KINETICS OF EARTH PROCESSES - Chemical Fundamentals of Geology and Environmental Geoscience - page 52

(a)

17

(b)

2.0

1.5

16

1.0

0.5

15

0.0

-0.5

14

2.5 3.0 3.5 4.0 4.5 5.0 3.10 3.15 3.20 3.25 3.30

1000 / T K -1 1000 / T K -1

Figure 3.5 (a) Arrhenius plot for reaction 3.1. The rate constant k is expressed in dm 3 mol −1 s −1 . (b) Arrhenius plot for the
photochemical oxidation of pyrite after here the rate k is expressed in units of μmol m −2 min. Note that temperature is
expressed in kelvins (K) in each case. (Sources: After Zhang (2008). © Princeton University Press, based on data of Borders &
Birks (1982); Schoonen et al . (2000). Reproduced with the authors' kind permission.)

of these oxygen free radicals with another oxygen
molecule generates an ozone (tri-oxygen) molecule, as
shown in reaction 3.9.

Ozone also forms photochemically in the lower trop-
osphere, but here the intensity of solar UV is much
lower, and ozone formation can only occur when catal-
ysed by nitrogen oxides arising from traffic pollution
(a factor in the infamous 'photochemical smog' that
afflicts many of the world's sunniest cities today, espe-
cially those at higher altitude like Mexico City).

OO O

• +→
2

(3.9)

3

Chemical reactions like 3.8 that are initiated by
energetic photons are described as photodissocia-
tion reactions. In kinetic terms they differ from con-
ventional reactions in that their rate constants depend
not on reactant temperature (as in Equation 3.5) but
on the UV photon flux to which the reactants are
exposed.
Solar UV radiation in a different wavelength range
stimulates a reverse reaction:

Diffusion

Any process that requires the input of thermal energy
to surmount an energy hurdle - a thermally activated
process - will show an Arrhenius-type temperature
dependence (Equation 3.5 and Figure 3.2). This prop-
erty is characteristic of a number of physical processes
in geology in addition to geochemical reactions.
When a component is unevenly distributed in a
phase - whether solid, liquid or gas - so that its con-
centration in one part is higher than in another, ran-
dom atomic motions tend to 'even out' the irregularities
over time, leading to a net migration, called diffusion ,
of the component 'down' the gradient to regions of
lower concentration. Given sufficient time, diffusion
leads eventually to a homogeneous distribution
(labeled t ∞ in Figure 3.6a). In fact, atoms diffuse

secondUVphoton

O + • → O O (3.10)

3 2

This is the key stratospheric reaction that strongly
attenuates the incoming UV flux in this wavelength
range and protects living things on the Earth's surface
from the most damaging effects of solar UV radiation.
This filter is highly efficient: we often refer to the
'ozone layer' protecting us, yet remarkably (in view of
the vital protection all living things on Earth derive
from it) the steady-state ozone concentration in the
stratosphere rarely exceeds 5 ppm .