OCR Text |
Show reactionsS• Starting at the lowest temperatures where there are no reactions, then as the temperature is increased, the kinetically least stable structures are destroyed. For some of these destruction processes, radicals are created. These radicals are molecular fragments and atoms with unpaired electrons and may be highly reactive. They will attack the other stable components in the system. This will generate other free radicals. In some cases more than one radical will be created. For example a reaction such as H + 02 -) OR + 0 a hydrogen radical is transformed into a hydroxyl and oxygen radical. If the reactions are exothermic, there will be a corresponding temperature rise. Since the rate constants for these processes are exponentially dependent on temperature there will be a cataclysmic increase in radical concentration and corresponding increase in rate of destruction. In addition, as the temperature is increased, new reaction channels are opened and this provide a further impetus for reaction. When radical concentrations get too high, then the radicals may well react with itself. A new stable structure is formed and this terminates the process. Generically, when we wish to describe chemical transformations in the gas phase there are two types of reactions that we must consider, unimolecular and bimolecular processes. For the former we are talking about molecules which has sufficient internal energy in excess of a certain threshold for decomposition. Bimolecular reactions are the processes involving radical attack or termination that we have discussed earlier. In the context of incineration the most important processes are the attack of OH and H and the unimolecular decomposition of the compound itself. This is because in high temperature systems the concentration of active radicals such as OH and H are high and their rate constants for attack are 4 |