Chemical equilibrium refers to the chemical equilibriumReversible reactionThe state in which the positive and reverse reaction rates of chemical reactions are equal, and the concentration of each component of reactants and products does not change any more.availableΔrGm=ΣνΑμΑ=0 judgment,μAIs that of substance A in the reactionChemical potential。according toLe Chatelier principleIf a system that has reached equilibrium is changed, the system will change accordingly to counter the change.Chemical balance is a kind of dynamic balance[1]。
In general, the change of the positive reaction rate and the reverse reaction rate in a reversible reaction is used to represent the process of establishing chemical equilibrium.The essence of chemical equilibrium: the positive reaction rate is equal to the reverse reaction rate.
The establishment of chemical equilibrium is based on reversible reaction.Reversible reaction refers to the reaction that can both go forward and backward under the same conditions.Most chemical reactions are reversible and can reach equilibrium to varying degrees.Chemical equilibrium refers to the state that in a reversible reaction under certain macro conditions, the forward and reverse reaction rates of chemical reaction are equal, and the concentrations of reactants and products no longer change.availableΔrGm=ΣνΑμΑ=0 judgment,μAIs the chemical potential of substance A in the reaction.According to Gibbs free energy criterion, whenΔrGm=At 0, the reaction reaches the maximum and is in equilibrium.according toLe Chatelier principleIf a system that has reached equilibrium is changed, the system will change accordingly to counter the change.
Generally speaking, the four chemical equilibrium are redox equilibrium, precipitation dissolution equilibrium, coordination equilibrium and acid-base equilibrium.
Chemical equilibrium plays an important role in analytical chemistry.
equilibrium constant
The chemical equilibrium constant refers to the ratio obtained by dividing the product of the stoichiometric power of the concentration of each product by the product of the stoichiometric power of the concentration of each reactant, which is a constant, when the reversible reaction reaches equilibrium at a certain temperature, whether starting from the positive reaction or the reverse reaction, and regardless of the initial concentration of the reactantKThis constant is calledChemical equilibrium constant。
Under chemical reaction conditions, the process of reversible reaction changing from one equilibrium state to another due to the change of reaction conditions is called the movement of chemical equilibrium.The fundamental reason for the shift of chemical equilibrium is that the forward and reverse reaction rates are not equal, and the result of the shift of equilibrium is that the reversible reaction reaches a new equilibrium state, at which time the forward and reverse reaction rates are again equal (may or may not be equal to the original rate).
The main factors that make the chemical balance move are concentration, temperature, pressure, etc.
Equilibrium process
1. Dynamic angle
From the kinetic point of view, at the beginning of the reaction, the concentration of reactants is larger and the concentration of products is smaller, so the positive reaction rate is greater than the reverse reaction rate.As the reaction proceeds, the concentration of reactants decreases and the concentration of products increases, so the positive reaction rate decreases and the reverse reaction rate increases.When the forward and reverse reaction rates are equal, the concentration of each substance in the system will not change, and the reaction will reach equilibrium.At this time, the system is in dynamic equilibrium, which does not mean that the reaction will stop completely
2. Micro perspective
From the microscopic point of view, it is the equilibrium phenomenon caused by the fact that the breaking rate of the chemical bond in the reactant molecule is equal to the breaking rate of the chemical bond of the product in the reversible reaction.
Equivalent equilibrium
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definition
For the same reversible reaction, under certain conditions, when the amount or concentration of reactants or products at the beginning is changed to reach equilibrium, the percentage composition of each component in the mixture is equal. Such equilibrium is called equivalent equilibrium.
Cause: balance, only related to temperature, pressure and concentration, not related to feeding sequence.
According to the gas state equation,pV=nRTIt can be found that if the temperature is kept constant and the constant volume system, as long asnThe pressure is the same and the equilibrium state is the same.If the temperature is kept constant and the system is under constant pressure, as long as after "one side down", each component n is in the same proportion and the concentration is the same, then the equilibrium state is the same.
Laws and judgments
1. Generally reversible reaction, constant temperature and constant volume, when the amount of starting reactants or products is the same through stoichiometric conversion, an equivalent balance is established.
As reaction
Establish equivalent balance under condition (A) and (B)
(A) Initial addition: 2mol SOtwo+ 1mol Otwo
(B) Initial addition: 2mol SOthree
Note: In this case, whether reactants or products, the amount of substances at the beginning must be the same as the stoichiometric ratio.
2. For general reversible reaction, at constant temperature and pressure, when the mass ratio of the initial reactant or product is the same (not necessarily required to be the same as the stoichiometric ratio), an equivalent balance is established.
As reaction
Establish equivalent balance at (C) and (D)
(C) Initial addition: 1mol SOtwo+ 1mol Otwo
(D) Initial addition: 2mol SOtwo+ 2mol Otwo
(3) For the gas reaction with constant volume before and after the reaction, when the volume ratio of the initial reactant or the product is the same (not necessarily the same as the stoichiometric ratio), the equivalent balance is established at constant temperature and volume.
As reaction
Establish equivalent balance at (E) (F)
(E) Initial addition: 1mol Htwo+ 2mol Itwo
(F) Initial addition: 2mol Htwo+ 4mol Itwo
Factors affecting chemical balance
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There are many factors affecting chemical equilibrium, such as pressure, temperature, concentration, etc.
Le Chatelier principle: if you change a condition (concentration pressure or temperature, etc.) that affects the balance, the balance will move in the direction that can weaken this change.
Concentration effect
Concentration effect
When other conditions remain unchanged, increasing the concentration of reactants or decreasing the concentration of products is conducive to the positive reaction, and the balance moves to the right;Increasing the concentration of products or decreasing the concentration of reactants is conducive to the balance of reverse reaction moving to the left.The change of the concentration of a single substance only changes the reaction rate of one reaction in the positive reaction or the reverse reaction, which leads to the unequal reaction rate of the positive reaction and the reverse reaction, thus breaking the balance.
Pressure effect
Pressure effect
For reversible reactions with unequal molecular numbers of gas reactants and gas products, when other conditions remain unchanged, increase the total pressure, and the equilibrium moves in the direction of reducing the number of gas molecules, that is, reducing the volume of gas;When the total pressure is reduced, the equilibrium moves in the direction of increasing the number of gas molecules, that is, the gas volume.If the total molecular number (total volume) of the gas before and after the reaction is unchanged, the change of pressure will not cause the shift of equilibrium.The change of pressure usually changes the forward and reverse reaction rates at the same time, which has a greater impact on the direction where the total volume of gas is large. For example, the gas involved in the positive reaction is three unit volumes, and the gas involved in the reverse reaction is two unit volumes. When the pressure is increased, the forward reaction rate will increase more, so thatvjust>vinverse, that is, the balance moves towards the positive reaction direction;When the pressure is reduced, the positive reaction rate decreases more and the equilibrium moves to the reverse reaction direction.
Temperature effect
Temperature effect
When other conditions remain unchanged, increasing the reaction temperature is conducive to endothermic reaction, and the equilibrium moves to the direction of endothermic reaction;Reducing the reaction temperature is conducive to exothermic reaction, and the balance moves towards exothermic reaction.Similar to the pressure, the change of temperature also changes the forward and reverse reaction rates at the same time. Heating up always increases the forward and reverse reaction rates at the same time, while cooling down always decreases the forward and reverse reaction rates at the same time.For endothermic reaction, the positive reaction rate increases more when the temperature rises, resulting invjust>vinverseResults;The reaction rate in the endothermic direction decreases more when the temperature drops.Different from the change of pressure, each chemical reaction will have a certain thermal effect, so changing the temperature will make the balance move, and there will be no no movement.
Catalyst effect
The catalyst can be changed to the same extentvjustandvinverse, has no effect on the movement of chemical equilibrium, but can shorten the time to reach equilibrium.
Balance judgment
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basic feature
The chemical equilibrium state has the characteristics of inverse, equal, dynamic, fixed, variable and equivalent.
Inverse: The object of chemical equilibrium study is reversible reaction.
Etc.: reversible reaction in a closed system, when in equilibrium, the forward and reverse reaction rates are equal, i.e. vjust=vinverse。(For the same substance,vjust=vinverseNumerically equal;For different substances,vA positive：vB inverse=a：b, i.e. equal to coefficient ratio)
Dynamic: during balance, the reaction is still in progress, which is dynamic balance, and the reaction is carried out to the maximum extent.（vjust=vinverseAnd not equal to 0)
Fixed: when the equilibrium state is reached, the (percent) content of each component in the reaction mixture remains unchanged, the reaction rate remains unchanged, and the conversion rate of reactants remains unchanged.
Change: Like all dynamic equilibrium, chemical equilibrium is conditional, temporary and relative. When conditions change, the equilibrium state will be destroyed, from equilibrium to imbalance, and then new equilibrium will be established under new conditions.
Same: for a certain reversible reaction, whether it starts from reactants, products or both, as long as the concentration of each component is equal, the same equilibrium state can be reached.
Basic model
There are two basic models for the chemical balance of gases, one is the constant volume device, the other is the constant pressure device.
For a reactionaA(g)+bB(g) =cC(g)+dD (g) wherea、b、c、dIs the measurement before the equation.
First category:VConstant, that is, when the volume is constant.
（1）a+b>c+dora+b<c+d
At this time, only equal balance can be established, which is based on balance, and the percentage content remains unchanged.
（2）a+b=c+d
At this time, an equivalent balance can be established, that is, the individual products added before the balance time are generated in proportion. At this time, the concentration of each substance will change with the change of the added ratio, but in both cases, the percentage content of each substance is unchanged.
Second category:PConstant, that is, the pressure of the container does not change
This kind of container is generally connected with the piston to ensure that the force of the gas in the piston remains unchanged.
（1）a+b>c+dora+b<c+d
This type of reversible reaction.Because the pressure will change with the change of reaction, but the external atmospheric pressure in this container will keep the reaction in equilibrium at all times.Therefore, under this balance, everything is constant, such as the concentration percentage of each substance.
（2）a+b=c+d
This type can be said to be all the above types of complexes.That is, nothing in the true sense remains unchanged.It's like the same reaction, one big bowl and one small bowl.
Limit judgement
(1) The concentration or volume fraction of the amount of substances in each component of the system and the amount fraction of substances remain unchanged;
(2) It is all a reversible reaction involving gas and changing the stoichiometric number before and after, and the pressure remains unchanged;
(3) It is all a reversible reaction involving gas and with the change of the stoichiometric number before and after, and the average relative molecular weight remains unchanged;
(4) For reversible reactions in which colored substances participate or form, the color does not change with time;
(5) For the same substance, the amount of the substance that breaks the chemical bond is equal to the amount of the substance that forms the chemical bond;
（6）vjust=vinverse。[2]
Research History
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In the 1950s and 1960s, the basic laws of thermodynamics had been clarified, but some thermodynamic concepts were still vague, and digital processing was tedious, which could not be used to solve slightly complicated problems, such as the direction of chemical reactions.At that time, most chemists were engaged in the research of organic chemistry, and some people tried to solve the direction of chemical reaction.In addition to the law of mass action, there are other people who try to explore the direction of reaction from other angles. Some empirical laws have been proposed.
During this period, Danish Thomson and Bertro tried to explain the direction of chemical reaction from the thermal effect of chemical reaction.They believe that reaction heat is a measure of chemical affinity of reactants. Every simple or complex pure chemical action is accompanied by the generation of heat.Betero more clearly expounded the same point of view, and called it "the principle of maximum work". He believed that any pure chemical change without external energy influence was carried out in the direction of producing the substance that released the maximum energy.Although he found that some endothermic reactions can also be carried out spontaneously, he subjectively assumed that there was an exothermic physical process.This wrong assertion was finally admitted by him in the 1930s, when he limited the application scope of the "maximum work principle" to the reactions between solids and put forward the concept of chemical heat, which is actually "free enthalpy".
In the 1960s and 1980s, HorstmannLe ChatelierandVan HoffHe has also made some contributions in this regard.First of all, in studying the sublimation process of ammonium chloride, Horstmann found that in the thermal decomposition reaction, the decomposition pressure and temperature had a certain relationship, which was consistent withClapeyron equation(Clausius Clapeyron equation): dp/dt=Q/T(V'-V)
amongQRepresents the heat of decomposition,V, V 'represents the total volume before and after decomposition.According to the above equation, Fan Hoff derived the following formula:
lnK=-(Q/RT)+c
This formula（Van Tohov equation）It can be applied to any reaction process, in whichQRepresents the heat absorbed by the system (i.eSublimation heat）。Fan Hoff called the above equation the principle of dynamic equilibrium and explained it. He said that any balance between two different states of matter moves towards the balance of the two systems generating heat due to the temperature drop.In 1874 and 1879, Moodier and Robin also put forward such principles.Moodier proposed that the increase of pressure is conducive to the reaction of corresponding reduction of volume.After that, Le Chatelier further explained this principle generally.He said that any system in chemical equilibrium will lead to a transformation in one direction due to the change of one of the multiple factors in the equilibrium. If this transformation is unique, it will lead to a change opposite to the change sign of the factor.
However, what has made outstanding contributions in this regard isGibbsHe played an extremely important role in the history of thermochemistry.Gibbs' contribution to the influence chemistry can be summarized in four aspects.First, on the basis of the second law established by Claudius et al., Gibbs derived the judgment basis of balance and correctly limited the judgment basis of entropy to the scope of isolated systems.It makes it possible to deal with general practical problems generally.Second, internal energy, entropy and volume are used to describe the system state instead of temperature, pressure and volume as variables.It is pointed out that Thomson's description of system state by temperature, pressure and volume is incomplete.He advocated the equation of state unfamiliar to scientists at that time, and gave a surface that fully describes all thermodynamic properties of the system in the three-dimensional coordinate diagram of internal energy, entropy and volume.Thirdly, Gibbs introduced the variable "concentration" in thermodynamics, and defined the derivative of the concentration of the component with respect to internal energy as "thermodynamic potential".In this way, thermodynamics can be used to deal with multi-component multiphase systems, and the problem of chemical equilibrium has conditions for treatment.Fourthly, he further discussed the equilibrium of the system under the influence of electricity, magnetism and surface.Moreover, he derived the simplest, most essential and most abstract thermodynamic relationship in thermodynamics, that is, the phase law. The equilibrium state is the state in which the degree of freedom indicated by the phase law is zero.
Gibbs's research results on balance are mainly published in his three articles.In 1873, he successively published the first two articles in the journal of Connecticut College, which immediately attracted Maxwell's attention.The first two articles of Gibbs can be said to be just a preparation. In 1876 and 1878, he published the third article - "On the Balance of Multiphase Matter" in two parts. The article is more than 300 pages long, including more than 700 formulas.The first two articles discuss single chemical system, while this article discusses multi-component multiphase system.Due to the introduction of thermodynamic potential, the problem of multi-component system can be solved by slightly changing the equation of state of single component system.
For Gibbs' work, Le Chatelier believes that this is the opening up of a new field, whose importance can be compared with the law of mass immortality.However, after the publication of Gibbs' three articles, their significance was not recognized by most scientists. It was not until 1891 that they were translated into German by Ostwald. After Le Chatelier's translation into French was published in 1899, the situation suddenly changed.After Gibbs, thermodynamics can only deal with systems in ideal state.At that time, American Louis published articles in 1901 and 1907, respectively, and put forward the concepts of "ease" and "activity".Lewis talked about the concept of "escape trend" and pointed out that some thermodynamic quantities, such as temperature, pressure, concentration, thermodynamic potential, etc. are all scales of escape trend measurement.
The concepts of fugacity and activity put forward by Lewis make Gibbs' theory get beneficial supplement and development, so that it is possible to unify the deviation of ideal system, and make the actual system have the same thermodynamic relationship with ideal system in form.
In conclusion, the chemical equilibrium state refers to the reversible reaction under certain conditions. The rate of positive reaction and reverse reaction is equal, and the concentration of each component in the reaction mixture remains unchanged.
