Question

When healthy, Earth’s stratosphere contains a low concentration of ozone (O₃)that absorbs potentially harmful ultraviolet (UV) radiation by the cycle shown at right.

Chlorofluorocarbon refrigerants, such as Freon 12 (CF₂Cl₂), are stable in the lower atmosphere, but in the stratosphere, they absorb high-energy UV radiation to generate chlorine radicals.

The presence of a small number of chlorine radicals appears to lower ozone concentrations dramatically. The following reactions are all known to be exothermic (except the one requiring light) and to have high-rate constants. Propose two mechanisms to explain how a small number of chlorine radicals can destroy large numbers of ozone molecules. Which of the two mechanisms is more likely when the concentration of chlorine atoms is very small?

Short answer

Mechanism 2 is more likely.

Step-by-step solution

Free radicals

An atom or group of atoms containing odd or unpaired electron is known as the free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

Steps involved in free radical chain reaction

In a free-radical chain reaction, free radicals are generally created in the initiation steps. A free radical and a reactant is combined to yield a product and another free radical in the propagation steps. Lastly, the number of free radicals generally decrease in the termination steps.

Mechanism

Both the mechanism explains how a small chlorine radicals can destroy large number of ozone molecules.

Mechanism 2 is more likely when the concentrations of chlorine atoms is very small.