Concise Physical Chemistry

c10 JWBS043-Rogers September 13, 2010 11:26 Printer Name: Yet to Come

156 CHEMICAL KINETICS

Ozone is depleted by about 4% per decade in its total volume in the Earth’s

stratosphere and to a much larger extent over Earth’s polar regions (the ozone hole).

The most important process is catalytic destruction of ozone by atomic halogens due

to photodissociation of chlorofluorocarbon compounds (commonly called freons)

and related compounds. Since the ozone layer prevents most harmful wavelengths

(270–315 nm) of ultraviolet light from passing through the Earth’s atmosphere, de-

creases in ozone have generated concern and led to banning the production of ozone

depleting chemicals. It is suspected that a variety of biological consequences such

as increases in skin cancer, damage to plants, and reduction of plankton populations

in the ocean’s photic zone may result from the increased UV exposure due to ozone

depletion (abstracted from Wikipedia).

10.6 THE INFLUENCE OF TEMPERATURE ON RATE

Variation of reaction rate with temperature is usually exponential, as you know if you

have ever been tempted to heat a reaction mixture “just a little bit more.” Observed

exponential rate curves tempt us to write equations of the same form as the van’t Hoff

equation to describe the rate constantk

lnk=−

aH

R

(

1

T

)

+const

The constantaHis theenthalpy of activation, often called the “energy” of activation.

It can be found empirically.

The activation process can be visualized by pushing over a block of wood, as in

Fig. 10.3. Positioncis more stable thana. Thermodynamically,ccorresponds to

the products in a spontaneous reaction accompanied by a decrease in free energy.

Even though the block in positionahas a higher energy thanc,adoes not go toc

spontaneously unless a force is applied to push it over, as inb. This is theactivation

process. The center of gravity ofbis higher than eitheraorc, hence it has higher

energy. Some energy—the push—must be put into the systema, and it must be

driven to a less stable configurationbbefore it spontaneously goes to the most stable

configurationc. The energy liberated in going frombtocis greater than the energy

put into the push, and the process liberates energy.

ab c

FIGURE 10.3 An activation energy barrier between an unstable position and a stable

position.