BioPHYSICAL chemistry

molecule. Nitrogen molecules contain a
triple bond and, in the gas phase, N 2 does
not easily accept or donate electrons. For
nitrogen, the triple bond is the most stable,
with a bond energy of 225 kcal mol−^1 com-
pared with 100 and 40 kcal mol−^1 for the
double and single bonds, respectively. The significant greater strength of
the triple bond in N 2 is more pronounced in nitrogen compared with other
gases; for example the triple-bond energy is less than 3-fold greater than
the single bond for oxygen. This stability of the triple bond for N 2 can be
seen in an examination of the enthalpies of formation for the different
states formed during the reaction:

N 2 +H 2 →N 2 H 2 ΔH°=+50.9 kcal mol−^1

N 2 H 2 +H 2 →N 2 H 4 ΔH°=−27.2 kcal mol−^1 (3.38)

N 2 H 4 +H 2 →2NH 3 ΔH°=+50.9 kcal mol−^1

The difficulties overcoming the triple bond and creating abundant amounts
of ammonia at atmospheric conditions and ambient temperature have been

Figure 3.11Representation of the nitrogen cycle.

CHAPTER 3 SECOND LAW OF THERMODYNAMICS 67

Nitrogen in

atmosphere (N 2 )

Decomposers

(aerobic and anaerobic

bacteria and fungi)

Plants

Nitrates (NO 3 )

Ammonium (NH 4 ) Nitrites (NO 2 )

Ammonification Nitrification

Nitrifying

bacteria

Nitrogen-fixing

soil bacteria

Nitrogen-fixing

bacteria in root

nodules of

legumes

Assimilation

Nitrifying

bacteria

Denitrifying

bacteria

Figure 3.12The use

of a metal surface

facilitates the

disruption of the

bonds in nitrogen

molecules.