1 / 17

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.

emerson-sloan
Télécharger la présentation

What is Photochemistry?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related