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An acid is a
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecule
or
ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
capable of donating a
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

proton
(hydrogen ion H+) (a Brønsted–Lowry acid), or, alternatively, capable of forming a
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
with an
electron pair In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...
(a
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty Non-bonding orbital, orbital which is capable of accepting an electron pair from a Lewis Base (chemistry), base to form a Lewis ad ...

Lewis acid
).IUPAC Gold Book - acid
/ref> The first category of acids are the proton donors, or Brønsted–Lowry acids. In the special case of
aqueous solutions 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 be rep ...
, proton donors form the
hydronium ion In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...
H3O+ and are known as Arrhenius acids. Brønsted and
LowryLowry may refer to: People Lowry is a common surname, and may refer to: * Calvin Lowry (born 1983), American football player * Dave Lowry (born 1965), Canadian ice hockey player * Desiree Lowry (born 1972), Puerto Rican beauty pageant titleholder ...

Lowry
generalized the Arrhenius theory to include non-aqueous solvents. A Brønsted or Arrhenius acid usually contains a hydrogen atom bonded to a chemical structure that is still energetically favorable after loss of H+. Aqueous Arrhenius acids have characteristic properties which provide a practical description of an acid. Acids form
aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute an ...
s with a sour taste, can turn blue
litmus Litmus is a water-soluble In chemistry, solubility is ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inabili ...
red, and react with bases and certain metals (like
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
) to form
salts In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
. The word ''acid'' is derived from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
''acidus/acēre'', meaning 'sour'. An aqueous solution of an acid has a less than 7 and is colloquially also referred to as "acid" (as in "dissolved in acid"), while the strict definition refers only to the
solute In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence i ...
. A lower pH means a higher acidity, and thus a higher concentration of positive hydrogen ions in the
solution Solution may refer to: * Solution (chemistry) In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prope ...
. Chemicals or substances having the property of an acid are said to be acidic. Common aqueous acids include
hydrochloric acid Hydrochloric acid +(aq) Cl−(aq) or H3O+ Cl− also known as muriatic acid, is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a salin ...

hydrochloric acid
(a solution of
hydrogen chloride The compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with ...

hydrogen chloride
which is found in
gastric acid Gastric acid, gastric juice, or stomach acid, is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of for ...
in the stomach and activates
digestive enzymes Digestive may refer to: Biology *Digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal orig ...
),
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
(vinegar is a dilute aqueous solution of this liquid),
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
(used in
car batteries An automotive battery or car battery is a rechargeable battery that is used to start a motor vehicle. Its main purpose is to provide an electric current to the electric-powered starting motor, which in turn starts the chemically-powered internal ...
), and
citric acid Citric acid is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, ...

citric acid
(found in citrus fruits). As these examples show, acids (in the colloquial sense) can be solutions or pure substances, and can be derived from acids (in the strict sense) that are solids, liquids, or gases.
Strong acid Acid strength is the tendency of an acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special ca ...

Strong acid
s and some concentrated weak acids are
corrosive A corrosive substance is one that will damage or destroy other substances with which it comes into contact by means of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical su ...
, but there are exceptions such as
carborane Carboranes are electron-delocalized (non-classically bonded) clusters composed of boron Boron is a chemical element with the Symbol (chemistry), symbol B and atomic number 5. Produced entirely by cosmic ray spallation and supernov ...
s and
boric acid Boric acid, also called hydrogen borate, boracic acid, and orthoboric acid is a weak, monobasic Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is ...

boric acid
. The second category of acids are
Lewis acids A Lewis acid is a chemical species that contains an empty orbital which is capable of accepting an electron pair In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds c ...
, which form a covalent bond with an electron pair. An example is
boron trifluoride Boron trifluoride is the inorganic compound with the chemical formula, 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 ...
(BF3), whose boron atom has a vacant orbital which can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
(NH3).
Lewis Lewis may refer to: Names * Lewis (given name) Lewis () is a masculine English given name, English-language given name. It was coined as an anglicisation of given names in other languages. "Lewis" has been used to anglicise the Irish language, Ir ...
considered this as a generalization of the Brønsted definition, so that an acid is a chemical species that accepts electron pairs either directly ''or'' by releasing protons (H+) into the solution, which then accept electron pairs. However, hydrogen chloride, acetic acid, and most other Brønsted–Lowry acids cannot form a covalent bond with an electron pair and are therefore not Lewis acids. Conversely, many Lewis acids are not Arrhenius or Brønsted–Lowry acids. In modern terminology, an ''acid'' is implicitly a Brønsted acid and not a Lewis acid, since chemists almost always refer to a Lewis acid explicitly as ''a Lewis acid''.


Definitions and concepts

Modern definitions are concerned with the fundamental chemical reactions common to all acids. Most acids encountered in everyday life are
aqueous solutions 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 be rep ...
, or can be dissolved in water, so the Arrhenius and Brønsted–Lowry definitions are the most relevant. The Brønsted–Lowry definition is the most widely used definition; unless otherwise specified, acid–base reactions are assumed to involve the transfer of a proton (H+) from an acid to a base. Hydronium ions are acids according to all three definitions. Although alcohols and amines can be Brønsted–Lowry acids, they can also function as
Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty Non-bonding orbital, orbital which is capable of accepting an electron pair from a Lewis Base (chemistry), base to form a Lewis ad ...
s due to the lone pairs of electrons on their oxygen and nitrogen atoms.


Arrhenius acids

In 1884,
Svante Arrhenius Svante August Arrhenius ( , ; 19 February 1859 – 2 October 1927) was a Swedish Swedish or ' may refer to: * Anything from or related to Sweden, a country in Northern Europe * Swedish language, a North Germanic language spoken primarily in S ...

Svante Arrhenius
attributed the properties of acidity to
hydrogen ion A hydrogen ion is created when a hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is ...
s (H+), later described as
protons A proton is a subatomic particle, symbol or , with a positive electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric cha ...
or hydrons. An Arrhenius acid is a substance that, when added to water, increases the concentration of H+ ions in the water. Note that chemists often write H+(''aq'') and refer to the
hydrogen ion A hydrogen ion is created when a hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is ...
when describing acid–base reactions but the free hydrogen nucleus, a
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

proton
, does not exist alone in water, it exists as the hydronium ion (H3O+) or other forms (H5O2+, H9O4+). Thus, an Arrhenius acid can also be described as a substance that increases the concentration of hydronium ions when added to water. Examples include molecular substances such as hydrogen chloride and acetic acid. An Arrhenius
base Base or BASE may refer to: Brands and enterprises *Base (mobile telephony provider) Base (stylized as BASE) is the third largest of Belgium Belgium ( nl, België ; french: Belgique ; german: Belgien ), officially the Kingdom of Belgium, ...
, on the other hand, is a substance which increases the concentration of
hydroxide Hydroxide is a diatomic anion with chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other sym ...
(OH) ions when dissolved in water. This decreases the concentration of hydronium because the ions react to form H2O molecules: H3O + OH ⇌ H2O(l) + H2O(l) Due to this equilibrium, any increase in the concentration of hydronium is accompanied by a decrease in the concentration of hydroxide. Thus, an Arrhenius acid could also be said to be one that decreases hydroxide concentration, while an Arrhenius base increases it. In an acidic solution, the concentration of hydronium ions is greater than 10−7 moles per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acidic solutions thus have a pH of less than 7.


Brønsted–Lowry acids

While the Arrhenius concept is useful for describing many reactions, it is also quite limited in its scope. In 1923 chemists
Johannes Nicolaus Brønsted Johannes Nicolaus Brønsted (; 22 February 1879 – 17 December 1947) was a Danish physical chemist Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be ...
and
Thomas Martin Lowry Thomas Martin Lowry (; 26 October 1874 – 2 November 1936) was an English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early ...

Thomas Martin Lowry
independently recognized that acid–base reactions involve the transfer of a proton. A Brønsted–Lowry acid (or simply Brønsted acid) is a species that donates a proton to a Brønsted–Lowry base. Brønsted–Lowry acid–base theory has several advantages over Arrhenius theory. Consider the following reactions of
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
(CH3COOH), the
organic acid An organic acid is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, beha ...
that gives vinegar its characteristic taste: : + + : + + Both theories easily describe the first reaction: CH3COOH acts as an Arrhenius acid because it acts as a source of H3O+ when dissolved in water, and it acts as a Brønsted acid by donating a proton to water. In the second example CH3COOH undergoes the same transformation, in this case donating a proton to ammonia (NH3), but does not relate to the Arrhenius definition of an acid because the reaction does not produce hydronium. Nevertheless, CH3COOH is both an Arrhenius and a Brønsted–Lowry acid. Brønsted–Lowry theory can be used to describe reactions of in nonaqueous solution or the gas phase.
Hydrogen chloride The compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with ...

Hydrogen chloride
(HCl) and ammonia combine under several different conditions to form
ammonium chloride Ammonium chloride is an inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, ...

ammonium chloride
, NH4Cl. In aqueous solution HCl behaves as
hydrochloric acid Hydrochloric acid +(aq) Cl−(aq) or H3O+ Cl− also known as muriatic acid, is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a salin ...

hydrochloric acid
and exists as hydronium and chloride ions. The following reactions illustrate the limitations of Arrhenius's definition: # H3O + Cl + NH3 → Cl + NH(aq) + H2O # HCl(benzene) + NH3(benzene) → NH4Cl(s) # HCl(g) + NH3(g) → NH4Cl(s) As with the acetic acid reactions, both definitions work for the first example, where water is the solvent and hydronium ion is formed by the HCl solute. The next two reactions do not involve the formation of ions but are still proton-transfer reactions. In the second reaction hydrogen chloride and ammonia (dissolved in
benzene Benzene is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organic matter, matter that has come from a once-living organ ...

benzene
) react to form solid ammonium chloride in a benzene solvent and in the third gaseous HCl and NH3 combine to form the solid.


Lewis acids

A third, only marginally related concept was proposed in 1923 by Gilbert N. Lewis, which includes reactions with acid–base characteristics that do not involve a proton transfer. A Lewis acid is a species that accepts a pair of electrons from another species; in other words, it is an electron pair acceptor. Brønsted acid–base reactions are proton transfer reactions while Lewis acid–base reactions are electron pair transfers. Many Lewis acids are not Brønsted–Lowry acids. Contrast how the following reactions are described in terms of acid–base chemistry: : In the first reaction a fluoride ion, F, gives up an
electron pair In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...
to
boron trifluoride Boron trifluoride is the inorganic compound with the chemical formula, 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 ...
to form the product
tetrafluoroborate Tetrafluoroborate is the anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyda ...

tetrafluoroborate
. Fluoride "loses" a pair of
valence electron In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s because the electrons shared in the B—F bond are located in the region of space between the two atomic
nuclei ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
and are therefore more distant from the fluoride nucleus than they are in the lone fluoride ion. BF3 is a Lewis acid because it accepts the electron pair from fluoride. This reaction cannot be described in terms of Brønsted theory because there is no proton transfer. The second reaction can be described using either theory. A proton is transferred from an unspecified Brønsted acid to ammonia, a Brønsted base; alternatively, ammonia acts as a Lewis base and transfers a lone pair of electrons to form a bond with a hydrogen ion. The species that gains the electron pair is the Lewis acid; for example, the oxygen atom in H3O+ gains a pair of electrons when one of the H—O bonds is broken and the electrons shared in the bond become localized on oxygen. Depending on the context, a Lewis acid may also be described as an
oxidizer An oxidizing agent, also known as an oxidant or oxidizer, is a substance that has the ability to oxidize (mild reducing agent) are added to powdered potassium permanganate (strong oxidizing agent), a violent redox reaction accompanied by ...

oxidizer
or an
electrophile In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...
. Organic Brønsted acids, such as acetic, citric, or oxalic acid, are not Lewis acids. They dissociate in water to produce a Lewis acid, H+, but at the same time also yield an equal amount of a Lewis base (acetate, citrate, or oxalate, respectively, for the acids mentioned). This article deals mostly with Brønsted acids rather than Lewis acids.


Dissociation and equilibrium

Reactions of acids are often generalized in the form HA H+ + A, where HA represents the acid and A is the
conjugate base A conjugate acid, within the 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 ...
. This reaction is referred to as protolysis. The protonated form (HA) of an acid is also sometimes referred to as the free acid. Acid–base conjugate pairs differ by one proton, and can be interconverted by the addition or removal of a proton (
protonation In chemistry, protonation (or hydronation) is the adding of a proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons ...
and
deprotonation Deprotonation (or dehydronation) is the removal (transfer) of a proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutr ...

deprotonation
, respectively). Note that the acid can be the charged species and the conjugate base can be neutral in which case the generalized reaction scheme could be written as HA+ H+ + A. In solution there exists an
equilibrium List of types of equilibrium, the condition of a system in which all competing influences are balanced, in a wide variety of contexts. Equilibrium may also refer to: Film and television * Equilibrium (film), ''Equilibrium'' (film), a 2002 scien ...
between the acid and its conjugate base. The
equilibrium constant The equilibrium constant of a chemical reaction is the value of its reaction quotientIn chemical thermodynamics Chemical thermodynamics is the study of the interrelation of heat In thermodynamics, heat is energy in transfer to or from a ...
''K'' is an expression of the equilibrium concentrations of the molecules or the ions in solution. Brackets indicate concentration, such that 2Omeans ''the concentration of H2O''. The
acid dissociation constant An acid dissociation constant, ''K''a, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength Physical strength *Physical strength, as in people or animals *Hysterical strength, extreme str ...
''K''a is generally used in the context of acid–base reactions. The numerical value of ''K''a is equal to the product of the concentrations of the products divided by the concentration of the reactants, where the reactant is the acid (HA) and the products are the conjugate base and H+. :K_a = \frac\ce\ce The stronger of two acids will have a higher ''K''a than the weaker acid; the ratio of hydrogen ions to acid will be higher for the stronger acid as the stronger acid has a greater tendency to lose its proton. Because the range of possible values for ''K''a spans many orders of magnitude, a more manageable constant, p''K''a is more frequently used, where p''K''a = −log10 ''K''a. Stronger acids have a smaller p''K''a than weaker acids. Experimentally determined p''K''a at 25 °C in aqueous solution are often quoted in textbooks and reference material.


Nomenclature

Arrhenius acids are named according to their
anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
s. In the classical naming system, the ionic suffix is dropped and replaced with a new suffix, according to the table following. The prefix "hydro-" is used when the acid is made up of just hydrogen and one other element. For example, HCl has
chloride The chloride ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects th ...

chloride
as its anion, so the hydro- prefix is used, and the -ide suffix makes the name take the form
hydrochloric acid Hydrochloric acid +(aq) Cl−(aq) or H3O+ Cl− also known as muriatic acid, is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a salin ...

hydrochloric acid
. ''Classical naming system:'' In the
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the authoritative power over chemist ...
naming system, "aqueous" is simply added to the name of the ionic compound. Thus, for hydrogen chloride, as an acid solution, the IUPAC name is aqueous hydrogen chloride.


Acid strength

The strength of an acid refers to its ability or tendency to lose a proton. A strong acid is one that completely dissociates in water; in other words, one
mole Mole (or Molé) may refer to: Animals * Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America * Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpidae ...
of a strong acid HA dissolves in water yielding one mole of H+ and one mole of the conjugate base, A, and none of the protonated acid HA. In contrast, a weak acid only partially dissociates and at equilibrium both the acid and the conjugate base are in solution. Examples of
strong acid Acid strength is the tendency of an acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special ca ...

strong acid
s are
hydrochloric acid Hydrochloric acid +(aq) Cl−(aq) or H3O+ Cl− also known as muriatic acid, is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a salin ...

hydrochloric acid
(HCl),
hydroiodic acid Hydroiodic acid (or hydriodic acid) is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table sa ...
(HI),
hydrobromic acid Hydrobromic acid is a strong acid Acid strength is the tendency of an acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the pr ...

hydrobromic acid
(HBr),
perchloric acid Perchloric acid is a mineral acid A mineral acid (or inorganic acid) is an acid An acid is a molecule or ion An ion () is a particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) ...
(HClO4), nitric acid (HNO3) and
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
(H2SO4). In water each of these essentially ionizes 100%. The stronger an acid is, the more easily it loses a proton, H+. Two key factors that contribute to the ease of deprotonation are the chemical polarity, polarity of the H—A bond and the size of atom A, which determines the strength of the H—A bond. Acid strengths are also often discussed in terms of the stability of the conjugate base. Stronger acids have a larger
acid dissociation constant An acid dissociation constant, ''K''a, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength Physical strength *Physical strength, as in people or animals *Hysterical strength, extreme str ...
, ''K''a and a more negative p''K''a than weaker acids. Sulfonic acids, which are organic oxyacids, are a class of strong acids. A common example is toluenesulfonic acid (tosylic acid). Unlike sulfuric acid itself, sulfonic acids can be solids. In fact, polystyrene functionalized into polystyrene sulfonate is a solid strongly acidic plastic that is filterable. Superacids are acids stronger than 100% sulfuric acid. Examples of superacids are fluoroantimonic acid, magic acid and
perchloric acid Perchloric acid is a mineral acid A mineral acid (or inorganic acid) is an acid An acid is a molecule or ion An ion () is a particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) ...
. Superacids can permanently protonate water to give ionic, crystalline hydronium "salts". They can also quantitatively stabilize carbocations. While ''K''a measures the strength of an acid compound, the strength of an aqueous acid solution is measured by pH, which is an indication of the concentration of hydronium in the solution. The pH of a simple solution of an acid compound in water is determined by the dilution of the compound and the compound's ''K''a.


Lewis acid strength in non-aqueous solutions

Lewis acids have been classified in the ECW model and it has been shown that there is no one order of acid strengths. The relative acceptor strength of Lewis acids toward a series of bases, versus other Lewis acids, can be illustrated by ECW model, C-B plots. It has been shown that to define the order of Lewis acid strength at least two properties must be considered. For Pearson's qualitative HSAB theory the two properties are HSAB theory, hardness and strength while for Drago’s quantitative ECW model the two properties are electrostatic and covalent.


Chemical characteristics


Monoprotic acids

Monoprotic acids, also known as monobasic acids, are those acids that are able to donate one
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

proton
per molecule during the process of dissociation (chemistry), dissociation (sometimes called ionization) as shown below (symbolized by HA): :HA(aq) + H2O(l) H3O + A         ''K''a Common examples of monoprotic acids in mineral acids include
hydrochloric acid Hydrochloric acid +(aq) Cl−(aq) or H3O+ Cl− also known as muriatic acid, is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a salin ...

hydrochloric acid
(HCl) and nitric acid (HNO3). On the other hand, for organic acids the term mainly indicates the presence of one carboxylic acid group and sometimes these acids are known as monocarboxylic acid. Examples in organic acids include formic acid (HCOOH),
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
(CH3COOH) and benzoic acid (C6H5COOH).


Polyprotic acids

Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule. Specific types of polyprotic acids have more specific names, such as diprotic (or dibasic) acid (two potential protons to donate), and triprotic (or tribasic) acid (three potential protons to donate). A diprotic acid (here symbolized by H2A) can undergo one or two dissociations depending on the pH. Each dissociation has its own dissociation constant, Ka1 and Ka2. : : The first dissociation constant is typically greater than the second; i.e., ''K''a1 > ''K''a2. For example,
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
(H2SO4) can donate one proton to form the bisulfate anion (HSO), for which ''K''a1 is very large; then it can donate a second proton to form the sulfate anion (SO), wherein the ''K''a2 is intermediate strength. The large ''K''a1 for the first dissociation makes sulfuric a strong acid. In a similar manner, the weak unstable carbonic acid can lose one proton to form bicarbonate anion and lose a second to form carbonate anion (CO). Both ''K''a values are small, but ''K''a1 > ''K''a2 . A triprotic acid (H3A) can undergo one, two, or three dissociations and has three dissociation constants, where ''K''a1 > ''K''a2 > ''K''a3. : : : An inorganic example of a triprotic acid is orthophosphoric acid (H3PO4), usually just called phosphoric acid. All three protons can be successively lost to yield H2PO, then HPO, and finally PO, the orthophosphate ion, usually just called phosphate. Even though the positions of the three protons on the original phosphoric acid molecule are equivalent, the successive ''K''a values differ since it is energetically less favorable to lose a proton if the conjugate base is more negatively charged. An organic compound, organic example of a triprotic acid is
citric acid Citric acid is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, ...

citric acid
, which can successively lose three protons to finally form the citrate ion. Although the subsequent loss of each hydrogen ion is less favorable, all of the conjugate bases are present in solution. The fractional concentration, ''α'' (alpha), for each species can be calculated. For example, a generic diprotic acid will generate 3 species in solution: H2A, HA, and A2−. The fractional concentrations can be calculated as below when given either the pH (which can be converted to the [H+]) or the concentrations of the acid with all its conjugate bases: :\begin \alpha_\ce &= \frac = \frac\\ \alpha_\ce &= \frac = \frac\\ \alpha_\ce&= \frac = \frac \end A plot of these fractional concentrations against pH, for given ''K''1 and ''K''2, is known as a Bjerrum plot. A pattern is observed in the above equations and can be expanded to the general ''n'' -protic acid that has been deprotonated ''i'' -times: : \alpha_= where ''K''0 = 1 and the other K-terms are the dissociation constants for the acid.


Neutralization

Neutralization (chemistry), Neutralization is the reaction between an acid and a base, producing a salt (chemistry), salt and neutralized base; for example,
hydrochloric acid Hydrochloric acid +(aq) Cl−(aq) or H3O+ Cl− also known as muriatic acid, is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a salin ...

hydrochloric acid
and sodium hydroxide form sodium chloride and water: :HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq) Neutralization is the basis of titration, where a pH indicator shows equivalence point when the equivalent number of moles of a base have been added to an acid. It is often wrongly assumed that neutralization should result in a solution with pH 7.0, which is only the case with similar acid and base strengths during a reaction. Neutralization with a base weaker than the acid results in a weakly acidic salt. An example is the weakly acidic
ammonium chloride Ammonium chloride is an inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, ...

ammonium chloride
, which is produced from the strong acid
hydrogen chloride The compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with ...

hydrogen chloride
and the weak base
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
. Conversely, neutralizing a weak acid with a strong base gives a weakly basic salt, e.g. sodium fluoride from hydrogen fluoride and sodium hydroxide.


Weak acid–weak base equilibrium

In order for a protonated acid to lose a proton, the pH of the system must rise above the p''K''a of the acid. The decreased concentration of H+ in that basic solution shifts the equilibrium towards the conjugate base form (the deprotonated form of the acid). In lower-pH (more acidic) solutions, there is a high enough H+ concentration in the solution to cause the acid to remain in its protonated form. Solutions of weak acids and salts of their conjugate bases form buffer solutions.


Titration

To determine the concentration of an acid in an aqueous solution, an acid–base titration is commonly performed. A strong base solution with a known concentration, usually NaOH or KOH, is added to neutralize the acid solution according to the color change of the indicator with the amount of base added. The titration curve of an acid titrated by a base has two axes, with the base volume on the x-axis and the solution's pH value on the y-axis. The pH of the solution always goes up as the base is added to the solution.


Example: Diprotic acid

For each diprotic acid titration curve, from left to right, there are two midpoints, two equivalence points, and two buffer regions.


Equivalence points

Due to the successive dissociation processes, there are two equivalence points in the titration curve of a diprotic acid. The first equivalence point occurs when all first hydrogen ions from the first ionization are titrated. In other words, the amount of OH added equals the original amount of H2A at the first equivalence point. The second equivalence point occurs when all hydrogen ions are titrated. Therefore, the amount of OH added equals twice the amount of H2A at this time. For a weak diprotic acid titrated by a strong base, the second equivalence point must occur at pH above 7 due to the hydrolysis of the resulted salts in the solution. At either equivalence point, adding a drop of base will cause the steepest rise of the pH value in the system.


Buffer regions and midpoints

A titration curve for a diprotic acid contains two midpoints where pH=pKa. Since there are two different Ka values, the first midpoint occurs at pH=pKa1 and the second one occurs at pH=pKa2. Each segment of the curve which contains a midpoint at its center is called the buffer region. Because the buffer regions consist of the acid and its conjugate base, it can resist pH changes when base is added until the next equivalent points.


Applications of acids

Acids exist universally in our life. There are both numerous kinds of natural acid compounds with biological functions and massive synthesized acids which are used in many ways.


In industry

Acids are fundamental reagents in treating almost all processes in today's industry. Sulfuric acid, a diprotic acid, is the most widely used acid in industry, which is also the most-produced industrial chemical in the world. It is mainly used in producing fertilizer, detergent, batteries and dyes, as well as used in processing many products such like removing impurities. According to the statistics data in 2011, the annual production of sulfuric acid was around 200 million tonnes in the world. For example, phosphate minerals react with sulfuric acid to produce phosphoric acid for the production of phosphate fertilizers, and zinc is produced by dissolving zinc oxide into sulfuric acid, purifying the solution and electrowinning. In the chemical industry, acids react in neutralization reactions to produce salts. For example, nitric acid reacts with
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
to produce ammonium nitrate, a fertilizer. Additionally, carboxylic acids can be Esterification, esterified with alcohols, to produce esters. Acids are often used to remove rust and other corrosion from metals in a process known as pickling (metal), pickling. They may be used as an electrolyte in a wet cell battery, such as
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
in a car battery.


In food

Tartaric acid is an important component of some commonly used foods like unripened mangoes and tamarind. Natural fruits and vegetables also contain acids. Citric acid is present in oranges, lemon and other citrus fruits. Oxalic acid is present in tomatoes, spinach, and especially in carambola and rhubarb; rhubarb leaves and unripe carambolas are toxic because of high concentrations of oxalic acid. Ascorbic acid (Vitamin C) is an essential vitamin for the human body and is present in such foods as amla (Phyllanthus emblica, Indian gooseberry), lemon, citrus fruits, and guava. Many acids can be found in various kinds of food as additives, as they alter their taste and serve as preservatives. Phosphoric acid, for example, is a component of cola drinks. Acetic acid is used in day-to-day life as vinegar. Citric acid is used as a preservative in sauces and pickles. Carbonic acid is one of the most common acid additives that are widely added in soft drinks. During the manufacturing process, CO2 is usually pressurized to dissolve in these drinks to generate carbonic acid. Carbonic acid is very unstable and tends to decompose into water and CO2 at room temperature and pressure. Therefore, when bottles or cans of these kinds of soft drinks are opened, the soft drinks fizz and effervesce as CO2 bubbles come out. Certain acids are used as drugs. Acetylsalicylic acid (Aspirin) is used as a pain killer and for bringing down fevers.


In human bodies

Acids play important roles in the human body. The hydrochloric acid present in the stomach aids digestion by breaking down large and complex food molecules. Amino acids are required for synthesis of proteins required for growth and repair of body tissues. Fatty acids are also required for growth and repair of body tissues. Nucleic acids are important for the manufacturing of DNA and RNA and transmitting of traits to offspring through genes. Carbonic acid is important for maintenance of pH equilibrium in the body. Human bodies contain a variety of organic and inorganic compounds, among those dicarboxylic acids play an essential role in many biological behaviors. Many of those acids are amino acids which mainly serve as materials for the synthesis of proteins. Other weak acids serve as buffers with their conjugate bases to keep the body's pH from undergoing large scale changes which would be harmful to cells. The rest of the dicarboxylic acids also participate in the synthesis of various biologically important compounds in human bodies.


Acid catalysis

Acids are used as catalysts in industrial and organic chemistry; for example,
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
is used in very large quantities in the alkylation process to produce gasoline. Some acids, such as sulfuric, phosphoric, and hydrochloric acids, also effect Dehydration reaction, dehydration and condensation reactions. In biochemistry, many enzymes employ acid catalysis.


Biological occurrence

Many biologically important molecules are acids. Nucleic acids, which contain acidic phosphate, phosphate groups, include DNA and RNA. Nucleic acids contain the genetic code that determines many of an organism's characteristics, and is passed from parents to offspring. DNA contains the chemical blueprint for the synthesis of proteins which are made up of amino acid subunits. Cell membranes contain fatty acid esters such as phospholipids. An α-amino acid has a central carbon (the α or alpha and beta carbon, ''alpha'' carbon) which is covalently bonded to a carboxyl group (thus they are carboxylic acids), an amine, amino group, a hydrogen atom and a variable group. The variable group, also called the R group or side chain, determines the identity and many of the properties of a specific amino acid. In glycine, the simplest amino acid, the R group is a hydrogen atom, but in all other amino acids it is contains one or more carbon atoms bonded to hydrogens, and may contain other elements such as sulfur, oxygen or nitrogen. With the exception of glycine, naturally occurring amino acids are Chirality (chemistry), chiral and almost invariably occur in the Chirality (chemistry)#By configuration: D- and L-, L-configuration. Peptidoglycan, found in some bacterial cell walls contains some D-amino acids. At physiological pH, typically around 7, free amino acids exist in a charged form, where the acidic carboxyl group (-COOH) loses a proton (-COO) and the basic amine group (-NH2) gains a proton (-NH). The entire molecule has a net neutral charge and is a zwitterion, with the exception of amino acids with basic or acidic side chains. Aspartic acid, for example, possesses one protonated amine and two deprotonated carboxyl groups, for a net charge of −1 at physiological pH. Fatty acids and fatty acid derivatives are another group of carboxylic acids that play a significant role in biology. These contain long hydrocarbon chains and a carboxylic acid group on one end. The cell membrane of nearly all organisms is primarily made up of a phospholipid bilayer, a micelle of hydrophobic fatty acid esters with polar, hydrophilic phosphate "head" groups. Membranes contain additional components, some of which can participate in acid–base reactions. In humans and many other animals,
hydrochloric acid Hydrochloric acid +(aq) Cl−(aq) or H3O+ Cl− also known as muriatic acid, is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a salin ...

hydrochloric acid
is a part of the
gastric acid Gastric acid, gastric juice, or stomach acid, is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of for ...
secreted within the stomach to help hydrolyze proteins and polysaccharides, as well as converting the inactive pro-enzyme, pepsinogen into the digestive enzyme, enzyme, pepsin. Some organisms produce acids for defense; for example, ants produce formic acid. Acid–base equilibrium plays a critical role in regulating mammalian breathing. molecular oxygen, Oxygen gas (O2) drives cellular respiration, the process by which animals release the chemical potential energy stored in food, producing carbon dioxide (CO2) as a byproduct. Oxygen and carbon dioxide are exchanged in the lungs, and the body responds to changing energy demands by adjusting the rate of ventilation (physiology), ventilation. For example, during periods of exertion the body rapidly breaks down stored carbohydrates and fat, releasing CO2 into the blood stream. In aqueous solutions such as blood CO2 exists in equilibrium with carbonic acid and bicarbonate ion. : CO2 + H2O H2CO3 H+ + HCO It is the decrease in pH that signals the brain to breathe faster and deeper, expelling the excess CO2 and resupplying the cells with O2. Cell membranes are generally impermeable to charged or large, polar molecules because of the lipophilicity, lipophilic fatty acyl chains comprising their interior. Many biologically important molecules, including a number of pharmaceutical agents, are organic weak acids which can cross the membrane in their protonated, uncharged form but not in their charged form (i.e. as the conjugate base). For this reason the activity of many drugs can be enhanced or inhibited by the use of antacids or acidic foods. The charged form, however, is often more soluble in blood and cytosol, both aqueous environments. When the extracellular environment is more acidic than the neutral pH within the cell, certain acids will exist in their neutral form and will be membrane soluble, allowing them to cross the phospholipid bilayer. Acids that lose a proton at the intracellular pH will exist in their soluble, charged form and are thus able to diffuse through the cytosol to their target. Ibuprofen, aspirin and penicillin are examples of drugs that are weak acids.


Common acids


Mineral acids (inorganic acids)

* Hydrogen halides and their solutions: hydrofluoric acid (HF),
hydrochloric acid Hydrochloric acid +(aq) Cl−(aq) or H3O+ Cl− also known as muriatic acid, is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a salin ...

hydrochloric acid
(HCl),
hydrobromic acid Hydrobromic acid is a strong acid Acid strength is the tendency of an acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the pr ...

hydrobromic acid
(HBr),
hydroiodic acid Hydroiodic acid (or hydriodic acid) is an aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table sa ...
(HI) * Halogen oxoacids: hypochlorous acid (HClO), chlorous acid (HClO2), chloric acid (HClO3),
perchloric acid Perchloric acid is a mineral acid A mineral acid (or inorganic acid) is an acid An acid is a molecule or ion An ion () is a particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) ...
(HClO4), and corresponding analogs for bromine and iodine ** Hypofluorous acid (HFO), the only known oxoacid for fluorine. * Sulfuric acid (H2SO4) * Fluorosulfuric acid (HSO3F) * Nitric acid (HNO3) * Phosphoric acid (H3PO4) * Fluoroantimonic acid (HSbF6) * Fluoroboric acid (HBF4) * Hexafluorophosphoric acid (HPF6) * Chromic acid (H2CrO4) * Boric acid (H3BO3)


Sulfonic acids

A sulfonic acid has the general formula RS(=O)2–OH, where R is an organic radical. * Methanesulfonic acid (or mesylic acid, CH3SO3H) * Ethanesulfonic acid (or esylic acid, CH3CH2SO3H) * Benzenesulfonic acid (or besylic acid, C6H5SO3H) * p-Toluenesulfonic acid (or tosylic acid, CH3C6H4SO3H) * Trifluoromethanesulfonic acid (or triflic acid, CF3SO3H) * Polystyrene sulfonic acid (sulfonated polystyrene, [CH2CH(C6H4)SO3H]n)


Carboxylic acids

A carboxylic acid has the general formula R-C(O)OH, where R is an organic radical. The carboxyl group -C(O)OH contains a carbonyl group, C=O, and a hydroxyl group, O-H. * Acetic acid (CH3COOH) * Citric acid (C6H8O7) * Formic acid (HCOOH) * Gluconic acid HOCH2-(CHOH)4-COOH * Lactic acid (CH3-CHOH-COOH) * Oxalic acid (HOOC-COOH) * Tartaric acid (HOOC-CHOH-CHOH-COOH)


Halogenated carboxylic acids

Halogenation at alpha and beta carbon, alpha position increases acid strength, so that the following acids are all stronger than acetic acid. * Fluoroacetic acid * Trifluoroacetic acid * Chloroacetic acid * Dichloroacetic acid * Trichloroacetic acid


Vinylogous carboxylic acids

Normal carboxylic acids are the direct union of a carbonyl group and a hydroxyl group. In vinylogous carboxylic acids, a carbon-carbon double bond separates the carbonyl and hydroxyl groups. * Ascorbic acid


Nucleic acids

* DNA, Deoxyribonucleic acid (DNA) * RNA, Ribonucleic acid (RNA)


References


Listing of strengths of common acids and bases
* *


External links



Acid–Base equilibria diagrams, calculation and titration curves simulation and analysis – freeware {{Authority control Acids, Acid–base chemistry