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What is Photochemistry?

What is Photochemistry?. Photochemistry: a chemical interaction involving radiation Why mention photochemistry? Photochemistry plays an important role in atmospheric processes. The wavelengths in important atmospheric photochemical reactions are shortwave – radiation from the sun.

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What is Photochemistry?

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  1. What is Photochemistry? • Photochemistry: a chemical interaction involving radiation • Why mention photochemistry? Photochemistry plays an important role in atmospheric processes. • The wavelengths in important atmospheric photochemical reactions are shortwave – radiation from the sun

  2. What is the Sun’s Source of Energy? Nuclear Energy = neutrons + protons minus binding energy Atomic Weight/(Neutrons+Protons) does not equal 1 because of the binding energy. More binding energy, lower ratio AW/(N+P), lower total energy. If two light nuclei join to form a heavier nucleus with higher binding energy (lower total energy), the extra energy is released as radiation – fusion.

  3. Fusion Fusion is a thermonuclear reaction. It releases energy but needs a high enough temperature to bring the two nuclei together. 2H+2H  4He+E 2x2.0144.0026+0.0254(as energy) Requirement, high temperature for activation ~25,000,000oC This is the temperature in the sun’s interior, but not the temperature at the sun’s surface (the temperature at which the sun emits). Can calculate Tsurface based on a steady state assumption (heat flow from interior balances heat loss from surface): dT/dt=C1MsTinterior-C2(S.A.)sTsurface=0 4/3pR3C’1Tinterior=4pR2C’2Tsurface Tsurface=C”RTinterior=~6000oK Solar surface temperature is determined from the ratio of surface to volume.

  4. Energy of Radiation Energy per photon: E=hn=hc/lh = 6.626 x 10-34 J s c = 3 x 108 m s-1

  5. Energy Per One Mole of Photons (Einstein) Energy per one mole of photons at: 100nm = 290 kcal/mole 400nm = 72.5 kcal/mole 700nm = 41.5 kcal/mole 1000nm = 29 kcal/mole Comparison to chemical bond strength: N-N = 225 kcal/mole Very strong O-O = 120 kcal/mole Strong C-Cl =75 kcal/mole Intermediate O-O2 = 35 kcal/mole Weak HO…..H=5 kcal/mole Very weak

  6. What Happens When Radiation Hits a Molecule? We learned in radiative transfer that two possible outcomes are: • scattering (no chemical interaction) • absorption: Following absorption, there are a number of possibilities.

  7. Pathways Following Absorption

  8. Pathways Following Absorption cont. • Dissociation/photolysis: breaking a chemical bond in the molecule Energy of radiation must be greater than bond energy. l=100-1000nm is sufficient to break any chemical bond. • Ionization: removing an electron from the molecule In general ionization energy is greater than chemical bond strength: He = 552 kcal/mole =l{52.6 nm} N2 = 398 kcal/mole = l{79.6 nm} Na = 120 kcal/mole = l{250 nm}

  9. Table of Ionization Energies

  10. Pathways Following Absorption cont. • Luminescence: re-emission of photon In atoms, re-emited photon is of same energy as excitation: lem=lex. In molecules, it can be less: lem>lex. Flourescence: visible wavelengths Phosphorescence: non-visible wavelengths

  11. Pathways Following Absorption cont. • Intramolecular energy transfer: conversion of the absorbed energy to several forms of lower energy (vibration, rotation and eventually to heat – typical for large molecules). § • Intermolecular energy transfer • Quenching • Reaction: conversion to more active state and undergo selective chemical reactions

  12. What Determines the Pathway? • wavelength – whether or not its possible • population of excited states – whether or not its probable • conservation of orbital angular momentum and spin – whether or not its probable

  13. Rate of a Photochemical Reaction rate of formation of AB* = J for a photochemical reaction is the equivalent of a rate constant for a chemical reaction J can be treated as a first order rate constant (units of time-1) but it depends on light intensity and spectral distribution.

  14. How is J Calculated? For a given wavelength: J{l}=PF{l} s{l} Y{l} PF{l}: PhotoFlux s{l} – Absorption cross section (population of excited states) Y{l} – Quantum yield (conservation of orbital angular momentum and spin) Y{l} = The quantum yield is sometimes also symbolized f. For a range of wavelengths (solar range): J =  PF{l} s{l} Y{l} dl

  15. Photolysis Rate Example NO2+hn NO + O(3P) J{NO2} The rate of O(3P) formation d[O(3P)]/dt = J{NO2} [NO2] The rate of O formation will change diurnally even at constant NO2.

  16. How Does J Vary With Latitude and Season? J=Jmax cos(f-d) cos(w) Jmaxa SRI/R2 SRI: solar radiation intensity w=2pt/60x60x24 • – Geographical Latitude • – Seasonal motion of the earth d=23.5 cos(2p {JD} /365) JD: Julian Day

  17. The Solar Spectrum

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