinc; purely basic metals such as magnesium will not work. All these reactions produce large amounts of heat and may pose a fire or explosion hazard.

Another way to produce hydrogen is to electrolyze water with a small amount of electrolyte. Hydrogen gas is produced at the anode and oxygen is produced at the cathode.

Projects

  • Hydrogen balloons
  • Metal hydrides
  • Reduction of organic compounds
  • Ammonia synthesis

Handling

Safety

Hydrogen poses an asphyxiant hazard at high concentrations, in closed environments. When mixed with air, it poses a great explosive hazard in concentrations from 5% to 95% by volume.

Storage

Compressed and cryogenic hydrogen should only be stored in cylinders made of metal that is not susceptible to embrittlement. The cylinders should be checked from time to time for signs of corrosion and to make sure the valves work properly. They must also be stored in dark cold places, away from any heat source, and if possible in a semi-open space to prevent a possible build-up.

Due to the very small size of diatomic hydrogen, hydrogen can easily diffuse out of many materials such as latex. Thus, filling a balloon with hydrogen should be done only when you plan to use the balloon.

Disposal

Hydrogen can be safely released in open air. Ignition of the hydrogen may be preferable to prevent an unexpected explosion from occurring. This however, should never be done in a closed environment, as even a small amount of hydrogen in the air can cause a fire or explosion. Continuous venting is highly recommended.

References

Relevant Sciencemadness threads