Chem 11 HL February 23, 2015
Catalysis of Ozone Depletion Ozone: • Ozone, O3 is a V-‐shaped (bent) molecule with a bond angle of 116.8°, and two equal O –
O bond lengths of 128 pm (1 pm = 10-‐12 m). • Contributing resonance forms for ozone:
• Bond order for the O – O bond on ozone: • Each bond order for the O – O bond = !"!#$ !"#$%& !" !!! !"#$%#& !"#$%
!"!#$ !"#$%& !" !!! !"#$%$"&#= !
!= 1.5
The Wavelength of Light Required to Dissociate Oxygen and Ozone • The energy, E, of a photon of light is inversely proportional to the wavelength λ, so the
greater the energy, the shorter the wavelength and vice versa.
𝐸 = ℎ𝜈 = !!!
Where, h = Planck’s constant = 6.63 X 10-‐34 J s ν = frequency of the radiation c = speed of light = 3.00 X 10-‐8 m s-‐1 λ = wavelength of the radiation
• The bond order in ozone (1.5) is lower than the bond order in oxygen (2), which means that the O = O double bond in oxygen is stronger.
• Radiation of shorter wavelength is required to break the stronger bond in oxygen. Bond enthalpy The bond enthalpy of a bond is defined as the energy required to break 1 mol of bonds in gaseous covalent molecules under standard conditions. It is also referred to as bond dissociation enthalpy (section 11 of the Data booklet). Bond enthalpy reflects the strength of a covalent bond.
Chem 11 HL February 23, 2015
Problem The average bond enthalpy in ozone is 362 kJ mol-‐1. Using the energy and wavelength relationship given above, and the bond enthalpy data given in section 11 of the Data Booklet, calculate the maximum wavelength, in nm, of the UV radiation required to break the O = O double bond in oxygen and the O – O bond in ozone. Solution Oxygen: the bond enthalpy for O = O is 498 kJ mol-‐1. To calculate the energy of photons in J, first convert kJ to J, and then divide by Avogadro’s constant:
𝐸 =498 × 1000 𝐽 𝑚𝑜𝑙!!
6.02 × 10!" 𝑚𝑜𝑙!! = 8.27 × 10!!"𝐽 𝐸 = !!
!, so 𝐸𝜆 = ℎ𝑐
∴ 𝜆 =ℎ𝑐𝐸 =
(6.63 × 10!!" 𝐽 𝑠)(3.00 × 10! 𝑚 𝑠!!)8.27 × 10!!" 𝐽 = 2.41 × 10!! 𝑚
Since 1 nm = 10-‐9 m, 𝝀 = 241 nm. Ozone: the bond enthalpy for the O – O bond in ozone is 362 kJ mol-‐1
𝐸 =362 × 1000 𝐽 𝑚𝑜𝑙!!
6.02 × 10!" 𝑚𝑜𝑙!! = 6.01 × 10!!"𝐽
∴ 𝜆 =ℎ𝑐𝐸 =
(6.63 × 10!!" 𝐽 𝑠)(3.00 × 10! 𝑚 𝑠!!)6.01 × 10!!" 𝐽 = 3.31 × 10!! 𝑚
∴ 𝝀 = 331 nm.
Chem 11 HL February 23, 2015
UV Radiation and Our Atmosphere • The ultraviolet radiation in the electromagnetic spectrum, ranging from 100 nm to 400
nm, can be further broken down into UVA, UVB, or UVC depending on their wavelength:
o Ultraviolet A, UVA: § 400 nm to 315 nm § long wave, black light § not absorbed by the ozone layer, as it does not have enough energy to
break apart the bonds in ozone § makes up ~95% of the sun’s UV radiation to reach the Earth § helps the skin form vitamin D § overexposure causes sunburns and cataracts
o ultraviolet B, UVB:
§ 315 nm to 280 nm § medium wave § mostly absorbed by the ozone layer, less than 5% reaches the Earth § harmful to DNA; damages the DNA molecules leading to a distorted
formation of proteins or even cell death § induces skin cancer and suppresses certain immune system activities
o ultraviolet C, UVC:
§ 280 nm to 100 nm § short wave § completely absorbed by the ozone layer and the atmosphere.
• Water vapour, oxygen, and carbon dioxide, along with clouds, smoke and dust in the
atmosphere also absorb and scatter radiation.