Ozone is a naturally occuring component of the stratosphere 15 to 45 km above the Earth's surface. It is a powerful oxidising agent and pale blueish gas with an acrid odour. Ozone can have harmful effects on living matter but is also essential to life and health (ozone layer).
Ozone is formed from the photo-dissociation of molecular oxygen by uv light. Free radicals are formed from the splitting of oxgyen molecules into oxygen atoms by high energy, short wavelength UV light from the sun. These oxygen atoms then react with other oxygen molecules to form ozone.
The resonance structure of ozone is as follows:
It suggests that the two bonding electrons from the pi bonds are spread over the entire structure of the molecule with a bond order of 1.5 (weaker bond than oxygen). As a result, less energy (i.e. longer wavelength light) is required to break the bonds in ozone than in oxygen.
The depletion of ozone hence occurs when the reverse reaction occurs and ozone absorbs longer wavelength UV light to form an oxygen molecule and an oxygen free radical. An oxygen free radical reacts with ozone to form two molecules of oxygen gas.
Ozone is hence constantly being formed and broken down, and acts as a shield by absorbing 93-99% of the sun's harmfulUV light of longer wavelength than that absorbed by oxygen and nitrogen.
E.4.2 List the ozone-depleting pollutants, and their sources.
Depletion of the ozone layer has been observed from satellite data. There are holes in the ozone layer over the north and south poles, although this is partially seasonal (e.g. with lowest levels in the antarctic spring).
Ozone is depleted by chlorofluorocarbons. CFCs are chemically inert when released but become chemically reactive chlorine atoms when they reach the unfiltered UV rays of the sun in the stratosphere.
The bond enthalphy for C-F is higher than that of the C-Cl bond, and as a result a chlorine free radical is formed, that reacts readily with ozone to produce oxygen.
CCl2F2 + uv light --> ∙Cl +CClF2
∙Cl + O3 --> ClO∙ + O2
Nitric oxide (from aircraft exhausts) react with ozone to form nitrogen dioxide and oxygen.
The depletion of ozone means that more harmful UV rays reach the Earth, increasing skin cancer, eye cataracts, genetic mutations, sunburn, damage to animals and plant (suppression of plant growth) and causing changes in the world's climate.
E.4.3 Discuss the alternatives to CFCs in terms of their properties.
Properties of CFCs that make it popularly used in aerosols and as solvents would be their lack of reactivity, low toxicity and low flammability. Alternatives to CFCs should have no C-Cl bonds and show little absorption of infrared radiation so as to to behave like a greenhouse gas.
1. Hydrocarbons are used as refrigerator coolants and do not cause ozone depletion, but are flammable and are greenhouse gases.
2. Hydrochloroflurocarbons decompose less easily because of the presence of hydrogen as the bond enthalpy for the C-H bond is higher than that of the C-Cl bond. However, ozone is still depleted because of the presence of a C-Cl bond.
3. Fluorocarbons have low reactivity, are neither toxic nor flammable and are stable to UV radiation because of the strong C-F bond. However, these are greenhouse gases.
4. Hydrofluorocarbons are also ideal as they do not contain chlorine atoms and have low reactivity, low toxicity and low flammability but contribute to global warming.
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