Hydrogen at ambient condition is present as a molecular hydrogen gas, H₂. The gas is colourless, odourless and insoluble in water. (In science it is important to understand the negatives. It should not be taken to mean totally or absolutely negative, unless emphasised. Insoluble in water means "almost insoluble".)

It would require 436 kJ of energy (a lot of energy) to break one mole of sigma bond to give the hydrogen atom.

ΔH^θ is known as the standard enthalpy. That is enthalpy (or heat) put into the system at standard conditions (STP) to execute the reaction, with the compounds involved (both reactants and products) in their natural states. We will take up this topic at the appropriate time. So do not worry about it if you are not too sure.

LIGHT & FLAMMABLE

Hydrogen gas has the lowest density of all gases. As such its first application was in floating balloons. In the early 1990s the street vendors would fill balloons with hydrogen gas. The floating balloons were in great demand by children. In 1917 a German, Count F. von Zeppelin, saw the possibility of using it to travel from one place to another in the sky. He filled a giant balloon with hydrogen gas, and designed a small passenger compartment hanging below. So airships are also known as zeppelins in his honour.

However hydrogen burnt by itself easily (flammable). In 1937 the airship Hindenberg went up in flame and put an end to the zeppelin as a mean of transport. Now there is renew interest in this idea. Instead of hydrogen, helium (a much, much more expensive gas) was used. The current practice is to transport heavy goods by big container ships, as it cost ten times more to transport them by airplane.

The top speed for ships at the moment is 25 knots (28.8 miles per hour). Technology is not available to reduce the drag of the vessels by the water. In Berlin, CargoLifter is developing giant airships known as CL-160 to carry load of 160 tonnes at 50mph. The global market is around 30 million tonnes a year. To serve just 10% of this market will require 200 airships. CargoLifter hope to go into operation in 2003.

The flammable nature of hydrogen is also being looked at, as an alternative fuel for automobile. Hydrogen burns in air to give water vapour. So it will solve the serious pollution caused by cars. Another advantage is that hydrogen can be generated from water. So it is a renewable (and easily available) source of energy.

Hydrogen is unique in its position in the Periodic Table since it has only 1 electron. Chemistry is driven by the need for atoms to achieve stability. The most stable electron configuration for an atom is given by the noble gases. Hydrogen can do this by one of three means: give up the one electron and has no electron, take one electron or share an electron and attain the electron configuration of helium, a noble gas. So it is placed above lithium in some Periodic Table, and above helium in others. Some even place it in the middle of the Table.

Give up an electron, to become a proton, the smallest cation possible.

The amount of energy needed to remove the electron (ionization potential) is 1312 kJ mol‾�, which is considered very high. The only element that can do so is fluorine to give H⁺F‾.

However the proton can exist in solution if it is stabilised by the solvent molecules. It can easily attach itself to a rich source of electron, like the oxygen in water molecule. In this case it forms the hydronium ion. This gives an acid, a widely used group of chemicals. The most popular reaction for hydrogen.

Hydrogen gas is often used to remove oxides from metal. Most metals we mined from the ground are in the form of oxides. For example rust is actually iron oxide. If we passed hydrogen gas over it at high temperature, the hydrogen can react with the oxygen to give water, leaving behind the metal - iron, copper, aluminum, etc. The removal of oxygen from a compound is a classical reduction reaction. (More about reduction reaction )

HYDROGENATION

Hydrogen can also be made to react with a pi-bond of a molecule by adding itself to a π-bond or a δ-bond. For example:

This is known as hydrogenation. Long chain hydrocarbons having pi-bonds (the lay people's term would be unsaturated hydrocarbons) is generally a liquid at room temperature. Once hydrogenated (or "saturated" in lay people's term) they become greasy solids, like margarine. For health reasons many people prefer unsaturated oil rather than saturated fats for cooking. More

Tutorial 1

• What do you understand by the following terms? Bromination, chlorination, fluorination, hydrogenation, oxidation and sulphuration.
• Would will consider hydrogenation as a reduction reaction?    Answer

SUMMARY

In this lesson I just want to highlight;

• That hydrogen can give away an electron to form a (ionic) molecule.
• That hydrogen can take an electron to form a (ionic) molecule.
• That hydrogen can share an electron to form a (covalent) molecule.
• Just because it can form a molecule do not mean the molecule is stable at ambient condition. The hydrides are stable at ambient temperature and pressure, but not to moisture present in the air. So when we refer to stability, we must be aware that there are many aspects to stability. In this case the hydride is unstable to the water moisture in the air (it is reactive to the water moisture in the air).
• In chemistry, stability is generally referred to the condition at ambient condition unless otherwise stated. And "not stable" is synonymous to reactive.