synonymAcid-base ion theory(acid-base ion theory) generally refers to acid-base theory
The acid-base theory is a theory to clarify what is acid-base and what is acid-base reaction.Robert Boyle first put forward the concept of acid-base. On the basis of his theory, the concept of acid-base is constantly updated and gradually improved, among which the most important ones are: acid-base ionization theory, acid-base proton theory and acid-base electron theory.There are many acid-base theories in history, but all of them are based on the views of a class of substances and reactions with common characteristics.They have their own characteristics, and they are interrelated and complementary to each other, improving people's understanding of the nature of acid-base.The concept of acid and alkali is embodied in all aspects of nature, which is of great significance to people's production and life.
Human beings have experienced a long time in understanding acid and alkali.At first, people called sour substances acid and astringent substances alkali.17th century British chemistBoyleThe plant juice is extracted as an indicator, and we have a preliminary understanding of acid and alkali.On the basis of a large number of experiments, Boyle put forward the original acid-base theory: substances whose aqueous solution can dissolve some metals will lose their original characteristics when contacted with alkali, and substances that can make litmus test solution red are called acids;The water solution of any substance has a bitter taste, which can corrode the skin, lose its original characteristics when contacting with acid, and can make litmus test solution blue is called alkali[1]。This definition is much more scientific than previous definitions, but there are still loopholes, for example, some products after the reaction of acid and base still have the properties of acid or base.Since then, scientists such as Lavoisier, David, and Libich have further supplemented this view, gradually touching on the essence of acid-base, but still have not been able to give a perfect theory.
Acid-base ionization theory
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Swedish scientistArenius(Arrhenius) summarized a large number of facts and put forward the essential point of view on acid-base - acid-base ionization theory (Arrhenius acid-base theory) in 1887.In the acid-base ionization theory, acid-base is defined as: all cations ionized in aqueous solution are H+Its substance is called acid;All ionized anions are OH-The substance of H is called alkali, and the essence of acid-base reaction is H+And OH-Reaction of combining to generate water[1]。(The form of hydrogen ion in water isHydrous hydrogen ion（HthreeO+）, but for the convenience of writing, it can be abbreviated as H without causing confusion+）
Scale
In the acid-base ionization theory, the acid-base property of aqueous solution is measured by the concentration of hydrogen ion and hydroxyl ion in the solution: the greater the concentration of hydrogen ion, the stronger the acidity;The greater the concentration of hydroxyl ion, the stronger the alkalinity.At 298K, it always exists in dilute solution
Therefore, when the concentration of one of the hydrogen ions or hydroxyl ions increases, the concentration of the other will inevitably decrease. Acids and bases are opposite.
For convenience of writing, define
, thenpHThe smaller, the stronger the acidity of the solution;pHThe greater the solution, the stronger the alkalinity.
Weak acid and strong acid
According to acid-base ionization theoryIonization degreeNot necessarily the same. Some reach more than 90%, some only 1%, so there arestrong acidandweak acid、Strong baseandweak alkaliThe strong acid and strong base are completely ionized in aqueous solution, while the weak acid and weak base are partially ionized[1]。The concentration of hydrogen ion (hydroxide ion) in strong acid (alkali) solution is equal to the concentration of strong acid itself, while the concentration of hydrogen ion (hydroxide ion) in weak acid (alkali) solution is less than that of weak acid.
Ionization equilibrium exists in acid. If the general formula of acid is HA, thenequilibrium constant
，
, strongly acidicKa>1，pKa<0, otherwise it is weak acid.
Progressive
one
The theory of acid-base ionization reveals the essence of acid-base reaction more profoundly;
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Since H in aqueous solution+And OH-The concentration of is measurable, so this theory is the first time to describe the properties of acids and bases and their behaviors in chemical reactions from a quantitative perspective. The acid-base ionization theory is applicable topHCalculation, ionization degree calculation, buffer solution calculation, solubility calculation, etc., and the calculation accuracy is relatively high, so it is still a very practical theory.
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Arenius also pointed out that,Polybasic acidAnd polybasic inaqueous solutionThe acid that can ionize multiple hydrogen ions is a polybasic acid, and the base that can ionize multiple hydroxyl ions is a polybasic. They are ionized in several steps[1]。
limitations
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In the absence of water, acid-base reactions can also occur, such asHydrogen chlorideGas andammoniaReactive formationammonium chlorideHowever, these substances are not ionized, and ionization theory cannot discuss such reactions.
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Dissolve ammonium chloride inliquid ammoniaThe solution has the characteristics of acid, can react with metal to produce hydrogen, and can change the color of indicatorNonaqueous solventThere is no ionized H in+The ionization theory cannot explain this.
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Sodium carbonate inaqueous solutionOH is not ionized from-However, it is alkaline. According to the ionization theory, carbonate ions occur in waterhydrolysisTo.
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In explaining NHthreeWhen the alkalinity of aqueous solution was caused, people once mistakenly believed that NH was formed firstfourOH，And then ionize out OH-[1]。
To solve these problems, the concept of acid-base must be separated from solvent (including water and otherNonaqueous solvent）It exists independently.At the same time, the concept of acid and base cannot exist in isolation from chemical reactions. Acid and base are interdependent and both are relative.
Acid base solvent theory
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Franklin put forward the acid-base solvent theory in 1905, which includes: the solute that produces the characteristic cation of the solvent in the solvent is called acid, and the solute that produces the characteristic anion of the solvent is called base[2]。
For example, the following balance exists in liquid ammonia:
Therefore, NH is ionized out of liquid ammoniafour+It is acid, such as NHfourCl，Ionization out of NHtwo-It is alkali, such as NaNH2。
Liquid NtwoOfourThe following balances exist in:
So in liquid NtwoOfourNO ionized out of+Acid, such as NOCl, ionizes NOthree-Alkali, such as AgNO3。
In the acid-base solvent theory, acid and base are not absolute. An acid in one solvent may be a base in another solvent.
Connection with ionization theory
The theory of acid-base solvents can be seen as the expansion of acid-base ionization theory in non-aqueous solvents -- the H produced by the autoionization of water in the acid-base ionization theory+And OH-In the acid-base solvent theory, it becomes the anion and cation ionized by the solvent self coupling. The acid-base solvent theory further develops the acid-base ionization theory, and has some influence on the later theory[3]。
Classification of solvents
According to the autoionization of solvents, solvents can be divided into two categories[3]：
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Proton type solvent: solvent with proton participation in the process of self coupling ionization, such as hydrogen fluoride, water, liquid ammonia, etc;
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Non proton solvent: solvent without proton in the process of autoionization, such as nitrogen tetroxide, sulfur trioxide, etc.
According to the acidity and alkalinity of the solvent, the solvent can be divided into[3]：
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Amphoteric neutral solvent: it can be used as both acid and alkali solvent.When the solute is strong acid, the solvent is alkaline;When the solute is a strong alkali, the solvent is acidic.[6]Common examples are water and alcohol;
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Acid solvent: This solvent is also an amphoteric solvent, but it is more acidic than water.Common examples include acetic acid, sulfuric acid, etc;
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Alkaline solvent: This solvent is also an amphoteric solvent, but it is more alkaline than water.Common examples are liquid ammonia, ethylene diamine, etc.
Leveling solvent and distinguishing solvent
In the acid-base ionization theory, the acid strength of strong acids cannot be distinguished because they are completely ionized in water, and it is impossible to distinguish which acid is stronger.However, a lot of experimental facts show that the strength of strong acids is still different. For example, when acetic acid is used as the solvent, it can be measured that the acidity of perchloric acid, sulfuric acid and nitric acid is HClOfour>HtwoSOfour>HNOthreeThe effect of such a solvent that can distinguish the strength of acid or alkali is calledDiscriminative effectThe corresponding solvent is called acidDifferentiate solvents。Correspondingly, in a relatively alkaline solvent such as water, the acidity of strong acids is reflected by hydrated hydrogen ions. The alkalinity of water eliminates the acidity difference of strong acids. This effect of flattening acids of different strengths to the level of solvated protons is calledFlattening effect, the corresponding solvent is called flattened acidLeveling solvent[3]。
Progressive
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Expanding the range of acid and alkali[3]In different solvents, there are corresponding different acids and bases;
It cannot indicate the shape of CaO+SOthree=CaSOfourThe reaction not carried out in solvent;
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The acid-base reaction in non ionizing solvent systems such as benzene, chloroform and ether cannot be explained.
Acid base proton theory
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J.N. Bronsted and Lowry put forward the acid base proton theory (Bronsted acid base theory) in 1923, and the corresponding acid base definition is:proton（H+）All substances are acids;All substances that can accept protons are bases.[1]It can be seen that the scope of acid and base is no longer limited to electrically neutral molecules or ionic compounds, and charged ions can also be called "acids" or "bases".If a substance can both give and receive protons, it is both an acid and a base, and is usually called "acid-base amphoteric substance". In order to distinguish the acid-base proton theory, sometimes the "acid" in this theory is called "protonic acid", and the "base" in this theory is called "protonic base".
Acid-base conjugation
Acid (HA) and base (A-）The following relationships exist between:
The acid and base in the above formula are calledConjugated acid base pair, base is the conjugate base of acid, and acid is the conjugate acid of base[1]This formula shows that acid and base are interdependent.
At the same time, it is easy to prove that in the aqueous solution,
Therefore, the stronger the acidity of the conjugate acid, the weaker the alkalinity of the conjugate base. On the contrary, the weaker the acidity of the conjugate acid, the stronger the alkalinity of the conjugate base.
Acid-base reaction
In the following reactions:
HCl and HthreeO+Can be releasedproton, they are all acids;HtwoO and Cl-Both can accept protons, and they are bases:
The above reaction is calledProton transferreaction[1], which can be expressed by a general formula:
acidone+Alkalitwo→ Acidtwo+Alkalione
acidoneAnd alkalione, acidtwoAnd alkalitwoIt's two pairsConjugated acid base pair。The above formula shows that when one molecule or ion loses its proton to play the role of acid, another molecule or ion must accept the proton to play the role of base, and a single pair of conjugated acids and bases cannot undergo acid base reaction.The shape is like "acid=alkali+H+”The reaction of cannot occur, calledAcid-base semireaction[1]。
In order to simplify writing, H in proton transfer reaction can bethreeO+Abbreviated as H+, get a reaction formula similar to the acid base semi reaction, for example:
This reaction formula is actually an abbreviation for the reaction of hydrogen chloride with water, not an acid-base semi reaction, although it looks exactly the same as the acid-base semi reaction.When it is necessary to distinguish between the abbreviated form and the semi reactive form, it is not difficult to analyze it in context.
It can also be seen from the general formula that whether a substance is an acid or a base depends on its role in the acid-base reaction.HCOthree-Released when reacting with NaOHprotonAt this time, it is an acid, HCOthree-When reacting with HCl, it receives protons, which is a base.For example, nitric acid is an acid in water, but it is a base when dissolved in pure sulfuric acid.It can be seen that the concepts of acid and base are relative.
Progressive
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The range of acid is expanded.As long as it can be releasedprotonNo matter in water solution or not, no matter in ion or neutral molecule, they are all acids, such as NHfour+、HCOthree-、HSOfour-、HS-、HPOfour2-、HtwoO。
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The scope of alkali is expanded.NHthree、NatwoCO3、F-、Cl-、Br-、I-、HSOfour-、SOfour2-Such substances can accept protons and are all bases.
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The range of acid-base reaction is expanded.In the acid-base ionization theory, the products of the reaction between acid and base must be "salt" and water, but in the acid-base proton theory, there is no such requirement.Neutralization in acid-base ionization theory, strong acid replacing weak acid, acid-base ionization, hydrolysis of salts, reaction between ammonia and hydrogen chloride, etc. are all acid-base reactions[1]。
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The relativity of acid and base is put forward.
limitations
Acid base proton theory still has unexplained reactions[3], such as the following reactions:
In this reaction, SOthreeIt obviously has the property of acid, but it does not release protons;CaO is obviously alkaline, but it does not accept protons.
For another example, experiments have proved that many compounds without hydrogen (they cannot release protons), such as AlClthree、BClthree、SnClfourBoth can react with bases, but the acid base proton theory does not consider them to be acids.
As another example, liquid NtwoOfourThe following balances exist:
The reaction is as follows:
This reaction is very similar to the acid-base reaction, but because there is no proton transfer, the acid-base proton theory cannot deal with it.Therefore, the acid-base theory needs further improvement.
Acid-base electron theory
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1923 American chemistGilbert Newton Lewis(Gilbert Newton Lewis) pointed out that there is no reason to believe that acids must be limited to hydrogen containing compounds, and his understanding stems from the fact that oxidation reaction does not necessarily involve oxygen.Louis isCovalent Bond Theory The founder of, he proposed the acid-base electronic theory (Lewis acid-base theory) with the view of structure: acid is the acceptor of electrons, and base is the donor of electrons.Acid base reaction is a process in which an acid receives a pair of electrons from a base to form a coordination bond and obtain an acid base adduct. The acid base reaction under this theoretical system is calledAcid-base addition reaction[2]。The general formula is:
Generally, the "acid" in the acid-base electronic theory is called Lewis acid, and the "base" is called Lewis base to show the difference.
Acid base reaction type
In the acid-base electronic theory, there are four basic reaction types[3]：
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Acid base addition reaction: the reaction of acid and base to produce acid base adducts;
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Acid substitution reaction: the reaction in which a strong acid displaces a weak acid from another acid-base adduct formed by a weak acid;
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Base substitution reaction: the reaction in which a strong base displaces a weak base from another acid-base adduct formed by a weak base;
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Double substitution reaction: the reaction in which two acid-base adducts exchange components to form two more stable acid-base adducts.
Comparison with proton theory
Lewis base includes all Brownst bases, while Lewis acid does not necessarily include Brownst acid[2]。For example, in the proton acid base theory, HCl is a proton acid, while in the Lewis acid base theory, HCl is a Lewis acid - H+And Lewis base - Cl-Acid base adduct formed by combination.
status
Lewis acid base reaction is one of the three basic reaction types in chemistry[3](The other two are redox reaction and free radical reaction).
Organic reaction mechanism
Lewis acid-base theory believes that many organic reactions are also acid-base reactions, such as CHthree+、CtwoHfive+、CHthreeCO+Both are acids, respectively with alkali H-、OH-、C2H5O-Synthetic adduct CHfour、CtwoHfiveOH、CHthreeCOOCtwoHfive。In organic reaction, Lewis acid isElectrophilic reagentLewis base isNucleophile。[2]
Progressive
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The scope of acid and base is further expanded, which can indicate that there is noprotonAcidity and alkalinity of substances, including metal cationsElectron deficient compound, polar double bond molecules (e.gcarbonylMolecules), valence layer expandable atomic compounds (complexes of some elements in the P region), neutral molecules with lone pair electrons, molecules containing C=C bonds (e.gChase salt）, the most widely used[1]；
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The scope of acid-base reaction is expanded, and the essence of acid-base reaction is pointed out more profoundly.
limitations
It is difficult to accurately describe the strength of acid-base reaction and judge the direction and limit of acid-base reaction[1]。
Soft and hard acid-base theory
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On the basis of previous work, Ralph G. Pearson put forward the soft and hard acid-base theory (HSAB) in 1963: small size, high positive charge number,PolarizabilityLowCentral atomIt is called hard acid. The central atom with large volume, low positive charge and high polarizability is called soft acid.It is difficult to oxidize those with high electronegativity and low polarizationCoordination atombe calledHard alkaliAnd vice versaSoft alkali；[2]Other acids and bases are boundary acids and bases.
Acid base hardness
In 1983, Pearson and Robert Parr jointly proposed a method to calculate the soft hardness of acid-base. The calculated soft hardness is calledChemical hardness[3]。
Reaction rule
Pearson's acid-base reaction law is: "hard acid has priority overHard alkaliCombination, soft acid has priority overSoft alkaliCombine. "[3]Although this is an empirical rule, the experiment proves that the following rule is completely consistent with HSAB theory:
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The substitution reactions tend to form hard hard and soft soft compounds.
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Soft soft and hard hard compounds are relatively stable, while soft hard compounds are not stable enough.
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Hard solvent preferentially dissolves hard solute, while soft solvent preferentially dissolves soft solute. Many organic compounds are not easily soluble in water, because water isHard alkali。
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Explain the reaction mechanism: the Friedel Crafts alkylation reaction in the organic reaction uses anhydrous aluminum chloride (AlClthree）As a catalyst, its mechanism is AlClthreeIt is a hard acid, and the hard base Cl in RCl-It is activated by combination.
Progressive
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Complexes suitable for discussing metal ions[3]；
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Accurately predict the direction of Lewis acid-base reaction[3]；
The scope of application cannot include the entire Lewis acid-base system[3]；
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It is only a qualitative rule, and the degree of reaction cannot be calculated quantitatively（K）；
matters needing attention
The hardness and softness of acid-base are different from the strength of acid-base. When dealing with acid-base reaction, it is necessary to comprehensively consider the hardness and softness of reactants.When the softness and hardness of acid and base are equal to the acid-base property, the softness and hardness of acid and base play a leading role in the direction of reaction[3]。
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Historically, acid-base theories have their own characteristics, but they are not completely irrelevant. The following Venn diagram shows the relationship between the four most commonly used acid-base theories[3](HSAB is regarded as a supplement to electronic theory and is not marked in the figure):
Relationship between acid-base theories[4]
It can be seen from the figure that the range of acid base electronic theory is the widest.In fact, almost all other existing theoretical concepts are included, and all acid-base reactions can be treated with acid-base electronic theory (combined with HSAB), but when quantitative calculation is required, electronic theory is powerless.
The acid-base proton theory has greater advantages in dealing with acid-base reactions with proton transfer, because compared with the ionization theory, the application scope of the proton theory is wider;Compared with the electronic theory, the quantitative calculation of the proton theory is more complete.
The acid-base solvent theory can be used to deal with acid-base reactions in aprotic solvents, and some quantitative calculations can also be done. However, due to its large restrictions and relatively small practicality, it is rarely used.
The acid-base ionization theory has the advantage of being easy to understand.In dilute aqueous solution, the calculation results obtained by ionization theory are identical with those obtained by proton theory, and it is simpler than proton theory in dealing with some calculation problems, so it is still widely used in quantitative calculation.
Therefore, when dealing with acid-base problems, it is necessary to reasonably select acid-base theory in order to draw correct conclusions, optimize the treatment process and save time.
In addition, after synthesizing the above theories, it is not difficult to find that in the above acid-base theories, most acid-base reactions are not redox reactions, because in these theories, acid-base reactions are always accompanied by the breaking of ionic bonds or covalent bondsHeteroschisisThere is basically no gain or loss of electrons or transfer of electron pairs.
Unusual theory
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Definition of Usanovich
This acid base definition comes from Mikhail Usanovich, a Russian chemist.According to this definition, as long as negative charge can be accepted or releasedpositive chargeIs acid, and vice versa.Because this definition is somewhat coincident with the definition of redox, chemists are not very inclined to use this definition.This is because redox mainly focuses on the physical electron transfer process, rather than the bond formation and fracture process, although it is impossible to completely distinguish the two.[5]
Definition of Lux Flood
This definition was proposed by German chemist Hermann Lux in 1939, and then further revised by H.kon Flood in 1947, mainly used in the geochemical and electrochemical research of modern molten salts.According to this theoretical definition: can accept oxygen ion O2-The substance that can provide oxygen ion is alkali[5]。
