Potential Surface and Transition State

Many of atmospheric reactions are the following type, involving the rearrangement 

Whether the Reaction (2.27) proceed to the right or, in reverse, to the left, is determined by the difference of the free energy AG between the reaction system and the product system. According to the first law of thermodynamics, the free energy change AG must be negative for the reaction to proceed. The AG is defined by 

Enthalpy of formation of typical atoms, molecules and free radicals relevant to atmospheric chemical reactions are cited in Table 2.5. The AH shown in Fig. 2.7 is the enthalpy change of reaction given by Eq. (2.29), showing the reaction to proceed from left to right since AH 0 in this figure. 

The rate constant of bimolecular reactions represented by Reaction (2.26) is represented as 

the quantities within the brackets [ ] represent concentration of each species, and the defined above is called a reaction rate constant. The reaction rate constant of a bimolecular reaction has the dimensimi of (concentration) (time) . In atmospheric chemistry, the concentration of gaseous species is expressed in general by the number density of molecules, molecules cm so that the unit of the rate constant of a bimolecular reaction is commonly expressed as cm molecule s .  

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