IB Chemistry on Resonance, Delocalization and Ozone Destruction

1. Delocalizationof electron
Resonance structuresozone
resonance structure 1 resonance structure 2
resonance hybrid
• All bond O-O are identical in length/ strength
• Hybrid of 2 resonance structure
• NO O-O (single) or O=O (double) bonds found
• Only O ----- O bond
• Intermediate in character bet single and double bond
• Bond Order = 1.5
Ozone
3O
Click here on video ozone
Resonance
• Describe delocalization of elec within a molecule/polyatomic ion
where bonding cant be express by ONE single Lewis structure
• Delocalization of π bond – π elec spread over more than 2 nuclei
• π elec are shared/spread – more stable
• Pale blue gas, polar, dimagnetic
• Oxidizing agent
• Potent respiratory hazard /pollutant at ground level
• Beneficialprevent UV B/C from reaching Earth
• Highest ozone level in stratosphere(10 km and 50 km)
Ozone at stratosphere
strongest radiation
UV radiation

2. FORMAL CHARGE (FC)
Formal Charge
• Tool/Model for comparing which Lewis structures is more acceptable
• Treat covalent bond with equal electron distribution no electronegativity differences bet atom
• Electronegative atom has negative while least electronegative atom has positive formal charge
Formula formalcharge
V - valence electrons of atom
L – lone pair electron
B – bonding electron molecule
Formal charge sulfur dioxide
formal charge for O
V- Valence electron O = 6
L- Lone pair electron O = 4
B- Bonding electron O = 4
L +
FC = 6 – (4 +2) = 0
formal charge for O
V- Valence electron O = 6
L- Lone pair electron O = 6
B- Bonding electron O = 2
FC = 6 – (6+1) = -1
formal charge for O
V- Valence electron O = 6
L - Lone pair electron O = 2
B - Bonding electron O = 6
FC = 6 – (2+3) = +1
All resonancestructure contributeto electronic structure.
Real structure is combinationof them.
Lowest FC (stable), contribute more than less stable structure.
Sum of FC must be zero for neutral or equal to charge on ion.0
+1
-1
L + L + L +

3. Ozone Good and Bad
Ozone in Strastophere
• blocks UV B + C
Ozone in Troposphere act as
• Greenhouse gas
Ozone in ground level act as
• Pollutant
• Photochemical
Click here on ozone depletion
substances ODS (phaseout)
Why ozone able to absorb UV B and UV C?
Breakdownof ozone – High UV radiation
– Skin cancer
- DNA mutation
Ozone depletion
UV Exposure
Ozone Hole
Bad SideGood Side

4. Ozone absorb UV radiation
Ozone absorb UV B/C
Ozone formation
O=O
Double
bond
O=O=O
Inter
mediate
Bond length/pm 121 127
Bond enthalpy/kJ
mol-1
498 363
Bond order 2 1.5
Dissociation by UV < 242nm < 330nm
Ozone weaker bond
• Absorb UV A/B
(wavelength 330 nm)
Bonding
O2 and O3
Oxygen stronger bond
• Absorb UV C
(wavelength 230 nm)
* Free radical -reactive species with unpair electron
How ozone layer protect life on earth?
Ozone Cycle
O2 split by
high UV to O· (radical)
O· radical combine to form ozone
Ozone absorb UV B/C
Ozone reform again
Ozone cycle
Ozone created/destroyed by Chapman cycle
Oxygen reform again
Weaker bond in ozone broken UV
O• + O2 → O3
O3 + hv → O2 + O•
O3 + O• → 2O2
O2 + hv → 2O•
O• + O2 → O3
O3 formation
O3 destruction
+
+
1
2
3
4

5. Ozone absorb UV radiation
Ozone formation
O=O O=O=O
Bond length/pm 121 127
Bond enthalpy/kJ mol-
1
498 363
Bond order 2 1.5
Dissociation by UV <242nm <330nm
Ozone weaker bond
• Absorb UV A/B
(wavelength 330nm)
Bonding
O2 and O3
Oxygen stronger bond
• Absorb UV C
(wavelength 230nm)
O2 split by
high UV to O· (radical)
O· radical combine to form ozone
Ozone absorb UV B/C
Ozone reform again
Ozone created/destroyed by Chapman cycle
Oxygen reform again
Weaker bond in ozone broken UV
O• + O2 → O3
O3 + hv → O2 + O•
O3 + O• → 2O2
O2 + hv → 2O•
O• + O2 → O3
BE O3 = 363 kJmol-1
λ need to break O3 bond
Energy 1 mole, 6.02 x 1023 = 363 kJ
Energy 1 photon - E = hf
In nature: Ozone formation = Ozone destruction (without CFC free radical)
Velocity light (c) = frequency(f) x wavelength(λ) - c = f λ
• Electromagnetic waves travel at speed of light (3.00 x 108
ms-1
)
• Radiation high ↑ frequency – short ↓ wavelength
• Electromagnetic radiation/photon- energy given by
E = hf

hc
E 
h = plank constant = 6.626 x 10-34
Js
f = frequency
λ = wavelength
BE O2 = 498 kJmol-1
λ need to break O3 bond
Energy 1 mole, 6.02 x 1023 = 498 kJ
Energy 1 photon - E = hf
JE 19
23
1003.6
1002.6
363000 



nm
E
hc
330
1003.6
1031063.6
19
834



 




hc
E 
JE 19
23
1027.8
1002.6
498000 



nm
E
hc
241
1027.8
1031063.6
19
834



 




hc
E 
easier to break ! harder to break !
1
2
3
4

6. Ozone absorb UV B/C
CFC breaks down
How CFC breaks down ?
High UV B/C
How CI· radical
destroy ozone?
✓
Catalytic destructionof ozone
Ozone Depleting Substances (ODS)
How CFC destroys ozone ?
CI free radicalsform
CI· react O3 form CIO· radical
CIO· react O· form CI· radical
Net - ozone break down.
Carbon
Fluorine
Chlorine
•CI + O3 → •CIO + O2
•CIO + O• → •CI + O2
O3 + O• → 2O2
+
F
‫׀‬
H – C – F
‫׀‬
CI
F
‫׀‬
F – C – CI
‫׀‬
CI
CI
‫׀‬
F – C – CI
‫׀‬
CI

7. Ozone absorb UV B/C
NOx breaks down
How CFC breaks down ?
High UV B/C
How NO· radical
destroy ozone?
Catalytic destructionof ozone
NO· free radicals form
NO· react O3 form
NO2· radical
NO2· react O· form
NO· radical
Net - ozone break down.
Oxides of Nitrogen
Nitrogen Dioxide
(NO2)
Nitrogen monoxide
(nitric oxide NO)
Nitrous oxide
N2O
Break down to form NO· free radical
(unpair electron)
Sources of NOx
NO• + O3 → NO2• + O2
NO2• + O• → NO• + O2
O3 + O• → 2O2
Ozone DepletingSubstances(ODS)

8. Catalytic destruction of ozone
Sourceof ODS
• Halogenatedsubstance
• Man-made halocarbon refrigerant,solvent, propellant
• Foam-blowingagent (CFC,HCFC,freon, halon)
MontrealProtocolban CFC, halon, and ODS like
carbon tetrachloride and trichloroethane.
CFC
• Contain chlorine,fluorine atom
• Extremelystablewith strong bond, long half life
• Stability allow CFC to stratosphere
• High UV radiation react with CFC
• High UV break CFC. Free Cl radicalform – destroy O3.
Trichloroflouromethane
CFC-11
Dichlorodifluoromethane
CFC-12
Chlorodifluoromethane
HCFC-22
Why
halogenated CFC
used?
Less harmful
Very harmful
Why fluorinatedis safer?
CI
‫׀‬
F – C – CI
‫׀‬
CI
CI
‫׀‬
F – C – CI
‫׀‬
F
H
‫׀‬
F – C – CI
‫׀‬
F
Bond length Bond strength
C – F 138 484
C – CI 176 338
C – Br 195 276
C – I 215 238
Ozone DepletingSubstances(ODS)

9. Conc O3 is shown below
i. O2 → 2 O∙
ii. O2 + O∙ → O3
iii. O3 → O2 + O∙
Identifywhich step is exothermic
Step ii, bondforming, exothermic,energygiven off
With referenceto bondingin O2 and O3, which step is most endothermic
Step i and ii. Bondbreaking.
Step i. O2, bondorder 2 – strong bond – more energy
Step ii. O3, bondorder1.5 – weak bond– less energy
Catalytic destruction of ozone
Ozone form from combustion of methane.
CH4 (g) + 5 O2 (g) → CO2 (g) + 2 H2O (g) + 2O2 (g)
Find ∆H for rxn in kJ mol -1
= - 591 + 74 = - 517 kJ mol -1
Find ∆S, in JK-1 mol-1
Find ∆G in kJ mol-1 at 298K
Deduce if rxn is spontaneous at 298K
CH4(g) + 5 O2 (g) → CO2(g) + 2 H2O(g) + 2O3 (g)
∆Hf
0 - 74 0 - 393 - 241 x 2 + 142 x 2
∆Hsys
θ = ∑∆Hf
θ
(pro) - ∑∆Hf
θ
(react)
CH4(g) + 5 O2 (g) → CO2(g) + 2 H2O(g) + 2 O3 (g)
S0 + 186 +205 x 5 + 213 + 188 x 2 + 237 x 2
- 74 - 591
Reactant Product
1
)tan()(
148
12111063




JKS
S
SSS
sys
sys
treacproductsys
+ 1211 + 1063
Reactant Product
syssyssys STHG 
1
472
)148.0(298517




kJmolG
G
STHG
1
472 
 kJmolG
CCl2F2 , cause ozone depletion.Formulate eqn for each step and explain initial
step by reference to bond in CCl2F2.
CCI2F2 → ∙CCIF2 + CI∙
CI∙ + O3 → CIO∙ + O2
CIO ∙ + O∙ → O2 + CI∙
CIO∙ + O3 → 2O2 + CI∙
hv