What Is Chemical Equilibrium?

Chemical equilibrium refers to a state in which the reaction rate of the chemical reaction is the same in the reversible reaction under certain macroscopic conditions, and the concentrations of each component of the reactant and the product are no longer changed. Judging by rGm = = 0, A is the chemical potential of substance A in the reaction. According to Le Chatelier's principle, if a balanced system is changed, the system will change to counteract the change.

There are many factors that affect chemical equilibrium, such as pressure, temperature, and concentration.

Le Chatelier principle: If a condition (concentration pressure or temperature, etc.) that affects the balance is changed, the balance moves in a direction that can weaken the change.

In the 1850s and 1960s, the basic laws of thermodynamics were clear, but some thermodynamic concepts were still vague, digital processing was cumbersome, and they could not be used to solve slightly more complex problems, such as the direction of chemical reactions. At that time, most chemists were working on organic chemistry, and some people were trying to solve the problem of the direction of chemical reactions. In addition to the law of mass action, there are other people trying to explore the direction of the reaction from other angles. Some of them have proposed some empirical rules.

During this period, Danish Thomson and Betro tried to explain the directionality of chemical reactions from the thermal effects of chemical reactions. They believe that the heat of reaction is a measure of the chemical affinity of the reactants, and that every simple or complex purification effect is accompanied by the generation of heat. Bertello elaborated the same view more clearly and called it the "maximum work principle". He believed that any kind of purely chemical change without external energy influence will proceed in the direction of producing a substance that releases the maximum energy. . Although at this time he discovered that some endothermic reactions can also proceed spontaneously, he made a subjective assumption that it was accompanied by an exothermic physical process. This wrong conclusion was finally acknowledged by him in the 1930s, at which time he limited the application of the "maximum work principle" to reactions between solids, and proposed the concept of chemical heat that is actually "free enthalpy" .

In the 1860s and 1980s, Horstmann, Le Chatelier, and Van Huff also made certain contributions in this regard. First of all, during the study of the sublimation of ammonium chloride, Horstman found that in the thermal decomposition reaction, the decomposition pressure and temperature had a certain relationship, which was in accordance with the Krapperon equation (Clautus-Clapeyron equation): dp / dt = Q / T (V'-V)

Where Q represents the heat of decomposition, and V and V 'represent the total volume before and after decomposition. Van Huff derived the following formula based on the above equation:

lnK =-(Q / RT) + c

This formula (Vanthov's equation) can be applied to any reaction process, where Q represents the heat absorbed by the system (ie, sublimation heat). Van Huff calls the above formula the principle of dynamic equilibrium and explains it. He said that any balance between two different states of matter, due to the temperature drop, moves toward the equilibrium of the two systems that generate heat. In 1874 and 1879, Mudiaier and Robin also proposed this principle, respectively. Moudier proposed that an increase in pressure is conducive to a corresponding reduction in volume. After that, Le Chatelier explained this principle further generally. He said that any system in chemical equilibrium will cause a transformation in one direction due to the change of one of the factors in the equilibrium. If this transformation is unique, it will cause a kind of and This factor changes in opposite signs.

However, Gibbs has made a significant contribution in this regard, and his position in the history of thermochemical development is extremely important. Gibbs' contribution to influence chemistry can be summarized in four aspects. First, on the basis of the second law established by Claudius et al., Gibbs led to a balanced judgment basis and restricted the entropy judgment basis correctly to the scope of the isolated system. It makes it possible to deal with general practical problems in general. Second, use the internal energy, entropy, and volume instead of temperature, pressure, and volume as variables to describe the state of the system. It was pointed out that Thomson used temperature, pressure and volume to describe the state of the system is incomplete. He advocated the equations of state that were unfamiliar to scientists at the time, and gave three-dimensional coordinate diagrams of internal energy, entropy, and volume to give surfaces that fully describe all the thermodynamic properties of the system. Third, Gibbs introduced the "concentration" variable in thermodynamics, and defined the derivative of the concentration of the component to the internal energy as "thermodynamic potential". In this way, thermodynamics can be used to deal with multi-component heterogeneous systems, and the problem of chemical equilibrium also needs to be handled. Fourth, he further discussed the balance of the system under the influence of electricity, magnetism and surface. In addition, he derived what is considered to be the simplest, most essential and most abstract thermodynamic relationship in thermodynamics, namely the phase law, and the equilibrium state is the state in which the degree of freedom indicated by the phase law is zero.

Gibbs' research on balance was published in three of his articles. In 1873, he published the first two articles in the journal of Connecticut College, which immediately caught Maxwell's attention. Gibbs's first two articles can only be said to be a preparation. In 1876 and 1878, a third article was published-"The Balance of Multiphase Matter". The article was more than 300 pages long and included more than 700 formulas. The first two articles discussed single chemical substance systems, and this article discussed multicomponent multiphase systems. Due to the introduction of thermodynamic potential, as long as the equation of state of the single-component system is slightly changed, the problem of the multi-component system can be dealt with.

Regarding Gibbs' work, Le Chatelier believes that this is the opening of a new field, and its importance can be compared with the law of immortality of quality. However, after the publication of three Gibbs articles, their significance was not recognized by most scientists. It was not translated into German by Ostwald until 1891, and published by Le Chatelle in French in 1899. After that, things changed suddenly. After Gibbs, thermodynamics can still only deal with ideal systems. At this time, American American Lewis published articles in 1901 and 1907, respectively, and proposed the concepts of "ease" and "activity". Louis talked about the concept of "escape trend", and pointed out that some thermodynamic quantities, such as temperature, pressure, concentration, and thermodynamic potential, are all scales of the escape trend measurement.

The concept of fugacity and activity put forward by Louis has helped Gibbs' theory to be usefully supplemented and developed, thereby making it possible for people to unify the deviation of the ideal system and make the actual system formally have the ideal system. Identical thermodynamic relationship.

In summary, the chemical equilibrium state refers to a reversible reaction under certain conditions, the rate of the forward reaction and the reverse reaction are equal, and the concentration of each component in the reaction mixture remains unchanged.