GREEN CHEMISTRY

Chap. 2, The Elements: Basic Building Blocks of Green Chemicals 31
Although hydrogen atoms do exist very high in the rarefied upper atmosphere and as
transient species in chemical reactions, under normal conditions elemental hydrogen
exists as H 2 molecules. It is convenient to have a means of showing the electrons in
molecules. This is done with Lewis formulas (electron-dot formulas). The Lewis
formula of H 2 is shown in Figure 2.1.
H . . H
H 2 H:H
Elemental hydrogen
does not exist as
individual H atoms.
Instead, it exists as
molecules made up of
2H atoms with the
chemical formula H 2 .
The covalent bond holding
the two H atoms together
consists of 2 shared electrons
shown in the Lewis
formula of H 2 above.
Figure 2.1. Lewis formula for hydrogen molecules.
Properties and Uses of Elemental Hydrogen
Pure elemental H 2 under normal conditions is a colorless, odorless gas that has
the lowest density of any pure substance. Liquified H 2 boils at a very cold -253˚ C and
solidifies at -259˚ C. Hydrogen gas is widely used in the chemical industry to react
chemically with a large number of substances. It burns readily with a large release
of energy and mixtures of hydrogen with oxygen or air are extremely explosive. The
chemical reaction for elemental hydrogen burning with oxygen (O 2 ) in air is
2H + O 2 2 → 2H 2O + energy (2.2.1)
The product of this reaction is water. Used as a fuel, elemental hydrogen is a very green
element because when it is burned or otherwise reacted to provide energy, the reaction
product is simply water, H 2 O. Furthermore, given a source of electrical energy, elemental
hydrogen and elemental oxygen can be produced at two separate electrodes by passing a
direct current through water in which an appropriate salt has been dissolved to make the
water electrically conducting:
2H 2 O Electrical
current 2H 2 + O 2 (2.2.2)
So elemental hydrogen generated by the application of electrical energy to water provides
a source of energy that can be moved from one place to another and utilized to produce
electricity in fuel cells (see below) or for other beneficial purposes.
Elemental hydrogen is widely used for chemical synthesis and other industrial
applications. Its preparation by electrolysis of water was mentioned above. It is now
most commonly prepared from methane, CH 4 , that is the main ingredient of natural gas
by steam reforming at high temperatures and pressures: