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Rate laws of complex photochemical reactions

المؤلف:  Peter Atkins، Julio de Paula

المصدر:  ATKINS PHYSICAL CHEMISTRY

الجزء والصفحة:  ص859-860

2025-12-28

373

Rate laws of complex photochemical reactions

As an example of how to write a rate law for a complex photochemical process, consider the photochemical activation of the reaction

H2(g) + Br2(g) →2 HBr(g)

 In place of the first step in the thermal reaction we have

Br2 + hν→Br·+Br·   v =Iabs

where I_abs is the number of photons of the appropriate frequency absorbed divided by the volume in which absorption occurs and the time interval. We are assuming a primary quantum yield of unity for the photodissociation of Br2. It follows that Iabs should take the place of ki[Br2][M] in the thermal reaction scheme, so from Example 23.1 we can write

We predict that the reaction rate should depend on the square root of the absorbed light intensity, which is confirmed experimentally.