Chemical Kinetics Essentials
Understanding chemical kinetics is crucial in studying reaction rates, mechanisms, and pathways. Explore bimolecular reactions, three-body reactions, thermolysis, photolysis, and more. Discover key concepts like rate constants, Arrhenius expressions, and quantum yields.
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Presentation Transcript
Bimolecular reaction A + B [A] [A][B] k dt C + D d In gas phase, [A] is in [molecules cm-3] = Rate: k is rate constant [cm3 molecule-1 s-1] Ea = ( ) k T exp[ / ] A E RT a Arrhenius expression Reaction is exothermic if heat is released: likely to occur, but may be slow if Eais large Reaction is endothermic if heat is absorbed: can be safely neglected
Three-body reaction: single product from two reactants k A + B AB* 1 Mechanism: k + M is a third body, generally N2 or O2; [M] is air density 1 k AB* + M M* Net: A + B + M AB + M* 2 M heat AB + M [A][B][M] [M] k + [AB] dt 1 2 k k k d = = From steady state for AB*, general solution: [AB*][M] k 2 1 2 ko [AB] dt 1 2 k k k d = = [A][B][M] [A][B][M] k Low-pressure limit: o 1 [AB] dt d = = [A][B] k [A][B] k k High-pressure limit: 1 [ ] k M [AB] dt d = o k M k = [A][B] k with General formulation: [ k ] + 1 o
Thermolysis: reverse of 3-body reaction Net: AB + M A + B + M k AB +M AB* + M 1 Mechanism: k 1 k AB* A+B 2 From steady state for AB*, general solution: [AB][M] [M] + [M] + [B] dt 2 1 k k k k k d = = = = [AB*] [AB] with k k k 1 2 [M] 2 k k k 1 2 1 2 = 1[M] k k k k ok Low-pressure limit: = k 1 2 High-pressure limit: 1
Photolysis [A] dt d + = A B + C [A] h k k [s-1] is the photolysis rate constant (also called photolysis frequency) wavelength [cm] = ( ) ( ) k q I d A A radiation flux: vertical 0 actinic flux [photons cm-2 s-1] actinic flux: all directions absorption cross-section [cm2 molecule-1] Absorption cross-section photon is not absorbed quantum yield: probability that photon absorption causes photolysis photon is absorbed Molecular cross-section A Probability of absorption for incoming photons = /A
Example of photolysis reaction: O3 + hv O2 + O(1D) excited state = ( ) ( ) k O(1D) q I d O3 0 solar flux I at sea level ozone absorption cross-section q I Photolysis at sea level is controlled by photons in 300-320 nm range O(1D) quantum yield q
Chemical radicals Chemical radicals are atoms or molecules with unpaired electrons Do not confuse with hydroxide ion OH-, which is not a radical Example: hydroxyl radical (OH) The unpaired electron makes radicals highly reactive, with atmospheric lifetimes of generally less than a minute. One can reliably assume chemical steady state for radicals (chemical production = chemical loss) To determine whether a species is a radical, add up the electrons contributed by each atom of the molecule. If the total number is odd, then the species is a radical; if it is even then the species is a non-radical. Very exceptionally, a species with an even number of electrons may be a biradical and hence very reactive, or in a singlet state and even more reactive. O(3P) and O(1D) are the only such cases that we will encounter.
Radical-assisted reaction chains Initiation: non-radical + energy radical + radical Propagation: radical + non-radical non-radical + radical X Termination: radical + radical radical + radical + M non-radical + non-radical non-radical propagation rate chain length = termination rate Radical family can generally be assumed to be in steady state: = [radical( )] i ~ minutes i initiation rate = termination rate radical family termination rate [X] dt d = chain length initiation rate Catalytic destruction of non-radical species X:
