Thermodynamic Factors in Pyrolytic Chemical Reactions

Pyrolytic reactions are not different in principle from any other chemical reactions. A few general considerations about the thermodynamic factors in chemical reactions are, therefore, useful for the understanding of the parameters affecting the pyrolytic process. 

In thermodynamics (see e.g. [1]) it is shown that for any isolated (closed) system (which does not exchange energy with its surroundings), spontaneous transformations take place with an increase in the entropy AS of the system. For a non-isolated system in isothermal conditions, spontaneous processes take place with a negative variation of free enthalpy AG where 

For a chemical reaction, the variation of enthalpy, entropy, and free enthalpy can be calculated from the particular values of reactants and products. If the values are considered at constant pressure of 1 atm., the standard values for the thermodynamic functions (noted °) will be obtained as follows  

Because AG, AH, and AS are temperature dependent, they must be calculated at a certain temperature. Values of thermodynamic functions at 298.15° K (or 25° C) for many chemical compounds are tabulated. For a large number of organic compounds, the decomposition reactions at standard temperature of 25° C have negative values for the standard free enthalpy AG °. These reactions should, therefore, occur spontaneously. However, their reaction rates are in most cases slow enough (see Section 3.2) such that this assures the chemical stability of numerous organic 

As mentioned before, each term in the expression of the equation for free enthalpy depends on temperature. At lower temperatures, enthalpy will play a more important role. At higher temperatures, the entropy term T AS will be more important. For most chemical reactions, an equilibrium is attained at a certain temperature vi/hen AG = 0 due to the opposite contribution of products and reactants to the total value of AG. From the expression for AG we can write the following equation for equilibrium  

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