Temperature effects on reaction

In this equation, ko is the frequency factor, also called the pre-exponential factor, and E the activation energy of the reaction in J mol1. Since the reaction rate is always expressed in md-vdume Hime the rate constant and the pre-exponential factor have dimensions depending on the order of the reaction volume1" mot ln Hime The universal gas constant used in this equation is 8.314 J mol"1 K"1. The van t Hoff rule, can be used as a rough approximation of the temperature effect on reaction rate ... 

In addition to chemical reaction, weak fluorescence was detected from 50 at room temperature (acxc 460 nm, Xem 552 nm, cj)f = 0.04). Temperature effects on reaction and fluorescence from 77-310 K have been studied 68). A steady decrease in quantum yield for reaction (r) and a complementary increase in fluorescence quantum yield (< )f) were observed down to about 150K where a sharp increase in f occurred. Photochemical reaction was negligible at 77 K (436 nm). The fluorescence lifetime at 77 K was a few nanoseconds and the estimated value at room temperature is on the order of 60 ps. Detailed analysis of the data showed that two thermally-activated processes are involved (1) chemical reaction of the singlet state with an Arrhenius activation energy of 1.5 kcal/mol and (2) radiationless decay of the singlet with Eact =1.1 kcal/mol. Both processes would appear to be associated with certain vibrational modes of the excited state which become progressively less populated with decreasing temperature. 

A similar slow evolution from energy to entropy with a final synthesis of both concepts can also be observed in the historical development of chemical kinetics. The energy factor was first pointed out by Arrhenius (1889) when he explained the temperature effect on reaction rates. But in spite of the early work of Kohnstamm and Scheffer (1911) who introduced the idea of activation entropy, the importance of entropy was generally recognized only after Eyring (1935) formulated clearly the thermodynamic treatment of the transition state method. 

Temperature Effects on Reaction Rates Chemical reactions typically have rate constants whose temperature dependence takes the mathematical form... 

Fig. 4.1. Temperature effects on reaction rate in a urethane. (Reaction of 4,4 -diphenylmethane diisocyanate with glycol adipate polyester in chlorobenzene.) (From Wright Gumming, 1969).

Figure 13.1 Surface metallopinacols on active titanium and uranium. Table 13.2 Temperature effect on reaction of benzophenone with U. ...

Concentration effect on reaction rate Temperature effect on reaction rate Surface area effect on reaction rate Chemical equilibrium conditions Simultaneous forward and reverse reaction Rate forward reaction = rate reverse reaction Reaction system is closed ... 

By reference to the appropriate sections of this book they can be used to demonstrate normally difficult to illustrate concepts such as quantisation, radical ion recombination, electron transfer, energy transfer, temperature effects on reactions and thermodynamics. Interesting discussions on their relationship to photosynthesis, vision and photochemistry can be provoked. 

---