Thermodynamic and Kinetic Constraints Upon Reactivity

One of the most fundamental concepts of chemistry is the distinction between kinetic and thermodynamic factors nonetheless, such arguments are frequently ignored, or at best only tacitly considered, in wider discussions of reactivity. Chemical thermodynamics is concerned with the energetic relationships between chemical species. The most useful parameter is the Gibbs free energy, G, which, like all thermodynamic terms, is based on an arbitrary scale placing a value of zero upon pure elements in their stable standard states at 298 K and 1 atmosphere pressure. Differences between free energies are denoted by AG, as shown in Eq. (1.1). 

Does a knowledge of G values for products and reactants, and hence AG for the reaction, enable us to make any predictions about the chemical behaviour of reactant systems To a certain extent, it does. Consider the reaction of metallic sodium with gaseous chlorine (Eq. 1.3), which is strongly exothermic. 

We would thus predict that chlorine gas reacts with sodium metal at 298 K to give sodium chloride, and, of course, our prediction is correct. However, it is also clear that the existence of a negative AG is not the sole factor affecting reactivity. 

Hydrogen bromide is a well-known chemical species, but AG for the dissociation reaction is —106 kJ mol 1, and yet hydrogen bromide is an isolable compound (Eq. 1.4)  

Although we cannot relate the thermodynamic free energy change for a reaction directly to the rate of the reaction, it is always useful to remember that the thermodynamic stability constant may be related to the rate constants for the forward (ftf) and backward (kb) reactions by Eq. (1.5). 

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