E9 Ozone depletion (HL)

E.9.1 Explain the dependence of O2 andO3 dissociation on the wavelength of light.

The amount of energy required to break an O=O bond is 496kJ/mol. Using the formulas E=hf and C=λf gives λ=hc/E, where h is Planck's constant and c is the velocity of light. To achieve this amount of energy, the wavelength of light required would be 242nm, as calculated from the formula, and is in the high energy region of the ultraviolet spectrum.

For ozone dissociation, ozone has two resonance hybrids, each with one double bond and one single bond, and as only the single bond is broken less energy is required (light of a lower wavelength) i.e. Ultraviolet light with wavelength 330nm. Working backwards, this means the strength of the O-O bond in ozone is 362kJ/mol.

E.9.2 Describe the mechanism in the catalysis of O3 depletion by CFCs and NOx.

CFCs catalyse the destruction of ozone because the high energy ultraviolet light in the stratosphere causes the homolytic fission of the C-Cl bond that produces chlorine free radicals, that then go one to break down ozone molecules and generate more radicals in a continuous process until the radicals eventually escape/terminate. One molecule of CFC can break down 100000 molecules of ozone.

CCI2F2 (g)-->CCIF•(g) + CI•(g)
CI•(g) + O3(g) --> ClO•(g) + O2(g)
CI O•(g) + O•(g) --> O2(g) + Cl•(g)

Evidence for this mechanism can be seen in how an increase in chlorine monoxide in the antarctic mirrors the decrease in ozone concentrations.

Nitrogen oxides also catalytically decompose ozone by a radical mechanism, whereby oxygen radicals are generated by the breakdown of NO2 in ultraviolet light.

NO2 (g) --> NO(g) + O•(g)
O•(g)+ O3(g) --> 2O2 (g)
NO(g) + O3(g) -->NO2 (g)+ O2(g)

Overall: 2O 3(g) --> 3O 2 (g) [NO2- homogenous catalyst]

E.9.3 Outline the reasons for greater ozone depletion in polar regions.

During winter, low temperatures cause small amounts of water vapour in the air to freeze and form ice crystals, which contain small amounts of chlorine compounds (e.g. HCl, ClONO2) that react in the presence of catalysts to form hypochlorous acid (HClO) and chlorine (Cl2). In spring, when the sun shines, these molecules break down to produce chlorine radicals which catalyse the destruction of ozone, with the largest holes in the ozone occuring during early spring. As the temperature increases and warmer winds blow into the region, the ice crystals disperse and the ozone concentration gradually increases.

OTHERS:

Sun-screening compounds --> Sunlight can be harmful; causes genetic mutations and damage to proteins. Ozone decreases more harmful ultraviolet light in reaching the Earth's surface. Other sun-screens include glass, melanin (pigment in body that is increased by the action of the sunlight on the skin), commercial substances (e.g. PABA [para-aminobenzoic acid) that absorb light of a particular frequency (e.g. for PABA, 265nm) as they contain delocalised π electrons with conjugated double bonds (alternate between C-C and C=C) which are excited to higher energy levels when they absorb UV light. Sunblocks include white pigments, like titanium dioxide or zinc oxide, which reflect and scatter sunlight.