ib chemistry on resonance, delocalization and ozone destruction

Download IB Chemistry on Resonance, Delocalization and Ozone Destruction

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  • Delocalization of electron

    Resonance structures ozone

    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 bond elec spread over more than 2 nuclei elec are shared/spread more stable

    Pale blue gas, polar, dimagnetic Oxidizing agent Potent respiratory hazard /pollutant at ground level Beneficial prevent UV B/C from reaching Earth Highest ozone level in stratosphere (10 km and 50 km)

    Ozone at stratosphere

    strongest radiation

    UV radiation

    https://www.youtube.com/watch?v=WT-ySp6WMu8https://www.youtube.com/watch?v=WT-ySp6WMu8

  • 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 formal charge

    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 resonance structure contribute to electronic structure. Real structure is combination of 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 +

  • 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?

    Breakdown of ozone High UV radiation Skin cancer - DNA mutation

    Ozone depletion

    UV Exposure

    Ozone Hole

    Bad Side Good Side

    http://www.epa.gov/ozone/strathome.htmlhttp://www.epa.gov/ozone/strathome.html

  • 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

  • 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

  • Ozone absorb UV B/C

    CFC breaks down

    How CFC breaks down ?

    High UV B/C

    How CI radical destroy ozone?

    .

    Catalytic destruction of ozone

    Ozone Depleting Substances (ODS)

    How CFC destroys ozone ?

    CI free radicals form

    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

  • Ozone absorb UV B/C

    NOx breaks down

    How CFC breaks down ?

    High UV B/C

    How NO radical destroy ozone?

    .

    Catalytic destruction of 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 Depleting Substances (ODS)

  • Catalytic destruction of ozone

    Source of ODS Halogenated substance Man-made halocarbon refrigerant, solvent, propellant Foam-blowing agent (CFC, HCFC, freon, halon)

    Montreal Protocol ban CFC, halon, and ODS like carbon tetrachloride and trichloroethane.

    CFC Contain chlorine, fluorine atom Extremely stable with strong bond, long half life Stability allow CFC to stratosphere High UV radiation react with CFC High UV break CFC. Free Cl radical form destroy O3.

    Trichloroflouromethane CFC-11

    Dichlorodifluoromethane CFC-12

    Chlorodifluoromethane HCFC-22

    Why

    halogenated CFC used?

    Less harmful

    Very harmful

    Why fluorinated is 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 Depleting Substances (ODS)

  • Conc O3 is shown below

    i. O2 2 O ii. O2 + O O3

    iii. O3 O2 + O

    Identify which step is exothermic Step ii, bond forming, exothermic, energy given off With reference to bonding in O2 and O3, which step is most endothermic Step i and ii. Bond breaking. Step i. O2, bond order 2 strong bond more energy Step ii. O3, bond order 1.5 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 = Hf

    (pro) - Hf

    (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

    1472

    )148.0(298517

    kJmolG

    G

    STHG

    1472 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

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