Acid-base reaction theories
Encyclopedia
An acid–base reaction is a chemical reaction
that occurs between an acid
and a base
. Several concepts that provide alternative definitions for the reaction mechanisms involved and their application in solving related problems exist. Despite several differences in definitions, their importance becomes apparent as different methods of analysis when applied to acid–base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent. The first of these scientific concepts of acids and bases was provided by the French chemist
Antoine Lavoisier
, circa 1776.
circa 1776. Since Lavoisier's knowledge of strong acids was mainly restricted to oxoacid
s, such as (nitric acid) and (sulphuric acid), which tend to contain central atoms in high oxidation states
surrounded by oxygen, and since he was not aware of the true composition of the hydrohalic acids
(HF
, HCl
, HBr
, and HI
), he defined acids in terms of their containing oxygen
, which in fact he named from Greek words meaning "acid-former" (from the Greek
οξυς (oxys) meaning "acid" or "sharp" and γεινομαι (geinomai) meaning "engender"). The Lavoisier definition was held as absolute truth for over 30 years, until the 1810 article and subsequent lectures by Sir Humphry Davy
in which he proved the lack of oxygen in H2S
, H2Te
, and the hydrohalic acids. However, Davy failed to develop a new theory, concluding that "acidity does not depend upon any particular elementary substance, but upon peculiar arrangement of various substances". One notable modification of oxygen theory was provided by Berzelius
, who stated that acids are oxides of nonmetals while bases are oxides of metals.
circa 1838, based on his extensive works on the chemical composition of organic acid
s. This finished the doctrinal shift from oxygen-based acids to hydrogen-based acids, started by Davy. According to Liebig, an acid is a hydrogen-containing substance in which the hydrogen could be replaced by a metal. Liebig's definition, while completely empirical, remained in use for almost 50 years until the adoption of the Arrhenius definition.
The Arrhenius definition of acid–base reactions is a development of the hydrogen theory of acids, devised by Svante Arrhenius
, which was used to provide a modern definition of acids and bases that followed from his work with Friedrich Wilhelm Ostwald in establishing the presence of ions in aqueous solution
in 1884, and led to Arrhenius receiving the Nobel Prize in Chemistry
in 1903 for "recognition of the extraordinary services... rendered to the advancement of chemistry by his electrolytic theory of dissociation".
As defined by Arrhenius, acid–base reactions are characterized by Arrhenius acids, which dissociate
in aqueous solution to form hydrogen ions , and Arrhenius bases, which form hydroxide ions. More recent IUPAC recommendations now suggest the newer term "hydronium" be used in favor of the older accepted term "oxonium" to illustrate reaction mechanisms such as those defined in the Brønsted–Lowry and solvent system definitions more clearly, with the Arrhenius definition serving as a simple general outline of acid–base character. The Arrhenius definition can be summarised as "Arrhenius acids form hydrogen ions in aqueous solution with Arrhenius bases forming hydroxide ions."
The universal aqueous acid–base definition of the Arrhenius concept is described as the formation of water from hydrogen and hydroxide ions, or hydrogen ions and hydroxide ions from the dissociation of an acid and base in aqueous solution:
(In modern times, the use of is regarded as a shorthand for
, since it is now known that the bare proton does not exist as a free species in solution.)
This leads to the definition that in Arrhenius acid–base reactions, a salt and water is formed from the reaction between an acid and a base. In other words, this is a neutralization reaction.
The positive ion from a base forms a salt with the negative ion from an acid. For example, two moles
of sodium ion from the base sodium hydroxide (NaOH) combine with one mole of sulfate ion from sulfuric acid to form one mole of sodium sulfate
. Two moles of water are also formed.
The Arrhenius definitions of acidity and alkalinity are restricted to aqueous solutions, and refer to the concentration of the solvent ions. Under this definition, pure or HCl dissolved in toluene are not acidic, and molten KOH and solutions of sodium amide in liquid ammonia are not alkaline.
Germann pointed out that in many solvents there is a certain concentration of a positive species, solvonium (earlier lyonium) cations and negative species, solvate (earlier lyate) anions, in equilibrium with the neutral solvent molecules. For example, water and ammonia
undergo such dissociation into hydronium
and hydroxide
, and ammonium
and amide
, respectively:
Some aprotic systems also undergo such dissociation, such as dinitrogen tetroxide
into nitrosonium
and nitrate
, antimony trichloride
into dichloroantimonium and tetrachloroantimonate, and phosgene
into chlorocarboxonium and chloride
.
A solute causing an increase in the concentration of the solvonium ions and a decrease in the solvate ions is defined as an acid and one causing the reverse is defined as a base. Thus, in liquid ammonia, (supplying ) is a strong base, and (supplying ) is a strong acid. In liquid sulfur dioxide
, thionyl
compounds (supplying ) behave as acids, and sulfites (supplying ) behave as bases.
The non-aqueous acid–base reactions in liquid ammonia are similar to the reactions in water:
Nitric acid can be a base in liquid sulfuric acid:
The unique strength of this definition shows in describing the reactions in aprotic solvents, for example in liquid :
Since solvent-system definition depends on the solvent as well as on the compound itself, the same compound can change its role depending on the choice of the solvent. Thus, is a strong acid in water, a weak acid in acetic acid, and a weak base in fluorosulfonic acid. This was seen as both a strength and a weakness, since some substances, such as and , were felt to be acidic or basic on their own right. On the other hand, solvent system theory was criticized as too general to be useful; it was felt that there is something intrinsically acidic about hydrogen compounds, not shared by non-hydrogenic solvonium salts.
in Denmark and Martin Lowry
in England, is based upon the idea of protonation
of bases through the de-protonation
of acids – that is, the ability of acids to "donate" hydrogen ions (H+) or protons to bases, which "accept" them. Unlike the previous definitions, the Brønsted–Lowry definition does not refer to the formation of salt and solvent, but instead to the formation of conjugate acids and conjugate bases, produced by the transfer of a proton from the acid to the base. In this approach, acids and bases are fundamentally different in behavior from salts, which are seen as electrolytes, subject to the theories of Debye
, Onsager
, and others. An acid and a base react not to produce a salt and a solvent, but to form a new acid and a new base. The concept of neutralization is thus absent.
According to Brønsted–Lowry definition, an acid is a compound that can donate a proton, and a base is a compound that can receive a proton. An acid–base reaction is, thus, the removal of a hydrogen ion from the acid and its addition to the base. This does not refer to the removal of a proton from the nucleus of an atom, which would require levels of energy not attainable through the simple dissociation of acids, but to removal of a hydrogen ion .
The removal of a proton (hydrogen ion) from an acid produces its conjugate base, which is the acid with a hydrogen ion removed, and the reception of a proton by a base produces its conjugate acid, which is the base with a hydrogen ion added.
For example, the removal of from hydrochloric acid
(HCl) produces the chloride ion , the conjugate base of the acid:
The addition of to the hydroxide ion , a base, produces water , its conjugate acid:
Although Brønsted–Lowry acid–base behavior is formally independent of any solvent, it encompasses Arrhenius and solvent system definitions in an unenforced way. For example, protonation of ammonia, a base, gives ammonium ion, its conjugate acid:
The reaction of ammonia, a base, with acetic acid
in absence of water can be described to give ammonium cation, an acid, and acetate anion, a base:
This definition also explains the dissociation of water into low concentrations of hydronium and hydroxide ions:
Water, being amphoteric, can act as both an acid and a base; here, one molecule of water acts as an acid, donating a ion and forming the conjugate base, , and a second molecule of water acts as a base, accepting the ion and forming the conjugate acid, .
Acid dissociation and acid hydrolysis are seen to be entirely similar phenomena:
as are basic dissociation and basic hydrolysis:
Thus, the general formula for acid–base reactions according to the Brønsted–Lowry definition is:
where AH represents the acid, B represents the base, represents the conjugate acid of B, and represents the conjugate base of AH.
Although Brønsted–Lowry calls hydrogen-containing substances like acids, and are not bases but salts containing the bases and . Also, some substances, which many chemists considered to be acids, such as or , are excluded from this classification due to lack of hydrogen. Gilbert Lewis wrote in 1938, "To restrict the group of acids to those substances that contain hydrogen interferes as seriously with the systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen."
in 1923, in the same year as Brønsted–Lowry, but it was not elaborated by him until 1938. Instead of defining acid–base reactions in terms of protons or other bonded substances, the Lewis definition defines a base (referred to as a Lewis base) to be a compound that can donate an electron pair
, and an acid (a Lewis acid) to be a compound that can receive this electron pair.
In this system, an acid does not exchange atoms with a base, but combines with it. For example, consider this classical aqueous acid–base reaction:
The Lewis definition does not regard this reaction as the formation of salt and water or the transfer of from HCl to . Instead, it regards the acid to be the ion itself, and the base to be the ion, which has an unshared electron pair. Therefore, the acid–base reaction here, according to the Lewis definition, is the donation of the electron pair from to the ion. This forms a covalent bond between and , thus producing water .
By treating acid–base reactions in terms of electron pairs instead of specific substances, the Lewis definition can be applied to reactions that do not fall under other definitions of acid–base reactions. For example, a silver cation behaves as an acid with respect to ammonia, which behaves as a base, in the following reaction:
The result of this reaction is the formation of an ammonia–silver adduct
.
In reactions between Lewis acid
s and bases, there is the formation of an adduct when the highest occupied molecular orbital (HOMO
) of a molecule, such as with available lone electron pair(s) donates lone pairs of electrons to the electron-deficient molecule's lowest unoccupied molecular orbital (LUMO
) through a co-ordinate covalent bond; in such a reaction, the HOMO-interacting molecule acts as a base, and the LUMO-interacting molecule acts as an acid. In highly-polar molecules, such as boron trifluoride
, the most electronegative
element pulls electrons towards its own orbitals, providing a more positive charge on the less-electronegative element and a difference in its electronic structure due to the axial or equatorial orbiting positions of its electrons, causing repulsive effects from lone pair – bonding pair (Lp–Bp) interactions between bonded atoms in excess of those already provided by bonding pair – bonding pair (Bp–Bp) interactions. Adducts involving metal ions are referred to as co-ordination compounds.
(oxidation-reduction) as a special case of acid-base reactions, and so did not become widespread, despite being easier to understand than Lewis theory, which required detailed familiarity with atomic structure.
Some examples of Usanovich acid-base reactions include:
in 1939, further improved by Håkon Flood
circa 1947 and is still used in modern geochemistry
and electrochemistry
of molten salt
s. This definition describes an acid as an oxide ion acceptor and a base as an oxide ion donor. For example:
proposed an advanced qualitative concept known as Hard Soft Acid Base principle, later made quantitative with help of Robert Parr
in 1984. 'Hard' applies to species that are small, have high charge states, and are weakly polarizable. 'Soft' applies to species that are large, have low charge states and are strongly polarizable. Acids and bases interact, and the most stable interactions are hard–hard and soft–soft. This theory has found use in organic and inorganic chemistry.
. When an acid reacts with an alkali it forms a metal salt and water. Acid–alkali reactions are also a type of neutralization reaction.
In general, acid–alkali reactions can be simplified to
by omitting spectator ions.
Acids are in general pure substances that contain hydrogen ions
or cause them to be produced in solutions. Hydrochloric acid and sulfuric acid are common examples. In water, these break apart into ions:
An alkali is a base, a base that contains a metal from column 1 or 2 of the periodic table (the alkali metals or the alkaline earth metals). Alkalis may be defined as soluble bases, which means they must be able to dissolve in water. In general, bases are defined as substances that contain hydroxide ion or produce it in solution. Therefore, one may also speak of hydroxide bases that dissolve in water, and thus these would also be alkalis. Some examples, then, of alkalis would be sodium hydroxide (NaOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)2), and calcium hydroxide (Ca(OH)2). Note that only hydroxides with an alkali metal —column 1 — are very soluble in water; hydroxides with an alkaline earth metal — column 2 — are not as soluble. Some sources will even say the alkaline earth metal hydroxides are insoluble.
To produce hydroxide ions in water, the alkali breaks apart into ions as below:
However, alkalies may also have a broader definition that includes carbonates bonded to a column 1 metal, an ammonium ion , or an amine (NHx radical) as the positive ion. Examples of alkalis would then also include , , and (NH4)2CO3.
There seems to be conflicting information on whether acid-base reactions are neutralization reactions. Some sources define a neutralization reaction as the reaction between an acid and a base that produces a salt and water. Yet, in the book Chemical Misconceptions: Prevention, Diagnosis and Cure by K. Tabor (2002), it is noted that “the term neutralization is usually reserved for acid–alkali reactions.” Thus, this does not make acid–alkali a type of neutralization reaction, but the only kind of neutralization reaction.
There are many uses of neutralization reactions that are acid-alkali reactions. A very common use is antacid tablets. These are designed to neutralize excess stomach acid (HCl) that may be causing discomfort in the stomach or lower esophagus. Also in the digestive tract, neutralization reactions are used when food is moved from the stomach to the intestines. In order for the nutrients to be absorbed through the intestinal wall, an alkaline environment is needed, so the pancreas produce an antacid bicarbonate to cause this transformation to occur.
Another common use, though perhaps not as widely known, is in fertilizers and control of soil pH. Slaked lime (calcium hydroxide) or limestone (calcium carbonate) may be worked into soil that is too acidic for plant growth. Fertilizers that improve plant growth are made by neutralizing sulfuric acid (H2SO4) or nitric acid (HNO3) with ammonia gas (NH3), making ammonium sulfate
or ammonium nitrate. These are salts utilized in the fertilizer.
Industrially, a by-product of the burning of coal, sulfur dioxide
gas may combine with water vapor in the air to eventually produce sulfuric acid, which falls as acid rain. To prevent the sulfur dioxide from being released, a device known as a scrubber gleans the gas from smoke stacks. This device first blows calcium carbonate into the combustion chamber where it decomposes into calcium oxide (lime) and carbon dioxide. This lime then reacts with the sulfur dioxide produced forming calcium sulfite. A suspension of lime is then injected into the mixture to produce a slurry, which removes the calcium sulfite
and any remaining unreacted sulfur dioxide.
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
that occurs between an acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
and a base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
. Several concepts that provide alternative definitions for the reaction mechanisms involved and their application in solving related problems exist. Despite several differences in definitions, their importance becomes apparent as different methods of analysis when applied to acid–base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent. The first of these scientific concepts of acids and bases was provided by the French chemist
Chemist
A chemist is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties such as density and acidity. Chemists carefully describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms...
Antoine Lavoisier
Antoine Lavoisier
Antoine-Laurent de Lavoisier , the "father of modern chemistry", was a French nobleman prominent in the histories of chemistry and biology...
, circa 1776.
Lavoisier's oxygen theory of acids
The first scientific concept of acids and bases was provided by Antoine LavoisierAntoine Lavoisier
Antoine-Laurent de Lavoisier , the "father of modern chemistry", was a French nobleman prominent in the histories of chemistry and biology...
circa 1776. Since Lavoisier's knowledge of strong acids was mainly restricted to oxoacid
Oxoacid
An oxoacid is an acid that contains oxygen. To be more specific, it is an acid that:#contains oxygen#contains at least one other element#has at least one hydrogen atom bound to oxygen#forms an ion by the loss of one or more protons....
s, such as (nitric acid) and (sulphuric acid), which tend to contain central atoms in high oxidation states
Oxidation number
In coordination chemistry, the oxidation number of a central atom in a coordination compound is the charge that it would have if all the ligands were removed along with the electron pairs that were shared with the central atom. Oxidation numbers are often confused with oxidation states.The...
surrounded by oxygen, and since he was not aware of the true composition of the hydrohalic acids
Hydrogen halide
Hydrogen halides are acids resulting from the chemical reaction of hydrogen with one of the halogen elements , which are found in Group 17 of the periodic table. Astatine is not included in the list because it is very rare, unstable and not found as the acid in substantial quantities...
(HF
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
, HCl
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
, HBr
Hydrogen bromide
Hydrogen bromide is the diatomic molecule HBr. HBr is a gas at standard conditions. Hydrobromic acid forms upon dissolving HBr in water. Conversely, HBr can be liberated from hydrobromic acid solutions with the addition of a dehydration agent, but not by distillation. Hydrogen bromide and...
, and HI
Hydrogen iodide
Hydrogen iodide is a diatomic molecule. Aqueous solutions of HI are known as iohydroic acid or hydroiodic acid, a strong acid. Gas and aqueous solution are interconvertible...
), he defined acids in terms of their containing oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, which in fact he named from Greek words meaning "acid-former" (from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
οξυς (oxys) meaning "acid" or "sharp" and γεινομαι (geinomai) meaning "engender"). The Lavoisier definition was held as absolute truth for over 30 years, until the 1810 article and subsequent lectures by Sir Humphry Davy
Humphry Davy
Sir Humphry Davy, 1st Baronet FRS MRIA was a British chemist and inventor. He is probably best remembered today for his discoveries of several alkali and alkaline earth metals, as well as contributions to the discoveries of the elemental nature of chlorine and iodine...
in which he proved the lack of oxygen in H2S
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
, H2Te
Hydrogen telluride
Hydrogen telluride is the inorganic compound with the formula H2Te. The simplest hydride of tellurium, it is rarely encountered because of it decomposes rapidly to the constituent elements. Most compounds with Te-H bonds are unstable with respect to loss of H2. H2Te is chemically and structurally...
, and the hydrohalic acids. However, Davy failed to develop a new theory, concluding that "acidity does not depend upon any particular elementary substance, but upon peculiar arrangement of various substances". One notable modification of oxygen theory was provided by Berzelius
Berzelius
Berzelius is a secret society at Yale University named for the Swedish scientist Jöns Jakob Berzelius, considered one of the founding fathers of modern chemistry...
, who stated that acids are oxides of nonmetals while bases are oxides of metals.
Liebig's hydrogen theory of acids
This definition was proposed by Justus von LiebigJustus von Liebig
Justus von Liebig was a German chemist who made major contributions to agricultural and biological chemistry, and worked on the organization of organic chemistry. As a professor, he devised the modern laboratory-oriented teaching method, and for such innovations, he is regarded as one of the...
circa 1838, based on his extensive works on the chemical composition of organic acid
Organic acid
An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are relatively stronger acids. The relative stability of the conjugate...
s. This finished the doctrinal shift from oxygen-based acids to hydrogen-based acids, started by Davy. According to Liebig, an acid is a hydrogen-containing substance in which the hydrogen could be replaced by a metal. Liebig's definition, while completely empirical, remained in use for almost 50 years until the adoption of the Arrhenius definition.
Arrhenius definition
Svante Arrhenius
Svante August Arrhenius was a Swedish scientist, originally a physicist, but often referred to as a chemist, and one of the founders of the science of physical chemistry...
, which was used to provide a modern definition of acids and bases that followed from his work with Friedrich Wilhelm Ostwald in establishing the presence of ions in aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
in 1884, and led to Arrhenius receiving the Nobel Prize in Chemistry
Nobel Prize in Chemistry
The Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in chemistry, physics, literature,...
in 1903 for "recognition of the extraordinary services... rendered to the advancement of chemistry by his electrolytic theory of dissociation".
As defined by Arrhenius, acid–base reactions are characterized by Arrhenius acids, which dissociate
Dissociation constant
In chemistry, biochemistry, and pharmacology, a dissociation constant is a specific type of equilibrium constant that measures the propensity of a larger object to separate reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt splits up into...
in aqueous solution to form hydrogen ions , and Arrhenius bases, which form hydroxide ions. More recent IUPAC recommendations now suggest the newer term "hydronium" be used in favor of the older accepted term "oxonium" to illustrate reaction mechanisms such as those defined in the Brønsted–Lowry and solvent system definitions more clearly, with the Arrhenius definition serving as a simple general outline of acid–base character. The Arrhenius definition can be summarised as "Arrhenius acids form hydrogen ions in aqueous solution with Arrhenius bases forming hydroxide ions."
The universal aqueous acid–base definition of the Arrhenius concept is described as the formation of water from hydrogen and hydroxide ions, or hydrogen ions and hydroxide ions from the dissociation of an acid and base in aqueous solution:
- (aq) + (aq)
(In modern times, the use of is regarded as a shorthand for
Hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
, since it is now known that the bare proton does not exist as a free species in solution.)
This leads to the definition that in Arrhenius acid–base reactions, a salt and water is formed from the reaction between an acid and a base. In other words, this is a neutralization reaction.
- acid + base → salt + water
The positive ion from a base forms a salt with the negative ion from an acid. For example, two moles
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
of sodium ion from the base sodium hydroxide (NaOH) combine with one mole of sulfate ion from sulfuric acid to form one mole of sodium sulfate
Sodium sulfate
Sodium sulfate is the sodium salt of sulfuric acid. When anhydrous, it is a white crystalline solid of formula Na2SO4 known as the mineral thenardite; the decahydrate Na2SO4·10H2O has been known as Glauber's salt or, historically, sal mirabilis since the 17th century. Another solid is the...
. Two moles of water are also formed.
- 2 NaOH + → + 2
The Arrhenius definitions of acidity and alkalinity are restricted to aqueous solutions, and refer to the concentration of the solvent ions. Under this definition, pure or HCl dissolved in toluene are not acidic, and molten KOH and solutions of sodium amide in liquid ammonia are not alkaline.
Solvent system definition
One of the limitations of Arrhenius definition was its reliance on water solutions. Edward C. Franklin studied the acid–base reactions in liquid ammonia in 1905 and pointed out the similarities to water-based Arrhenius theory, and Albert F. O. Germann, working with liquid , generalized Arrhenius definition to cover aprotic solvents and formulated the solvent system theory in 1925.Germann pointed out that in many solvents there is a certain concentration of a positive species, solvonium (earlier lyonium) cations and negative species, solvate (earlier lyate) anions, in equilibrium with the neutral solvent molecules. For example, water and ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
undergo such dissociation into hydronium
Hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
and hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
, and ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
and amide
Amide
In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
, respectively:
- 2 +
- 2 +
Some aprotic systems also undergo such dissociation, such as dinitrogen tetroxide
Dinitrogen tetroxide
Dinitrogen tetroxide is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium mixture with nitrogen dioxide; some call this mixture dinitrogen tetroxide, while some call it nitrogen dioxide.Dinitrogen tetroxide is a powerful oxidizer, making it highly...
into nitrosonium
Nitrosonium
The nitrosonium ion is NO+, in which the nitrogen atom is bonded to an oxygen atom with a bond order of 3, and the overall diatomic species bears a positive charge. This ion is usually obtained as the following salts: NOClO4, NOSO4H , and NOBF4. The ClO and BF salts are slightly soluble in CH3CN...
and nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
, antimony trichloride
Antimony trichloride
Antimony trichloride is the chemical compound with the formula SbCl3. The soft colorless solid with a pungent odor was known to the alchemists as butter of antimony.-Preparation:...
into dichloroantimonium and tetrachloroantimonate, and phosgene
Phosgene
Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a chemical weapon during World War I. It is also a valued industrial reagent and building block in synthesis of pharmaceuticals and other organic compounds. In low concentrations, its odor resembles...
into chlorocarboxonium and chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
.
- +
- 2 +
- +
A solute causing an increase in the concentration of the solvonium ions and a decrease in the solvate ions is defined as an acid and one causing the reverse is defined as a base. Thus, in liquid ammonia, (supplying ) is a strong base, and (supplying ) is a strong acid. In liquid sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
, thionyl
Thionyl
The thionyl group is SO, a sulfur atom plus an oxygen atom.It occurs in compounds such as thionyl fluoride, SOF2.Thionyl chloride, SOCl2, is a common reagent used in organic synthesis to convert carboxylic acids to acyl chlorides....
compounds (supplying ) behave as acids, and sulfites (supplying ) behave as bases.
The non-aqueous acid–base reactions in liquid ammonia are similar to the reactions in water:
- 2 (base) + (amphiphilic amide) →
- 2 (acid) + (amphiphilic amide) →
Nitric acid can be a base in liquid sulfuric acid:
- (base) + 2 → + + 2
The unique strength of this definition shows in describing the reactions in aprotic solvents, for example in liquid :
- (base) + (acid) → (solvent) + (salt)
Since solvent-system definition depends on the solvent as well as on the compound itself, the same compound can change its role depending on the choice of the solvent. Thus, is a strong acid in water, a weak acid in acetic acid, and a weak base in fluorosulfonic acid. This was seen as both a strength and a weakness, since some substances, such as and , were felt to be acidic or basic on their own right. On the other hand, solvent system theory was criticized as too general to be useful; it was felt that there is something intrinsically acidic about hydrogen compounds, not shared by non-hydrogenic solvonium salts.
Brønsted–Lowry definition
The Brønsted–Lowry definition, formulated in 1923, independently by Johannes Nicolaus BrønstedJohannes Nicolaus Brønsted
Johannes Nicolaus Brønsted born in Varde was a Danish physical chemist.He received a degree in chemical engineering in 1899 and his Ph. D. in 1908 from the University of Copenhagen. He was immediately appointed professor of inorganic and physical chemistry at Copenhagen.In 1906 he published his...
in Denmark and Martin Lowry
Martin Lowry
Thomas Martin Lowry CBE FRS was an English physical chemist. Independently from Johannes Nicolaus Brønsted he has developed the Brønsted–Lowry acid–base theory and was as a founder-member and president of the Faraday Society.-Biography:Lowry was born in Low Moor, Bradford, West Yorkshire,...
in England, is based upon the idea of protonation
Protonation
In chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...
of bases through the de-protonation
Deprotonation
Deprotonation is the removal of a proton from a molecule, forming the conjugate base.The relative ability of a molecule to give up a proton is measured by its pKa value. A low pKa value indicates that the compound is acidic and will easily give up its proton to a base...
of acids – that is, the ability of acids to "donate" hydrogen ions (H+) or protons to bases, which "accept" them. Unlike the previous definitions, the Brønsted–Lowry definition does not refer to the formation of salt and solvent, but instead to the formation of conjugate acids and conjugate bases, produced by the transfer of a proton from the acid to the base. In this approach, acids and bases are fundamentally different in behavior from salts, which are seen as electrolytes, subject to the theories of Debye
Peter Debye
Peter Joseph William Debye FRS was a Dutch physicist and physical chemist, and Nobel laureate in Chemistry.-Early life:...
, Onsager
Lars Onsager
Lars Onsager was a Norwegian-born American physical chemist and theoretical physicist, winner of the 1968 Nobel Prize in Chemistry.He held the Gibbs Professorship of Theoretical Chemistry at Yale University....
, and others. An acid and a base react not to produce a salt and a solvent, but to form a new acid and a new base. The concept of neutralization is thus absent.
According to Brønsted–Lowry definition, an acid is a compound that can donate a proton, and a base is a compound that can receive a proton. An acid–base reaction is, thus, the removal of a hydrogen ion from the acid and its addition to the base. This does not refer to the removal of a proton from the nucleus of an atom, which would require levels of energy not attainable through the simple dissociation of acids, but to removal of a hydrogen ion .
The removal of a proton (hydrogen ion) from an acid produces its conjugate base, which is the acid with a hydrogen ion removed, and the reception of a proton by a base produces its conjugate acid, which is the base with a hydrogen ion added.
For example, the removal of from hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
(HCl) produces the chloride ion , the conjugate base of the acid:
- → +
The addition of to the hydroxide ion , a base, produces water , its conjugate acid:
- + →
Although Brønsted–Lowry acid–base behavior is formally independent of any solvent, it encompasses Arrhenius and solvent system definitions in an unenforced way. For example, protonation of ammonia, a base, gives ammonium ion, its conjugate acid:
- + →
The reaction of ammonia, a base, with acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
in absence of water can be described to give ammonium cation, an acid, and acetate anion, a base:
- + → +
This definition also explains the dissociation of water into low concentrations of hydronium and hydroxide ions:
- + +
Water, being amphoteric, can act as both an acid and a base; here, one molecule of water acts as an acid, donating a ion and forming the conjugate base, , and a second molecule of water acts as a base, accepting the ion and forming the conjugate acid, .
Acid dissociation and acid hydrolysis are seen to be entirely similar phenomena:
- HCl (acid) + (base) (acid) + (base)
- (acid) + (base) (acid) + (base)
as are basic dissociation and basic hydrolysis:
- (base) + (acid) (acid) + (base)
- (base) + (acid) (acid) + (base)
Thus, the general formula for acid–base reactions according to the Brønsted–Lowry definition is:
- AH + B → +
where AH represents the acid, B represents the base, represents the conjugate acid of B, and represents the conjugate base of AH.
Although Brønsted–Lowry calls hydrogen-containing substances like acids, and are not bases but salts containing the bases and . Also, some substances, which many chemists considered to be acids, such as or , are excluded from this classification due to lack of hydrogen. Gilbert Lewis wrote in 1938, "To restrict the group of acids to those substances that contain hydrogen interferes as seriously with the systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen."
Lewis definition
The hydrogen requirement of Arrhenius and Brønsted–Lowry was removed by the Lewis definition of acid–base reactions, devised by Gilbert N. LewisGilbert N. Lewis
Gilbert Newton Lewis was an American physical chemist known for the discovery of the covalent bond , his purification of heavy water, his reformulation of chemical thermodynamics in a mathematically rigorous manner accessible to ordinary chemists, his theory of Lewis acids and...
in 1923, in the same year as Brønsted–Lowry, but it was not elaborated by him until 1938. Instead of defining acid–base reactions in terms of protons or other bonded substances, the Lewis definition defines a base (referred to as a Lewis base) to be a compound that can donate an electron pair
Electron pair
In chemistry, an electron pair consists of two electrons that occupy the same orbital but have opposite spins.Because electrons are fermions, the Pauli exclusion principle forbids these particles from having exactly the same quantum numbers. Therefore the only way to occupy the same orbital, i.e....
, and an acid (a Lewis acid) to be a compound that can receive this electron pair.
In this system, an acid does not exchange atoms with a base, but combines with it. For example, consider this classical aqueous acid–base reaction:
- (aq) + (aq) → (l) + (aq)
The Lewis definition does not regard this reaction as the formation of salt and water or the transfer of from HCl to . Instead, it regards the acid to be the ion itself, and the base to be the ion, which has an unshared electron pair. Therefore, the acid–base reaction here, according to the Lewis definition, is the donation of the electron pair from to the ion. This forms a covalent bond between and , thus producing water .
By treating acid–base reactions in terms of electron pairs instead of specific substances, the Lewis definition can be applied to reactions that do not fall under other definitions of acid–base reactions. For example, a silver cation behaves as an acid with respect to ammonia, which behaves as a base, in the following reaction:
- + 2 : → :Ag:
The result of this reaction is the formation of an ammonia–silver adduct
Adduct
An adduct is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is considered a distinct molecular species...
.
In reactions between Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
s and bases, there is the formation of an adduct when the highest occupied molecular orbital (HOMO
Homo
Homo may refer to:*the Greek prefix ὅμο-, meaning "the same"*the Latin for man, human being*Homo, the taxonomical genus including modern humans...
) of a molecule, such as with available lone electron pair(s) donates lone pairs of electrons to the electron-deficient molecule's lowest unoccupied molecular orbital (LUMO
Lumo
Lumo is a 2007 documentary film about twenty-year-old Lumo Sinai, a woman who fell victim to "Africa's First World War." While returning home one day, Lumo and another woman were gang-raped by a group of soldiers fighting for control of the Democratic Republic of the Congo during the 1994 Rwandan...
) through a co-ordinate covalent bond; in such a reaction, the HOMO-interacting molecule acts as a base, and the LUMO-interacting molecule acts as an acid. In highly-polar molecules, such as boron trifluoride
Boron trifluoride
Boron trifluoride is the chemical compound with the formula BF3. This pungent colourless toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds.-Structure and bonding:...
, the most electronegative
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
element pulls electrons towards its own orbitals, providing a more positive charge on the less-electronegative element and a difference in its electronic structure due to the axial or equatorial orbiting positions of its electrons, causing repulsive effects from lone pair – bonding pair (Lp–Bp) interactions between bonded atoms in excess of those already provided by bonding pair – bonding pair (Bp–Bp) interactions. Adducts involving metal ions are referred to as co-ordination compounds.
Usanovich definition
Simultaneously with Lewis, a Soviet chemist Mikhail Usanovich from Tashkent, developed a general theory that does not restrict acidity to hydrogen-containing compounds, but his approach, published in 1938, was even more general than Lewis theory. Usanovich's theory can be summarized as defining an acid as anything that accepts negative species or donates positive ones, and a base as the reverse. This pushed the concept of acid–base reactions to its logical limits, and even redefined the concept of redoxRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
(oxidation-reduction) as a special case of acid-base reactions, and so did not become widespread, despite being easier to understand than Lewis theory, which required detailed familiarity with atomic structure.
Some examples of Usanovich acid-base reactions include:
- (base) + (acid) → 2 + (species exchanged: anion )
- 3 (base) + (acid) → 6 + 2 (species exchanged: anion )
- Na (base) + Cl (acid) → + (species exchanged: electron)
Lux–Flood definition
This acid–base theory was a revival of oxygen theory of acids and bases, proposed by German chemist Hermann LuxHermann Lux
Hermann Lux , was a prominent inorganic chemist from Munich, Germany.Lux studied chemistry in the University of Karlsruhe where he graduated with honors in 1928 and then completed his education in the University of Bonn in 1929...
in 1939, further improved by Håkon Flood
Håkon Flood
Håkon Flood was a professor of inorganic chemistry at the Norwegian Institute of Technology in Trondheim, Norway, from 1953 to 1975. He also worked as the director of the Institute of Silicate Research at NTH...
circa 1947 and is still used in modern geochemistry
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
and electrochemistry
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
of molten salt
Molten salt
Molten salt refers to a salt that is in the liquid phase that is normally a solid at standard temperature and pressure . A salt which is normally liquid at STP is usually called a room temperature ionic liquid, although technically molten salts are a class of ionic liquids.-Uses:Molten salts have...
s. This definition describes an acid as an oxide ion acceptor and a base as an oxide ion donor. For example:
- (base) + (acid) →
- (base) + (acid) →
- (base) + (acid) → + 2
Pearson definition
In 1963, Ralph PearsonRalph Pearson
Ralph G. Pearson is a physical inorganic chemist best known for the development of the concept of hard and soft acids and bases ....
proposed an advanced qualitative concept known as Hard Soft Acid Base principle, later made quantitative with help of Robert Parr
Robert Parr
Robert Ghormley Parr is a theoretical chemist. He is a chemistry professor at the University of North Carolina at Chapel Hill.-Career:...
in 1984. 'Hard' applies to species that are small, have high charge states, and are weakly polarizable. 'Soft' applies to species that are large, have low charge states and are strongly polarizable. Acids and bases interact, and the most stable interactions are hard–hard and soft–soft. This theory has found use in organic and inorganic chemistry.
Acid–alkali reaction
An acid–alkali reaction is a special case of an acid–base reaction, where the base used is also an alkaliAlkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
. When an acid reacts with an alkali it forms a metal salt and water. Acid–alkali reactions are also a type of neutralization reaction.
In general, acid–alkali reactions can be simplified to
- (aq)HydroxideHydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
+ (aq) →
by omitting spectator ions.
Acids are in general pure substances that contain hydrogen ions
Hydron (chemistry)
In chemistry, a hydron is the general name for a cationic form of atomic hydrogen : most commonly a "proton". However, hydron includes cations of hydrogen regardless of their isotopic composition: thus it refers collectively to protons , deuterons , and tritons...
or cause them to be produced in solutions. Hydrochloric acid and sulfuric acid are common examples. In water, these break apart into ions:
- → (aq) + (aq)
- → (aq) + (aq)
An alkali is a base, a base that contains a metal from column 1 or 2 of the periodic table (the alkali metals or the alkaline earth metals). Alkalis may be defined as soluble bases, which means they must be able to dissolve in water. In general, bases are defined as substances that contain hydroxide ion or produce it in solution. Therefore, one may also speak of hydroxide bases that dissolve in water, and thus these would also be alkalis. Some examples, then, of alkalis would be sodium hydroxide (NaOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)2), and calcium hydroxide (Ca(OH)2). Note that only hydroxides with an alkali metal —column 1 — are very soluble in water; hydroxides with an alkaline earth metal — column 2 — are not as soluble. Some sources will even say the alkaline earth metal hydroxides are insoluble.
To produce hydroxide ions in water, the alkali breaks apart into ions as below:
- → (aq) + (aq)
However, alkalies may also have a broader definition that includes carbonates bonded to a column 1 metal, an ammonium ion , or an amine (NHx radical) as the positive ion. Examples of alkalis would then also include , , and (NH4)2CO3.
There seems to be conflicting information on whether acid-base reactions are neutralization reactions. Some sources define a neutralization reaction as the reaction between an acid and a base that produces a salt and water. Yet, in the book Chemical Misconceptions: Prevention, Diagnosis and Cure by K. Tabor (2002), it is noted that “the term neutralization is usually reserved for acid–alkali reactions.” Thus, this does not make acid–alkali a type of neutralization reaction, but the only kind of neutralization reaction.
There are many uses of neutralization reactions that are acid-alkali reactions. A very common use is antacid tablets. These are designed to neutralize excess stomach acid (HCl) that may be causing discomfort in the stomach or lower esophagus. Also in the digestive tract, neutralization reactions are used when food is moved from the stomach to the intestines. In order for the nutrients to be absorbed through the intestinal wall, an alkaline environment is needed, so the pancreas produce an antacid bicarbonate to cause this transformation to occur.
Another common use, though perhaps not as widely known, is in fertilizers and control of soil pH. Slaked lime (calcium hydroxide) or limestone (calcium carbonate) may be worked into soil that is too acidic for plant growth. Fertilizers that improve plant growth are made by neutralizing sulfuric acid (H2SO4) or nitric acid (HNO3) with ammonia gas (NH3), making ammonium sulfate
Ammonium sulfate
Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...
or ammonium nitrate. These are salts utilized in the fertilizer.
Industrially, a by-product of the burning of coal, sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
gas may combine with water vapor in the air to eventually produce sulfuric acid, which falls as acid rain. To prevent the sulfur dioxide from being released, a device known as a scrubber gleans the gas from smoke stacks. This device first blows calcium carbonate into the combustion chamber where it decomposes into calcium oxide (lime) and carbon dioxide. This lime then reacts with the sulfur dioxide produced forming calcium sulfite. A suspension of lime is then injected into the mixture to produce a slurry, which removes the calcium sulfite
Calcium sulfite
Calcium sulfite, or calcium sulphite, is a chemical compound which is the calcium salt of sulfurous acid with the molecular formula CaSO3. As a food additive it is used as a preservative under the E number E226...
and any remaining unreacted sulfur dioxide.
See also
- AlkaliAlkaliIn chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
- Electron configurationElectron configurationIn atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
- Lewis structureLewis structureLewis structures are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule. A Lewis structure can be drawn for any covalently bonded molecule, as well as coordination compounds...
- Resonance structure
- ProtonationProtonationIn chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...
and DeprotonationDeprotonationDeprotonation is the removal of a proton from a molecule, forming the conjugate base.The relative ability of a molecule to give up a proton is measured by its pKa value. A low pKa value indicates that the compound is acidic and will easily give up its proton to a base... - Nucleophilic substitutionNucleophilic substitutionIn organic and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which an electron nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms called the leaving group; the positive or partially positive...
and Redox reactions - Acid–base titration