Lecture 61 Kinetics IV, Chemistry and Light

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Lecture 61 - Kinetics IV,Chemistry and Light
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Review
k increases with T (if Ea gt 0)
A catalyst changes the mechanism, and lowers
Ea, thus k is larger
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Today...

• Catalysis
• Explosions
• Interactions of light with matter

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Heterogeneous Catalysis

• Rate limiting step
• gas phase H2(g) 2 H(g)
• catalyzed adsorption

lower Ea, thus faster
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Other Catalyzed Processes
Fe, K2O, Al2O3
N2(g) 3 H2(g)
2 NH3(g)
industrially
Pt or V2O5
2 SO2(g) O2(g)
SO3(g)
(dust, water)
naturally
(then SO3(g) H2O(l) H2SO4)
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Explosions

• A. Thermal explosions

k
assume A B
- as reaction proceeds, T rises - thus k rises -
reaction goes faster, T rises faster - k rises -
BOOM
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Explosions

• B. Chain Reactions
• e.g. 2 H2(g) O2(g) 2 H2O(g)
• radicals contain un-paired electrons
• e.g. H, O, HO2, HO

½
½
½
½
mucho reactive!
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Chain Reactions

• 1. Initiation step radicals are produced
• H2 O2 HO2 H
• 2. Propagation steps new radicals formed
• H2 HO2 HO H2O
• H2 OH H H2O
• H O2 HO O
• O H2 HO H

½
½
D
½
½

no net radical production
½
½
½
½
½

branching occurs, number of radicals
increases
½
½
½
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Chain Reactions

• more radicals
• higher rate
• more radicals
• BOOM

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Chemistry and Light

• Light can be
• - a reaction initiator
• - a reaction product
• - a tool to probe atoms, molecules, materials,
  reactions

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Light Electromagnetic Radiation
moving this charge
makes EM radiation
which makes this charge move
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Light and Matter
Luminescence, l
Absorbance by matter
Incident Light, l
Transmitted Light, l (less intense)
Scattered Light, l
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Absorbance

• e.g. the greenhouse effect

UV from Sun
Infrared Light
CO2 hn(IR) ? heat
Earth
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Absorbance

• e.g. smog
• NO2(g) hn(UV) NO2
• NO2 NO O

NO2 with excess energy
one possible result photolysis
i.e. if hn gt N-O bond energy
½
½
½
(then O2 O O3)
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Other Light Initiated Processes

• Vision
• Photography

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Vision
11-cis retinal
hn
11-trans retinal
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Vision
opsin
hn
trans-retinal
cis-retinal
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Vision

• opsin nerve cell depolarization of nerve
  cell
• Na ions rush in
• nerve impulse travels to brain

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Photography
dye hn ? dye
in film
dye AgBr ? dye Ag Br
(AgBr-)
opaque (black)
clear, colorless
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Spectroscopy
Transmitted Light (less intense)
Incident Light
Sample
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Infrared Spectroscopy
Aspirin
Transmittance
Wavenumbers, cm-1
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Other Forms of Spectroscopy

• Microwave (? rotations of molecules)
• IR (? vibrations of molecules
• UV/VIS (? electronic energies)
• X-ray (? crystal structure)

waves differ only in frequency
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Processes Which Produce Light

• Chemiluminescence
• (Bioluminescence)
• Auroras

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Chemilumiscence
H2O2, OH-
h?
Luminol
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in the upper atmosphere...
½
½
NO O ? NO2
NO2 ? NO2 hn
mostly in IR
air glow