OZONE: A CASE STUDY IN RESONANCE, MOLECULAR POLARITY AND
FORMAL CHARGE
INTRO:
*Ozone is an interesting molecule because of its role in:
- the __________________ (12-50 km above Earth / lower part of stratosphere = ____________ layer)
- air ___________________(Ozone is described as ‘good up high, bad nearby’.)
- _______________ purification.
Review of Ozone’s Structures & Properties:
1. Draw: (a) two resonance forms (represent shape) of ozone; and (b) resonance hybrid for ozone.
2. Deduce all formal charges.
OZONE:
ELECTRON DOMAIN GEOMETRY
MOLECULAR GEOMETRY
BOND ANGLE
HYBRIDIZATION
POLARITY
A. EXPLANATION OF THE WAVELENGTH OF LIGHT REQUIRED TO DISSOCIATE OXYGEN AND
OZONE
O•(g) = oxygen free ____________; that is, a highly reactive species that contains an ______________ electron. (Any
species with an unpaired electron is called a free radical.)
UV light,   242 nm
(a) OXYGEN Dissociation: O2(g)      O•(g) + O•(g)
(atomic oxygen)
UV light,   330 nm
(b) OZONE Dissociation: 1. O3(g)      O•(g) + O2(g)
 2O2(g) + energy (exothermic rxn)
2. O3(g) + O•(g) 
O-O single bond
O=O double bond
Oxygen-Oxygen bonds in
Ozone, O3
Bond Length / pm
Bond Order
Bond Enthalpy / kJ mol-1
Dissociated by UV light (λ)
*Oxygen contains _______________bonds than ozone, and so is dissociated by light of ______________ wavelength.
(Recall: __ wavelength = __ frequency = __ energy)
*BOND ORDER = (# of bonding pairs)  (# of bonding positions)
B. DESCRIPTION OF THE MECHANISM OF THE CATALYSIS OF OZONE DEPLETION WHEN
CATALYZED BY CFCs and NOx
(i) NOx Catalyzed Depletion of Ozone
*Nitrogen monoxide a free radical produced in vehicle engines.
MECHANISM:
NO•(g) + O3(g)  NO2•(g) + O2(g)
NO2•(g) + O•(g)  NO•(g) + O2(g)
NET CHANGE:
*NO•(g) has acted as a ______________because it is ___________________ during the reaction.
(ii) CFC Catalyzed Depletion of Ozone
*Chlorofluorocarbons were widely used in aerosols, refrigerants, solvents and plastics. In the stratosphere, higher energy
UV radiation breaks them down releasing highly reactive chlorine free radicals.
MECHANISM:
UV
CCl2F2(g)  CClF2•(g) + Cl•(g)
Cl• + O3(g)  O2(g) + ClO•(g)
ClO•(g) + O•(g)  O2(g) + Cl•(g)
NET CHANGE:
*Cl•(g) has acted as a _______________because it is ________________during the reaction.