The behavior of organic chemical reactions as well as chemical reactions that involve inorganic compounds can usually be predicted and formulated theoretically through the use of basic functions of thermodynamics and by the use of the transition state theory of chemical reactions which is a part of the subject of chemical kinetics. These functions from thermodynamics and kinetics can be applied to the study of chemical reactions and for predicting its chemical progress either theoretically or experimentally. Usually the theoretical investigation of chemical reactions is simpler than the experimental reactions. This is so due to the usual involvement of solvents in the investigations of chemical reactions which can affect the progress of the reaction and its rate. For example SN2 reactions in organic reactions are concerted reactions that do not develop charged intermediates in the reaction. Therefore solvents that are nonpolar usually accelerate this type of reactions. On the other hand SN1 organic reactions usually involve charged intermediates. Therefore this type of reactions is usually accelerated by polar solvents. For the aforementioned discussion investigation of organic chemical reactions is preferably done in the gas phase where this solvent effect is absent.
The most well known thermodynamic functions that have effect on the progress of chemical reactions in organic chemistry are the enthalpy and the free energy of Gibbs in addition to the function that is called the entropy. In the kinetic theory of chemical organic reactions the most important concept for the progress of the reaction is the activation energy of the reaction.
The enthalpy of a chemical reaction is the difference between the enthalpies of the products of the reaction to that of the initial reactants. This is a typical thermodynamic function that measures the amount of heat that is either released or is absorbed in a given chemical reaction by the reacting components in the flask. Usually positive net value of the enthalpy between the products and the reactants of the reaction signifies an endothermic reaction. This means that the reaction has absorbed heat in the process of the reaction. This type of reactions is usually not favored thermodynamically. According to Hammond postulate this type of reaction usually is also kinetically disfavored as manifested by a high energy barrier for this reaction.
A net negative value of the enthalpy difference between the products and the reactants of the reaction usually signifies an exothermic reaction. This means that the reaction proceeds with the release of heat. This type of reactions is usually thermodynamically favored and also is thermodynamically driven. This is so due to the formation of a more stable compound than the reacting molecule. According to the Hammond postulate this type of exothermic reactions is also kinetically favored due to the low energy barrier that characterizes this type of reactions.
Thermodynamically driven reactions can also be kinetically disfavored due to the presence of high energy barrier for this type of reactions. So Hammond postulate may not be applied to all kind of reactions. Also a phenomenon that is called rapid equilibrium in organic reactions is characterized by enthalpy difference between the products and the reacting molecules that is zero in addition to the presence of very low energy barrier for this type of reactions.
The other thermodynamic function that has effect on the behavior and progress of chemical reactions is the free energy of Gibbs. This is one type of basic functions of thermodynamics that is defined by controlling the temperature and pressure of the chemical system. The free energy of Gibbs usually tells if the reaction is spontaneous of not or if it is in a state of chemical equilibrium. This will basically depend on the sign whether positive or negative of the difference in value of this function between the products and the reactants.
An equilibrium state for a given reaction usually has zero value of the free energy of Gibbs. A spontaneous reaction usually has negative net value of the free energy of Gibbs of the reaction. A positive value of the free energy of Gibbs difference between the products and the reactant usually signifies a nonspontaneous progress of the reaction.
The third thermodynamic function that has effect of the behavior of chemical reactions is the entropy. This function was developed in thermodynamics after the investigation of the behavior of thermodynamic machines and its governing thermodynamic functions. The entropy simply speaking is a measure of the disorder in a given system. The tendency in our world is to maximize the entropy. Systems that tend toward increased entropy are thermodynamically favored. This is also applicable to organic chemical reactions. Reactions that have more components in their products are entropically favored. As an example to manifest the use of this function is in phase transitions of the same compound. Usually the gas phase of a given compound has more entropy than the liquid phase. Therefore the transformation of liquid into the gas phase is also entropically driven.
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Thermodynamic and Kinetic Functions which control Chemical Reactions
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