Physical and chemical properties of oxygen and hydrogen

Some see hydrogen gas as the clean fuel of the future – generated from water and returning to water when it is oxidised. Hydrogen-powered fuel cells are increasingly being seen as ‘pollution-free’ sources of energy and are now being used in some buses and cars.

Hydrogen also has many other uses. In the chemical industry it is used to make ammonia for agricultural fertiliser (the Haber process) and cyclohexane and methanol, which are intermediates in the production of plastics and pharmaceuticals. It is also used to remove sulfur from fuels during the oil-refining process. Large quantities of hydrogen are used to hydrogenate oils to form fats, for example to make margarine.

In the glass industry hydrogen is used as a protective atmosphere for making flat glass sheets. In the electronics industry it is used as a flushing gas during the manufacture of silicon chips.

The low density of hydrogen made it a natural choice for one of its first practical uses – filling balloons and airships. However, it reacts vigorously with oxygen (to form water) and its future in filling airships ended when the Hindenburg airship caught fire.

Hydrogen is an essential element for life. It is present in water and in almost all the molecules in living things. However, hydrogen itself does not play a particularly active role. It remains bonded to carbon and oxygen atoms, while the chemistry of life takes place at the more active sites involving, for example, oxygen, nitrogen and phosphorus.

Hydrogen is easily the most abundant element in the universe. It is found in the sun and most of the stars, and the planet Jupiter is composed mostly of hydrogen.

On Earth, hydrogen is found in the greatest quantities as water. It is present as a gas in the atmosphere only in tiny amounts – less than 1 part per million by volume. Any hydrogen that does enter the atmosphere quickly escapes the Earth’s gravity into outer space.

Most hydrogen is produced by heating natural gas with steam to form syngas (a mixture of hydrogen and carbon monoxide). The syngas is separated to give hydrogen. Hydrogen can also be produced by the electrolysis of water.

History

History

In the early 1500s the alchemist Paracelsus noted that the bubbles given off when iron filings were added to sulfuric acid were flammable. In 1671 Robert Boyle made the same observation. Neither followed up their discovery of hydrogen, and so Henry Cavendish gets the credit. In 1766 he collected the bubbles and showed that they were different from other gases. He later showed that when hydrogen burns it forms water, thereby ending the belief that water was an element. The gas was given its name hydro-gen, meaning water-former, by Antoine Lavoisier.

In 1931, Harold Urey and his colleagues at Columbia University in the US detected a second, rarer, form of hydrogen. This has twice the mass of normal hydrogen, and they named it deuterium.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.
Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.
Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.
Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.
First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic data

Bond enthalpy (kJ mol −1 )
A measure of how much energy is needed to break all of the bonds of the same type in one mole of gaseous molecules.

Bond enthalpies

Bond enthalpies

Glossary
Common oxidation states The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.
Isotopes Atoms of the same element with different numbers of neutrons.
Key for isotopes

Oxidation states and isotopes

Oxidation states and isotopes

Glossary Data for this section been provided by the British Geological Survey .
Relative supply risk An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.
Crustal abundance (ppm) The number of atoms of the element per 1 million atoms of the Earth’s crust.
Recycling rate The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.
Substitutability The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.
Reserve distribution The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.
Political stability of top producer A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.
Political stability of top reserve holder A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.

Supply risk

Supply risk

Glossary

Specific heat capacity (J kg −1 K −1 )

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. It provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Pressure and temperature data – advanced

Podcasts

Podcasts

Chemistry in its element: hydrogen

You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

This week we hear what its like to be at the top, and number one, as we meet the King of the Elements. Here's Brian Clegg.

Forget 10 Downing Street or 1600 Pennsylvania Avenue, the most prestigious address in the universe is number one in the periodic table, hydrogen. In science, simplicity and beauty are often equated - and that makes hydrogen as beautiful as they come, a single proton and a lone electron making the most compact element in existence.

Hydrogen has been around since atoms first formed in the residue of the Big Bang, and is the most abundant element by far. Despite billions of years of countless stars fusing hydrogen into helium it still makes up 75 per cent of the detectable content of the universe.

This light, colourless, highly flammable gas carries on its uniqueness by having the only named isotopes (and some of the best known at that), deuterium with an added neutron in the nucleus and tritium with two neutrons.

Hydrogen is an essential for life, the universe and just about everything. Life, in fact, is multiply dependent on it. Without hydrogen we wouldn't have the Sun to give us heat and light. There would be no useful organic compounds to form the building blocks of life. And that most essential substance for life's existence, water, would not exist.

It's only thanks to a special trick of hydrogen's that we can use water at all. Hydrogen forms weak bonds between molecules, latching onto adjacent oxygen, nitrogen or fluorine atoms. It's these hydrogen bonds that give water many of its properties. If they didn't exist, the boiling point of water would be below -70 degrees Celsius. Liquid water would not feature on the Earth.

Hydrogen was the unwitting discovery of Paracelsus, the sixteenth century Swiss alchemist also known as Theophrastus Philippus Aureolus Bombastus von Hohenheim. He found that something flammable bubbled off metals that were dropped into strong acids, unaware of the chemical reaction that was forming metal salts and releasing hydrogen, something a number of others including Robert Boyle would independently discover over the years.

However, the first person to realize hydrogen was a unique substance, one he called 'inflammable air,' was Henry Cavendish, the noble ancestor of William Cavendish who later gave his name to what would become the world's most famous physics laboratory in Cambridge. Between the 1760s and 1780s, Henry not only isolated hydrogen, but found that when it burned it combined with oxygen (or 'dephlogisticated air' as it was called) to produce water. These clumsy terms were swept aside by French chemist Antoine Lavoisier who changed chemical naming for good, calling inflammable air 'hydrogen', the gene, or creator, of hydro, water.

Because hydrogen is so light, the pure element isn't commonly found on the Earth. It would just float away. The prime components of air, nitrogen and oxygen, are fourteen and sixteen times heavier, giving hydrogen dramatic buoyancy. This lightness of hydrogen made it a natural for one of its first practical uses - filling balloons. No balloon soars as well as a hydrogen balloon.

The first such aerial vessel was the creation of French scientist Jacques Charles in 1783, who was inspired by the Montgolfier brothers' hot air success a couple of months before to use hydrogen in a balloon of silk impregnated with rubber. Hydrogen seemed to have a guaranteed future in flying machines, reinforced by the invention of airships built on a rigid frame, called dirigibles in the UK but better known by their German nickname of Zeppelins, after their enthusiastic promoter Graf Ferdinand von Zeppelin.

These airships were soon the liners of the sky, carrying passengers safely and smoothly across the Atlantic. But despite the ultimate lightness of hydrogen it has another property that killed off airships - hydrogen is highly flammable. The destruction of the vast zeppelin the Hindenburg, probably by fire caused by static electricity, was seen on film by shocked audiences around the world. The hydrogen airship was doomed.

Yet hydrogen has remained a player in the field of transport because of the raw efficiency of its combustion. Many of NASA's rockets, including the second and third stages of the Apollo Program's Saturn V and the Space Shuttle main engines, are powered by burning liquid hydrogen with pure oxygen.

More recently still, hydrogen has been proposed as a replacement for fossil fuels in cars. Here it has the big advantage over petrol of burning to provide only water. No greenhouse gasses are emitted. The most likely way to employ hydrogen is not to burn it explosively, but to use it in a fuel cell, where an electrochemical reaction is used to produce electricity to power the vehicle.

Not everyone is convinced that hydrogen fuelled cars are the future, though. We would need a network of hydrogen fuel stations, and it remains a dangerous, explosive substance. At the same time, it is less efficient than petrol, because a litre of petrol has about three times more useful energy in it than a litre of liquid hydrogen (if you use compressed hydrogen gas that can go up to ten times more). The other problem is obtaining the hydrogen. It either comes from hydrocarbons, potentially leaving a residue of greenhouse gasses, or from electrolysing water, using electricity that may not be cleanly generated.

But even if we don't get hydrogen fuelled cars, hydrogen still has a future in a more dramatic energy source - nuclear fusion, the power source of the sun. Fusion power stations are tens of years away from being practical, but hold out the hope of clean, plentiful energy.

However we use hydrogen, though, we can't take away its prime position. It is numero uno, the ultimate, the king of the elements.

So it's the most abundant element, is essential for life on earth, fuels space rockets and could resolve our fossil fuel dependents. You can see why Brian Clegg classes hydrogen as number one. Now next week we meet the time keeper of the periodic table.

One current use is in atomic clocks, though rubidium is considered less accurate than caesium. The rubidium version of the atomic clock employs the transition between two hyperfine energy states of the rubidium-87 isotope. These clocks use microwave radiation which is tuned until it matches the hyperfine transition, at which point the interval between wave crests of the radiation can be used to calibrate time itself.

And to find out more of the roles of Rubidium join Tom Bond on next week's Chemistry in its Element. Until then I'm Meera Senthilingam, thanks for listening and goodbye.

Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists.com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld.org/elements.

Video

Video

Resources

Resources

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