Page 96 - Physical chemistry understanding our chemical world

CREATING FORMAL CHEMICAL BONDS 63

condenses to form liquid water; and the water is a liquid because of the formation of
intermolecular forces in the form of hydrogen bonds.

Why does land become more fertile after a
thunderstorm?

Breaking bonds requires an input of energy

A plant accumulates nutrients from the soil as it grows. Such accumulation depletes
the amount of nutrient remaining in the soil; so, harvesting an arable crop, such as
maize, barley or corn, removes nutrients from the field. A farmer
needs to replenish the nutrients continually if the land is not to The reaction of elemen-
become ‘exhausted’ after a few seasons. tal nitrogen to form
In the context here, ‘nutrients’ principally comprise compounds compounds that can
of nitrogen, most of which come from bacteria that employ natu- be readily metabolized
rally occurring catalysts (enzymes) which feed on elemental nitro- by a plant is termed
gen – a process known as fixing. An example is the bacterium ‘fixing’. All the principal
Rhizobium which lives on beans and peas. The bacteria convert means of fixing nitro-
atmospheric nitrogen into ammonia, which is subsequently avail- gen involve bacteria.
able for important biological molecules such as amino acids, pro-
teins, vitamins and nucleic acids.
Other than natural fixing, the principal sources of nutrients are the man-made fertil-
izers applied artificially by the farmer, the most common being inorganic ammonium
nitrate (NH 4 NO 3 ), which is unusually rich in nitrogen.
But lightning is also an efficient fertilizer. The mixture of gases
we breathe comprises nitrogen (78 per cent), oxygen (21 per cent) Notice the difference
and argon (1 per cent) as its principal components. The nitro- between the two words
gen atoms in the N 2 molecule are bound together tightly via a ‘princiPAL’ (meaning
triple bond, which is so strong that most reactions occurring dur- ‘best’, ‘top’ or ‘most
ing plant growth (photosynthesis) cannot cleave it: N 2 is inert. But important’) and ‘prin-
the incredible energies unleashed by atmospheric lightning are able ciPLE’ (meaning ‘idea’,
‘thought’ or ‘concept’).
to overcome the N≡N bond.
The actual mechanism by which the N≡N molecule cleaves is
very complicated, and is not fully understood yet. It is nevertheless
clear that much nitrogen is oxidized to form nitrous oxide, NO. This NO dissolves in
the water that inevitably accompanies lightning and forms water-soluble nitrous acid
HNO 2 , which further oxidizes during the storm to form nitric acid, HNO 3 . Nitric acid
functions as a high-quality fertilizer. It has been estimated that a thunderstorm can
yield many tonnes of fertilizer per acre of land.
To summarize, the N≡N bond in the nitrogen molecule is very
strong and cannot be cleaved unless a large amount of energy is We require energy to
cleave bonds: bond
available to overcome it. Whereas bacteria can fix nitrogen, the
energies are discussed
biological processes within crops, such as corn and maize, can-
on p. 114.
not provide sufficient energy. But the energy unleashed during a