Effect of Temperature, Pressure, and Concentration on Reaction Spontaneity

5 EFFECT OF TEMPERATURE. PRESSURE. AND CONCENTRATION ON REACTION SPONTANEITY 

A change in reaction conditions can, and often does, change the direction in which a reaction occurs spontaneously. The Gibbs-Helmholtz equation in the form 

When the temperature of a reaction system is increased, the sign of AG°, and hence the direction in which the reaction proceeds spontaneously, may or may not change. Whether it does or does not depends on the signs of AH° and AS°. The four possible situations, deduced from the Gibbs-Helmholtz equation, are summarized in Table 17.2 (p. 464). 

5 Effect of Temperature, Pressure, and Concentration on Reaction Spontaneity 

If AfT and AS° have opposite signs (Table 17.2,1 and n), it is impossible to reverse the direction of spontaneity by a change in temperature alone. The two terms Afi° and — TAS° reinforce one another. Hence AG° has the same sign at all temperatures. Reactions of this type are rather uncommon. One such reaction is 

Clearly AG° is positive at all temperatures. The reaction cannot take place spontaneously at 1 atm regardless of temperature. 

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The proposed approach leads directly to practical results such as the prediction—based upon the chemical potential—of whether or not a reaction runs spontaneously. Moreover, the chemical potential is key in dealing with physicochemical problems. Based upon this central concept, it is possible to explore many other fields. The dependence of the chemical potential upon temperature, pressure, and concentration is the gateway to the deduction of the mass action law, the calculation of equilibrium constants, solubilities, and many other data, the construction of phase diagrams, and so on. It is simple to expand the concept to colligative phenomena, diffusion processes, surface effects, electrochemical processes, etc. Furthermore, the same tools allow us to solve problems even at the atomic and molecular level, which are usually treated by quantum statistical methods. This approach allows us to eliminate many thermodynamic quantities that are traditionally used such as enthalpy H, Gibbs energy G, activity a, etc. The usage of these quantities is not excluded but superfluous in most cases. An optimized calculus results in short calculations, which are intuitively predictable and can be easily verified. 

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