An acid–base reaction is a

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...

that occurs between an acid and a base. It can be used to determine pH via

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant ...

. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their application in solving related problems; these are called the acid–base theories, for example,

Brønsted–Lowry acid–base theory The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. The fundamental concept of this the ...

. Their importance becomes apparent in analyzing acid–base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent. The first of these concepts was provided by the French

chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe ...

Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS ( – Table of discoveries attributes Antoine Lavoisier as the first to posit a scientific theory in relation to oxyacids. It is important to think of the acid-base reaction models as theories that complement each other. For example, the current Lewis model has the broadest definition of what an acid and base are, with the Brønsted-Lowry theory being a subset of what acids and bases are, and the Arrhenius theory being the most restrictive.

Acid–base definitions

Historic development

The concept of an acid-base reaction was first proposed in 1754 by

Guillaume-François Rouelle Guillaume François Rouelle (, 15 September 1703 – 3 August 1770) was a French chemist and apothecary. In 1754 he introduced the concept of a base into chemistry as a substance which reacts with an acid to form a salt). He is known as ''l'Aîn ...

, who introduced the word " base" into chemistry to mean a substance which reacts with an acid to give it solid form (as a salt). Bases are mostly bitter in nature.

Lavoisier's oxygen theory of acids

The first scientific concept of acids and bases was provided by

Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS ( strong acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutio ...

s was mainly restricted to oxoacids, such as (nitric acid) and (sulfuric acid), which tend to contain central atoms in high

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. ...

s 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 Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

'', which in fact he named from Greek words meaning "acid-former" (from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

ὀξύς (''oxys'') meaning "acid" or "sharp" and γεινομαι (''geinomai'') meaning "engender"). The Lavoisier definition held for over 30 years, until the 1810 article and subsequent lectures by

Sir Humphry Davy Sir Humphry Davy, 1st Baronet, (17 December 177829 May 1829) was a British chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, several elements for th ...

in which he proved the lack of oxygen in , 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

Jöns Jacob Berzelius Baron Jöns Jacob Berzelius (; by himself and his contemporaries named only Jacob Berzelius, 20 August 1779 – 7 August 1848) was a Swedish chemist. Berzelius is considered, along with Robert Boyle, John Dalton, and Antoine Lavoisier, to be ...

, who stated that acids are oxides of nonmetals while bases are oxides of metals.

Liebig's hydrogen theory of acids

In 1838,

Justus von Liebig Justus Freiherr von Liebig (12 May 1803 – 20 April 1873) was a German scientist who made major contributions to agricultural and biology, biological chemistry, and is considered one of the principal founders of organic chemistry. As a profess ...

proposed that an acid is a hydrogen-containing compound whose hydrogen can be replaced by a metal. This redefinition was based on his extensive work on the chemical composition of

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 ...

s, finishing the doctrinal shift from oxygen-based acids to hydrogen-based acids started by Davy. Liebig's definition, while completely empirical, remained in use for almost 50 years until the adoption of the Arrhenius definition.

Arrhenius definition

The first modern definition of acids and bases in molecular terms was devised by

Svante Arrhenius Svante August Arrhenius ( , ; 19 February 1859 – 2 October 1927) was a Swedish scientist. Originally a physicist, but often referred to as a chemist, Arrhenius was one of the founders of the science of physical chemistry. He received the N ...

.Miessler G.L. and Tarr D.A. ''Inorganic Chemistry'' (2nd ed., Prentice-Hall 1999) p. 154 A hydrogen theory of acids, it followed from his 1884 work with Friedrich Wilhelm Ostwald in establishing the presence of ions in

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would ...

and led to Arrhenius receiving the

Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...

in 1903. As defined by Arrhenius: *''an Arrhenius acid'' is a substance that dissociates in water to form hydrogen ions (H⁺); that is, an acid increases the concentration of H⁺ ions in an aqueous solution. This causes the

protonation In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...

of water, or the creation of the

hydronium In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is di ...

(H₃O⁺) ion. Thus, in modern times, the symbol H⁺ is interpreted as a shorthand for H₃O⁺, because it is now known that a bare proton does not exist as a free species in aqueous solution. This is the species which is measured by

pH indicator A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hen ...

s to measure the acidity or basicity of a solution. *''an Arrhenius base is a substance that dissociates in water to form hydroxide (OH⁻) ions; that is, a base increases the concentration of OH⁻ ions in an aqueous solution." The Arrhenius definitions of

acidity In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

and

alkalinity Alkalinity (from ar, القلوي, al-qaly, lit=ashes of the saltwort) is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale. Alkalinity is the strengt ...

are restricted to aqueous solutions and are not valid for most non-aqueous solutions, and refer to the concentration of the solvent ions. Under this definition, pure H₂SO₄ and HCl dissolved in toluene are not acidic, and molten NaOH and solutions of calcium amide in liquid ammonia are not alkaline. This led to the development of the Brønsted-Lowry theory and subsequent Lewis theory to account for these non-aqueous exceptions. The reaction of an acid with a base is called a neutralization reaction. The products of this reaction are a

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...

and water. :acid + base → salt + water In this traditional representation an acid–base neutralization reaction is formulated as a double-replacement reaction. For example, the reaction of

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dig ...

, HCl, with

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkal ...

, NaOH, solutions produces a solution of

sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35 ...

, NaCl, and some additional water molecules. :HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O The modifier ( aq) in this equation was implied by Arrhenius, rather than included explicitly. It indicates that the substances are dissolved in water. Though all three substances, HCl, NaOH and NaCl are capable of existing as pure compounds, in

s they are fully dissociated into the aquated ions H⁺, Cl⁻, Na⁺ and OH⁻.

Example: Baking Powder

When combined with water, the sodium bicarbonate and acid salts react to produce gaseous

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

. Whether commercially or domestically prepared, the principles behind baking powder formulations remain the same. The acid-base reaction can be generically represented as shown: :NaHCO₃ + H⁺ → Na⁺ + CO₂ + H₂O The real reactions are more complicated because the acids are complicated. For example, starting with baking soda and monocalcium phosphate, the reaction produces carbon dioxide by the following

stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...

:John Brodie, John Godber "Bakery Processes, Chemical Leavening Agents" in Kirk-Othmer Encyclopedia of Chemical Technology 2001, John Wiley & Sons. :14 NaHCO₃ + 5 Ca(H₂PO₄)₂ → 14 CO₂ + Ca₅(PO₄)₃OH + 7 Na₂HPO₄ + 13 H₂O A typical formulation (by weight) could call for 30% sodium bicarbonate, 5–12% monocalcium phosphate, and 21–26%

sodium aluminium sulfate Sodium aluminium sulfate is the inorganic compound with the chemical formula NaAl(SO4)2·12H2O (sometimes written Na2SO4·Al2(SO4)3·24H2O). Also known as soda alum, sodium alum, or SAS, this white solid is used in the manufacture of baking pow ...

. Alternately, a commercial baking powder might use sodium acid pyrophosphate as one of the two acidic components instead of sodium aluminium sulfate. Another typical acid in such formulations is

cream of tartar Potassium bitartrate, also known as potassium hydrogen tartrate, with formula K C4 H5 O6, is a byproduct of winemaking. In cooking, it is known as cream of tartar. It is processed from the potassium acid salt of tartaric acid (a carboxylic ...

( K C₄ H₅ O₆), a derivative of

tartaric acid Tartaric acid is a white, crystalline organic acid that occurs naturally in many fruits, most notably in grapes, but also in bananas, tamarinds, and citrus. Its salt Salt is a mineral composed primarily of sodium chloride (NaCl), a c ...

Brønsted–Lowry definition

The Brønsted–Lowry definition, formulated in 1923, independently by Johannes Nicolaus Brønsted in Denmark and Martin Lowry in England, is based upon the idea of

of bases through the

deprotonation Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju. ...

of acids – that is, the ability of acids to "donate" hydrogen ions (H⁺)—otherwise known as protons—to bases, which "accept" them. – According to this page, the original definition was that "acids have a tendency to lose a proton""Removal and addition of a proton from the nucleus of an atom does not occur – it would require very much more energy than is involved in the dissociation of acids." An acid–base reaction is, thus, the removal of a hydrogen ion from the acid and its addition to the base. The removal of a hydrogen ion from an acid produces its ''

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...

'', which is the acid with a hydrogen ion removed. The reception of a proton by a base produces its ''

conjugate acid A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...

'', which is the base with a hydrogen ion added. 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 The debye (symbol: D) (; ) is a CGS unit (a non- SI metric unit) of electric dipole momentTwo equal and opposite charges separated by some distance constitute an electric dipole. This dipole possesses an electric dipole moment whose value is g ...

, 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. Brønsted–Lowry acid–base behavior is formally independent of any solvent, making it more all-encompassing than the Arrhenius model. The calculation of pH under the Arrhenius model depended on alkalis (bases) dissolving in water (

). The Brønsted–Lowry model expanded what could be pH tested using insoluble and soluble solutions (gas, liquid, solid). The general formula for acid–base reactions according to the Brønsted–Lowry definition is: :HA + B → BH⁺ + A⁻ where HA represents the acid, B represents the base, BH⁺ represents the conjugate acid of B, and A⁻ represents the conjugate base of HA. For example, a Brønsted–Lowry model for the dissociation of

(HCl) in

would be the following: :HCl + H₂O H₃O⁺ + Cl⁻ The removal of H⁺ from the HCl produces the

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...

ion, Cl⁻, the conjugate base of the acid. The addition of H⁺ to the H₂O (acting as a base) forms the

ion, H₃O⁺, the conjugate acid of the base. Water is

amphoteric In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphip ...

—that is, it can act as both an acid and a base. The Brønsted–Lowry model explains this, showing the dissociation of water into low concentrations of hydronium and

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water ...

ions: :H₂O + H₂O H₃O⁺ + OH⁻ This equation is demonstrated in the image below: Bronsted lowry 3d diagram

Here, one molecule of water acts as an acid, donating an H⁺ and forming the conjugate base, OH⁻, and a second molecule of water acts as a base, accepting the H⁺ ion and forming the conjugate acid, H₃O⁺. As an example of water acting as an acid, consider an aqueous solution of

pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakl ...

, C₅H₅N. :C₅H₅N + H₂O ₅H₅NHsup>+ + OH⁻ In this example, a water molecule is split into a hydrogen ion, which is donated to a pyridine molecule, and a hydroxide ion. In the Brønsted–Lowry model, the solvent does not necessarily have to be water, as is required by the Arrhenius Acid-Base model. For example, consider what happens when

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

, CH₃COOH, dissolves in liquid ammonia. : + + An H⁺ ion is removed from acetic acid, forming its conjugate base, the

acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...

ion, CH₃COO⁻. The addition of an H⁺ ion to an ammonia molecule of the solvent creates its conjugate acid, the ammonium ion, . The Brønsted–Lowry model calls hydrogen-containing substances (like HCl) acids. Thus, some substances, which many chemists considered to be acids, such as SO₃ or BCl₃, are excluded from this classification due to lack of hydrogen.

Gilbert N. Lewis Gilbert Newton Lewis (October 23 or October 25, 1875 – March 23, 1946) was an American physical chemist and a Dean of the College of Chemistry at University of California, Berkeley. Lewis was best known for his discovery of the covalent bond a ...

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 An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...

to substances containing

." Furthermore, KOH and KNH₂ are not considered Brønsted bases, but rather salts containing the bases OH⁻ and .

Lewis definition

The hydrogen requirement of Arrhenius and Brønsted–Lowry was removed by the Lewis definition of acid–base reactions, devised by

in 1923, – Table of discoveries attributes the date of publication/release for the Lewis theory as 1924. 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. For example,

boron trifluoride Boron trifluoride is the inorganic compound with the formula BF3. This pungent, colourless, and 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 bondin ...

, BF₃ is a typical Lewis acid. It can accept a pair of electrons as it has a vacancy in its

octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 com ...

. The

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ...

ion has a full octet and can donate a pair of electrons. Thus :BF₃ + F⁻ → is a typical Lewis acid, Lewis base reaction. All compounds of

group 13 The Group 13 network ( pl, Trzynastka, Yiddish: ''דאָס דרײַצענטל'') was a Jewish Nazi collaborationist organization in the Warsaw Ghetto during the German occupation of Poland in World War II. The rise and fall of the Gro ...

elements with a formula AX₃ can behave as Lewis acids. Similarly, compounds of group 15 elements with a formula DY₃, such as

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent su ...

s, NR₃, and

phosphine Phosphine ( IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotti ...

s, PR₃, can behave as Lewis bases. Adducts between them have the formula X₃A←DY₃ with a dative covalent bond, shown symbolically as ←, between the atoms A (acceptor) and D (donor). Compounds of group 16 with a formula DX₂ may also act as Lewis bases; in this way, a compound like an

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again b ...

, R₂O, or a

thioether In organic chemistry, an organic sulfide (British English sulphide) or thioether is an organosulfur functional group with the connectivity as shown on right. Like many other sulfur-containing compounds, volatile sulfides have foul odors. A sul ...

, R₂S, can act as a Lewis base. The Lewis definition is not limited to these examples. For instance,

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

acts as a Lewis base when it forms an adduct with boron trifluoride, of formula F₃B←CO. Adducts involving metal ions are referred to as co-ordination compounds; each ligand donates a pair of electrons to the metal ion. The reaction : g(H₂O)₄sup>+ + 2NH₃ → g(NH₃)₂sup>+ + 4H₂O can be seen as an acid–base reaction in which a stronger base (ammonia) replaces a weaker one (water) The Lewis and Brønsted–Lowry definitions are consistent with each other since the reaction :H⁺ + OH⁻ H₂O is an acid–base reaction in both theories.

Solvent system definition

One of the limitations of the Arrhenius definition is its reliance on water solutions. Edward Curtis Franklin studied the acid–base reactions in liquid ammonia in 1905 and pointed out the similarities to the water-based Arrhenius theory. Albert F.O. Germann, working with liquid

phosgene Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, es ...

, , formulated the solvent-based theory in 1925, thereby generalizing the Arrhenius definition to cover aprotic solvents. Germann pointed out that in many solutions, there are ions in equilibrium with the neutral solvent molecules: *solvonium ions: a generic name for positive ions. (The term ''solvonium'' has replaced the older term ''

lyonium ion In chemistry, a lyonium ion is the cation derived by the protonation of a solvent molecule. For example, a hydronium ion is formed by the protonation of water, and is the cation formed by the protonation of methanol. Its counterpart is a lyate i ...

s'': positive ions formed by protonation of solvent molecules.) *solvate ions: a generic name for negative ions. (The term ''solvate'' has replaced the older term ''

lyate ion In chemistry, a lyate ion is the anion derived by the deprotonation of a solvent molecule. For example, a hydroxide ion is formed by the deprotonation of water, and methoxide () is the anion formed by the deprotonation of methanol. Its coun ...

s'': negative ions formed by deprotonation of solvent molecules.) For example, water and

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...

undergo such dissociation into

and

, and

ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternar ...

and

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...

, respectively: :2 + :2 + Some aprotic systems also undergo such dissociation, such as

dinitrogen tetroxide Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russia rocket engineers) as amyl, is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an Chemical equi ...

into nitrosonium and

nitrate Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insoluble ...

, antimony trichloride into dichloroantimonium and tetrachloroantimonate, and phosgene into chlorocarboxonium and

: : + :2 + : + A solute that causes an increase in the concentration of the solvonium ions and a decrease in the concentration of solvate ions is defined as an ''acid''. A solute that causes an increase in the concentration of the solvate ions and a decrease in the concentration of the solvonium ions 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 ( IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic ...

(), thionyl compounds (supplying ) behave as acids, and

sulfites Sulfites or sulphites are compounds that contain the sulfite ion (or the sulfate(IV) ion, from its correct systematic name), . The sulfite ion is the conjugate base of bisulfite. Although its acid ( sulfurous acid) is elusive, its salts are wid ...

(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: : + 2 → + + 2 The unique strength of this definition shows in describing the reactions in aprotic solvents; for example, in liquid : : + → + Because the solvent system definition depends on the solute as well as on the solvent itself, a particular solute can be either an acid or a base depending on the choice of the solvent: is a strong acid in water, a weak acid in acetic acid, and a weak base in fluorosulfonic acid; this characteristic of the theory has been seen as both a strength and a weakness, because some substances (such as and ) have been seen to be acidic or basic on their own right. On the other hand, solvent system theory has been criticized as being too general to be useful. Also, it has been thought that there is something intrinsically acidic about hydrogen compounds, a property not shared by non-hydrogenic solvonium salts.

Lux–Flood definition

This acid–base theory was a revival of the oxygen theory of acids and bases proposed by German chemist Hermann Lux in 1939, further improved by

Håkon Flood Håkon Flood (25 September 1905 – 9 October 2001) 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 (''Inst ...

circa 1947 and is still used in modern

geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...

and

electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...

molten salt Molten salt is salt which is solid at standard temperature and pressure but enters the liquid phase due to elevated temperature. Regular table salt has a melting point of 801 °C (1474°F) and a heat of fusion of 520 J/g.Proc. Roy. Soc. Bibli ...

s. This definition describes an acid as an oxide ion () acceptor and a base as an oxide ion donor. For example: : + → : + → : + → + 2 This theory is also useful in the systematisation of the reactions of

noble gas compound In chemistry, noble gas compounds are chemical compounds that include an element from the noble gases, group 18 of the periodic table. Although the noble gases are generally unreactive elements, many such compounds have been observed, particula ...

s, especially the xenon oxides, fluorides, and oxofluorides.

Usanovich definition

Mikhail Usanovich 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 defined the concept of

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

(oxidation-reduction) as a special case of acid–base reactions Some examples of Usanovich acid–base reactions include: : + → 2 + (species exchanged: anion) : + → 6 + 2 (species exchanged: 3 anions) : + → 2 + 2 (species exchanged: 2 electrons)

Rationalizing the strength of Lewis acid–base interactions

HSAB theory

In 1963, Ralph Pearson proposed a qualitative concept known as the Hard and Soft Acids and Bases 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.

ECW model

The ECW model created by Russell S. Drago is a quantitative model that describes and predicts the strength of Lewis acid base interactions, −Δ''H''. The model assigned ''E'' and ''C'' parameters to many Lewis acids and bases. Each acid is characterized by an ''E''_A and a ''C''_A. Each base is likewise characterized by its own ''E''_B and ''C''_B. The ''E'' and ''C'' parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. The equation is : −Δ''H'' = ''E''_A''E''_B + ''C''_A''C''_B + ''W'' The ''W'' term represents a constant energy contribution for acid–base reaction such as the cleavage of a dimeric acid or base. The equation predicts reversal of acids and base strengths. The graphical presentations of the equation show that there is no single order of Lewis base strengths or Lewis acid strengths.

Acid–base equilibrium

The reaction of a strong acid with a strong base is essentially a quantitative reaction. For example, :HCl_(aq) + Na(OH)_(aq) → H₂O + NaCl_(aq) In this reaction both the sodium and chloride ions are spectators as the neutralization reaction, :H⁺ + OH⁻ → H₂O does not involve them. With weak bases addition of acid is not quantitative because a solution of a weak base is a

buffer solution A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is ...

. A solution of a weak acid is also a buffer solution. When a weak acid reacts with a weak base an equilibrium mixture is produced. For example,

adenine Adenine () (symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivativ ...

, written as AH, can react with a hydrogen

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

ion, . :AH + A⁻ + The equilibrium constant for this reaction can be derived from the acid dissociation constants of adenine and of the dihydrogen phosphate ion. : ⁻ ⁺= ''K''_a1 H:[] ⁺= ''K''_a2[] The notation [X] signifies "concentration of X". When these two equations are combined by eliminating the hydrogen ion concentration, an expression for the equilibrium constant, ''K'' is obtained. : ⁻[] = ''K''[AH] []; ''K'' =

Acid–alkali reaction

An acid–alkali reaction is a special case of an acid–base reaction, where the base used is also an alkali. When an acid reacts with an alkali salt (a metal hydroxide), the product is a metal

and water. Acid–alkali reactions are also neutralization reactions. In general, acid–alkali reactions can be simplified to : _(aq) + _(aq) → by omitting spectator ions. Acids are in general pure substances that contain hydrogen cations () 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) The alkali breaks apart in water, yielding dissolved hydroxide ions: : → _(aq) + _(aq)

Notes

References

Sources

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External links

Acid-base Physiology: an on-line text
{{DEFAULTSORT:Acid-Base Reaction Acids Bases (chemistry) Acid-base chemistry Equilibrium chemistry