Page 105 - Physical chemistry understanding our chemical world

72 INTRODUCING INTERACTIONS AND BONDS

Why does lightning conduct through air?

Electron affinity

Lightning is one of the more impressive manifestations of the power in nature: the
sky lights up with a brilliant flash of light, as huge amounts of electrical energy pass
through the air.
As an excellent generalization, gases may be thought of as electrical insulators, so
why do we see the lightning travel through the air? How does it conduct? Applying a
huge voltage across a sample of gas generates an electric discharge, which is apparent
by the appearance of light. In fact, the colour of the light depends on the nature of the
gas, so neon gives a red colour, krypton gives a green colour and helium is invisible
to the eye, but emits ultraviolet light.
The source of the light seen with an electric discharge is the plasma formed by
the electricity, which is a mixture of ions and electrons, and unionized atoms. If,
for example, we look solely at nitrogen, which represents 78 per cent of the air, an
+
+
−
electric discharge would form a plasma comprising N 2 ,N , electrons e , nitrogen
radicals N , as well as unreacted N 2 . Incidentally, the formation of these ions explains
•
how air may conduct electricity.
Very soon after the electric discharge, most of the electrons and nitrogen cations
reassemble to form uncharged nitrogen, N 2 . The recombination produces so much
energy that we see it as visible light – lightning. Some of the electrons combine with
−
nitrogen atoms to form nitrogen anions N , via the reaction
− −
N (g) + e (g) −−→ N (g) (2.10)

+
and, finally, some, N 2 cations react with water or oxygen in the
Care:do not con- air to form ammonium or hydroxylamine species.
fuse the symbols for
The energy exchanged during the reaction (in Equation (2.10))
electron affinity E (ea) is called the ‘electron affinity’ E (ea) . This energy (also called the
and activation energy
E a from kinetics (see electron attachment energy) is defined as the change in the inter-
Chapter 8). nal energy that occurs when 1 mol of atoms in the gas phase are
converted by electron attachment to form 1 mol of gaseous ions.
The negative ions formed in Equation (2.10) are called anions.
The negative value of Most elements are sufficiently electronegative that their electron
E (ea) illustrates how the affinities are negative, implying that energy is given out during
energy of the species the electron attachment. For example, the first electron affinity
−1
X (g) is lower than that of nitrogen is only 7 kJ mol , but for chlorine E (ea) =
−
of its precursor, X (g) . −364 kJ mol . Table 2.9 lists the electron affinities of gaseous
−1
halogens, and Figure 2.16 depicts the electron affinities for the first
20 elements (hydrogen–calcium).
The electron affinity measures the attractive force between the incoming electron
and the nucleus: the stronger the attraction, the more energy there is released. The
factors that affect this attraction are exactly the same as those relating to ionization