Page 204 - Instant notes

Physical chemistry 190

high pressure, when the surface is saturated (completely covered) and θ=1 (i.e.
),
rate of reaction=k 2

and the reaction is zero order; there is no dependence of rate on concentration of NH 3.
At low pressure, when surface coverage θ is small (i.e. ),

and the reaction is first order in NH 3.

Photochemical rate laws

A photochemical reaction is one initiated by absorption of one or more photons of
electromagnetic radiation. Obvious examples are reactions initiated by solar radiation
such as the absorption of red and blue light by molecules of chlorophyll leading to
production of carbohydrates through photosynthesis, or the absorption of ultraviolet light
by molecules of oxygen in the upper atmosphere to produce Earth’s protecting ozone
layer (see Topic I7).
The rate of a photochemical elementary reaction is directly proportional to the
concentration of absorbing species and is therefore described by first order kinetics. The
constant of proportionality is called the photochemical rate constant, and usually given
the symbol J. Using the photodissociation of O 2 into two O atoms as an example:

the rate of removal of O 2 is:

and the half-life for O 2 removal is, t 1/2=ln2/J.
The magnitude of the photochemical rate constant is a function of the intensity of the
incident light causing the photochemical process and the intrinsic ability of the molecule
to absorb photons (known as the absorption coefficient) for all appropriate wavelengths
of incident light. Therefore the value of a solar photochemical rate constant varies with
time of day, latitude and season, etc, because the intensity of solar radiation varies with
these parameters. But for any particular set of irradiation conditions the constant J may
be treated analogously to first order thermal rate constants, k.
The quantum yield, Φ, of a photochemical reaction is equal to the ratio of the number
of molecules or radicals of the product under consideration to the number of photons
absorbed: