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Some special topics in kinetics

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1 Some important catalytic processes

### Destruction of stratospheric ozone by chlorofluorocarbons

We usually think of catalysts as something we intentionally add in order to speed up a reaction, but atmospheric pollutants can lead to potentially disasterous catalytic processes. The best-known example of this is associated with the release of chlorofluorocarbons into the atmosphere. These substances (also known as Freons) have been widely used as refrigerants and aerosol propellants. Freons are released either during manufacture or handling, or as they gradually leak out of discarded refrigerators and air conditioners. Owing to their chemical inertness, these substances remain in the atmosphere for around 100 years, and some of them eventually drift up into the stratosphere.

Ozone, chlorofluorocarbons, and the stratosphere

The intense ultraviolet solar radiation that impinges on the uppermost region of the atmosphere (60-200 km above the earth's surface) is able to decompose virtually all of the atmospheric gas molecules that manage to diffuse to these heights. In doing so, much of this radiation, which would destroy life on the earth's surface, is absorbed. But a particularly intense component is able to penetrate down into the stratosphere (elevation 10–50 km) where it is absorbed by oxygen and dissociates it into O atoms. The latter react with intact O2 molecules to form ozone, O3. The net process can be summarized as

3/2 O2 → O3 ΔG°f = +163 kJ mol–1

Ozone itself is even a stronger absorber of uv light which causes it to dissociate back into O2 and O, so the concentration of ozone in the stratosphere is controlled by the balance between these two processes which proceed at different rates in different regions. Overall, however, stratospheric ozone acts as a filter which reduces the intensity of uv light at the earth's surface to biologically tolerant levels.

Beginning in the mid-1980s, it was noticed that stratospheric ozone in the antarctic regios was greatly reduced during the local winters, and smaller depletions were observed in the arctic as well. Further observations revealed that these periods of ozone deplection were accompanied by hightened concentrations of chlorine species. This fact drew attention to the suggestion made by Mario Molina and F.S. Rowland of UC-San Diego in 1974 of the potentially severe consequences Freon chlorofluorocarbon releases could have on the environment.

Once a chlorofluorocarbon molecule reaches the stratosphere, absorption of uv lighcan decompose it into free chlorine atoms:

CF2Cl2 → ·CF2Cl + Cl·

The atomic chlorine initiates a chain reaction which destroys an ozone molecule and produces a chain carrier ClO that regenerates the chlorine atom:

Cl· + O3 → ·ClO + O2

·ClO + O· → Cl· + O2

It is now believed that a key step in the process is the activation of chlorine species by adsorption onto ice crystals that form in the extremely cold stratospheric air during the polar night. This air tends to circulate in a pattern known as the polar vortex that tends to isolate it from surrounding regions. After the vortex breaks up in the summer, most of the ozone is restored, but partly at the expense of ozone from other regions of the stratosphere, leading to a worldwide depletion of stratospheric ozone and the prospect a greater incidence of skin cancer and damage to plant crops.

Molina and Rowland (along with Paul Crutzen) were awarded the 1995 Nobel Prize in Chemistry for their work. Woldwide fforts to phase out the manufacture and use of chloroflurocarbons have been underway for some time.

2 Flames

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3 Isotopic effects on reaction rates

Molecules that are chemically identical except for the presence of different isotopes of the same atom can react at different rates. The magnitude of this effect depends on

- the ratio of the isotopic masses
- whether the bond that is broken to form the activated complex or product involves the isotopic atom

How can this be? You may recall from elementary physics that the natural vibrational frequecny of two bodies joined by a spring depends on the masses of the bodies; the lighter they are, the higher is the natural vibrational frequency. It is much the same way with the vibrational stretching frequency of a chemical bond. But unlike macroscopic objects, the vibrations of chemical bonds are quantized, and as such, they possess so-called zero point energies. What this means is that even at very low temperatures, some vibrational motion is present. The lighter the bonded atoms, the greater this energy is.

Consider, for example, the dihydrogen molecules H2, HD, and HT, in which D represents deuterium 2H1 and T is tritium 3H1 (H is of course 1H1). The minimum vibrational energy each molecule can possess is highest for H and smallest for T. But the energy required to cleave the bond between the two atoms depends mainly on electronic factors that are not affected by isotopic mass, so as the diagram shows, the energy that must be added in order to break the pair of atoms is smallest for H–H and greatest for H–T.

The kinetic isotope effect forms the basis for an experimental way of determining if the breaking of a particular bond is the rate-determining step in a mechanism. By examining the effect of a deuterated solvent such as HDO, D2O or C2H5OD, one can establish the role of proton transfer involving the solvent in a mechanism.

In the case of dihydrogen, the dissociation energy is so much greater than these zero-point energies that the differences are of little practical importance. But for deuterium-substituted C–H or O–H bonds which are much weaker, the effect is much greater. A reaction in which one of these bonds breaks to form the transition state will be significantly slowed down if the H is replaced by D.

Because the magnitude of this effect, known as the kinetic isotope effect, depends on the relative masses of the isotopes, it is most apparent when H and D isotopes are compared.

4 Timeline of chemical dynamics
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1797 | The first recorded use of clay minerals as heterogeneous catalysts by Bondt et. al. who studied the dehydration of alcohols.
1835 | Swedish chemist Jöns Jacob Berzelius (1779-1848) introduced the term "catalyst" which he defined as "substances which by their mere presence evoke chemical reactions that would not otherwise take place." (He also coined the words "catalysis", "polymer", "isomer" and "allotrope", and named several chemical elements.
1877 | German physiologist Wilhelm Kühne (1837-1900) introduces the word "enzyme" in connection with his discovery of trypsin.
1907 | German chemist Eduard Buchner (1860-1917) receives Nobel Prize in Chemistry for showing that juice squeezed out of yeast cells can ferment sugars — thus finally overturning the old vitalistic idea that biological reactions can only take place within living cells.