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METO 621Photochemical ChangeSlide 3Slide 4PhotodissociationSlide 6Slide 7Slide 8Slide 9Slide 10Wavelength Threshold for Dissociation of OzoneQuantum yield for OzoneSlide 13Potential Energy SurfaceSlide 15Chemical KineticsSlide 17Bimolecular reactionsBimolecular reactionsSlide 20Slide 21Slide 22Transition State TheorySlide 24Liquid Phase ReactionsHeterogeneous ReactionsSlide 27METO 621Lesson 20Photochemical Change•A quantum of radiative energy is called a photon, and is given the symbol hHence in a chemical equation we write: O3 + h→O2 + O•The energy of a photon in terms of its wavelength  is E=119625/kJ mol-1 or 1239.8/  eV •To get enough energy to break up a molecule (dissociation) the wavelength must be in or below the ultraviolet. Thus dissociation typically occurs as the result of electronic transitions•Small, light chemical species generally have electronic transitions at wavelengths shorter than those for more complex compounds, e.g. <200 nm for O2,.Photochemical Change•Atmospheres tend to act as filters cutting out short wavelength radiation, since the absorptions of their major constituents are generally strong at the short wavelengths.•As a result, photochemically active radiation that penetrates into an atmosphere is of longer wavelengths, and the chemistry is characterized by lower energies. For example, the dissociation of molecular oxygen, the ultimate source of ozone in the stratosphere, is limited to altitudes above 30 km.•Absorption of a photon of photochemically active radiation leads to electronic excitation, represented asAB + h →AB*•If the excited molecule then breaks apart, the quantum yield of such a reaction is defined as the number of reactant molecules decomposed for each quantum of radiation absorbedPhotochemical ChangePhotodissociation• Two main mechanisms are recognized for dissociation, optical dissociation and pre-dissociation. These processes will be illustrated in the O2 and O3 molecules.• Optical dissociation occurs within the electronic state to which the dissociation first occurs. The absorption spectrum leading to dissociation is a continuum.• At some longer wavelength the spectrum shows vibrational bands. The bands get closer together as the limit is approached – the restoring force for the vibration gets weaker.•The absorption from the ‘X’ to the ‘B’ state in O2 , is an example.PhotodissociationPhotodissociationPhotodissociationPhotodissociation•Note that when the B state dissociates, one of the two atomic fragments is excited. One atom is left in the ground state (3P) and the other in an excited state (1D).•Some fragmentation occurs in the B→X (Schumann-Runge) system before the dissociation limit. This occurs because a repulsive state crosses the B electronic state and a radiationless transition takes place. The repulsive state is unstable and dissociation takes place. Note that both atomic fragments are 3P.•Although molecular oxygen has many electronic states, not all of the possible transitions between the states are allowed. The magnitude of the photon energy is not the only criteria•Consideration of things such as the need to conserve quantum spin and orbital angular momentum indicate if the transition is possible.Photodissociation•Let us take the reactionO3 + h→ O2 + O•The O2 molecule and atom can be left in several states if we consider energy alone, because any ‘extra’ energy can be used for kinetic energy of the products.•For wavelengths about 310 nm or less, spin conservation allows the transition O3 + h→ O2(1g) + O(1D)•The O(1D) atom formed in this reaction plays a major role in atmospheric chemistry, for exampleO(1D) + H2O → OH + OH•OH, the hydroxyl radical, can break down hydrocarbons.Wavelength Threshold for Dissociation of OzoneQuantum yield for OzoneQuantum yield for Ozone•Note that the onset of dissociation is not abrupt. •The shape of the curve can be explained if the internal energy of the molecule (vibration and rotation) can be added to the photon energy to induce transitions.•Transitions from vibrationally excited states can be important in the atmosphere.•The solar spectrum shows a rapid increase above 310 nm, so any extension of the absorption cross section above this limit can lead to a significant increase in say the quantum yield of O1DPotential Energy Surface•We have already considered the potential energy diagram for a diatomic surface, which can be represented as a two dimensional surface.•But for a polyatomic molecule we must consider a three dimensional potential energy surface.•The next figure represents the potential energy surface for the reactionA + BC → ABC* → AB + C•The symbol * indicating that ABC has energy above that of the reactants A and BC, and therefore ABC* is unstable.•ABC* will either drop back to A + BC or drop down to AB + CPotential Energy SurfaceChemical Kinetics•A reactionA + B → products proceeds at a rate proportional to the concentrations raised to some power][][][][ BAkdtBddtAdRate • k is the rate coefficient (rate constant). The powers and  are the order of the reaction with respect to the reactants i.e. A + B → products• If for example   then the reaction is called a second order reaction () .Chemical Kinetics•If the concentration of B is very much greater then A then [B] can be considered a constant.•One can now combine [B] with k to form a first order reaction rate ][]}[][{][ 12AkABkdtAdRate • k1 is called a pseudo first order rate coefficientBimolecular reactions •As two reactants approach each other closely enough, the energy of the reaction system rises ( see the previous figure).•The contours of the surface show that there is a valley that provides the lowest energy approach of the reactants, the dotted line in the figure is that lowest path.•There comes a point , marked ‘*’ beyond which the energy starts to decrease again, and so product formation is now energetically favorable.•The next figure shows the energy of the ABC system as a function of distance traveled along the lowest path for an exothermic reaction.Bimolecular reactionsBimolecular reactions•In principle, if the potential surface is known, it is possible to calculate the rate coefficient, but in practice this a difficult


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UMD AOSC 621 - Lesson 20 Photochemical Change

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