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the (353), acid (289), and (167), acids (134), are (75), base (60), for (54), that (40), proton (36), lewis (36), can (35), with (34), form (32), solution (31), brønsted (31), hydrogen (24), #example (23), from (22), one (22), arrhenius (22), reactions (21), lowry (21), aqueous (20), used (20), also (20), water (20), other (19), concentration (19), two (18), chemical (17), weak (17), which (17), this (16), has (16), ion (16), group (16), conjugate (16), amino (15), sulfuric (15), carboxylic (15), strength (15), many (15), acidic (15), equilibrium (15), dissociation (15), chloride (15), pair (15), isbn (14), titration (14), organic (14), than (14), cooh (14), when (14), such (14), hydronium (14), chemistry (13), solutions (13), hydrochloric (13), added (13), reaction (13), article (12), bases (12), diprotic (12), where (12), their (12), hcl (12), first (12), strong (12), all (11), acetic (11), known (11), not (11), more (11), its (11), electron (11), bond (10), because (10), produce 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ordfriisk gaeilge kriyòl gwiyannen gàidhlig galego hausa 客家語 hak kâ ngî עברית हिन्दी fiji hindi hrvatski kreyòl ayisyen magyar հայերեն interlingua bahasa indonesia ilokano ido íslenska italiano 日本語 patois jawa ქართული qaraqalpaqsha taqbaylit jju қазақша ಕನ್ನಡ yerwa kanuri 한국어 کٲشُر kurdî кыргызча latina lëtzebuergesch lingua franca nova limburgs lombard ລາວ lietuvių latviešu मगही мокшень malagasy македонски മലയാളം монгол ꯃꯤꯇꯩ ꯂꯣꯟ मराठी bahasa melayu မြန်မာဘာသာ napulitano plattdüütsch नेपाली नेपाल भाषा nederlands norsk nynorsk norsk bokmål novial occitan oromoo ਪੰਜਾਬੀ pangasinan polski piemontèis پنجابی português runa simi română русский русиньскый संस्कृतम् саха тыла sicilianu سنڌي srpskohrvatski српскохрватски simple english slovenčina سرائیکی slovenščina chishona shqip српски srpski sunda svenska kiswahili தமிழ் తెలుగు тоҷикӣ ไทย tagalog türkçe татарча tatarça тыва дыл ئۇيغۇرچە uyghurche українська اردو oʻzbekcha ўзбекча vèneto vepsän kel tiếng việt walon winaray 吴语 ייִדיש yorùbá vahcuengh 文言 閩南語 bân lâm gí 粵語 中文 edit links article talk english read view source view history tools tools move to sidebar hide actions read view source view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia redirected from acidity this article is about acids in chemistry for other uses see acid disambiguation acidity and acidic redirect here for the novelette see acidity novelette for the band see acidic band chemical compound giving a proton or accepting an electron pair zinc a typical metal reacting with hydrochloric acid a typical acid acids and bases acceptor number acid acid base reaction acid base homeostasis acid strength acidity function amphoterism base buffer solutions dissociation constant donor number equilibrium chemistry extraction hammett acidity function ph proton affinity self ionization of water titration lewis acid catalysis frustrated lewis pair chiral lewis acid ecw model acid types brønsted lowry lewis mineral organic oxide strong superacids weak solid base types brønsted lowry lewis organic oxide strong superbases non nucleophilic weak v t e an acid is a molecule or ion capable of either donating a proton i e hydrogen cation h known as a brønsted lowry acid or forming a covalent bond with an electron pair known as a lewis acid 1 the first category of acids are the proton donors or brønsted lowry acids in the special case of aqueous solutions proton donors form the hydronium ion h 3 o and are known as arrhenius acids brønsted and lowry generalized the arrhenius theory to include non aqueous solvents a brønsted lowry 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 that provide a practical description of an acid 2 acids form aqueous solutions with a sour taste can turn blue litmus red and react with bases and certain metals like calcium to form salts the word acid is derived from the latin acidus meaning sour 3 an aqueous solution of an acid has a ph 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 1 a lower ph means a higher acidity and thus a higher concentration of hydrogen cations in the solution chemicals or substances having the property of an acid are said to be acidic common aqueous acids include hydrochloric acid a solution of hydrogen chloride that is found in gastric acid in the stomach and activates digestive enzymes acetic acid vinegar is a dilute aqueous solution of this liquid sulfuric acid used in car batteries and 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 1 sense that are solids liquids or gases strong acids and some concentrated weak acids are corrosive but there are exceptions such as carboranes and boric acid the second category of acids are lewis acids which form a covalent bond with an electron pair an example is boron trifluoride bf 3 whose boron atom has a vacant orbital that 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 nh 3 lewis 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 hydrogen chloride acetic acid and most other brønsted lowry acids cannot form a covalent bond with an electron pair however and are therefore not lewis acids 4 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 such 4 definitions and concepts main article acid base reaction modern definitions are concerned with the fundamental chemical reactions common to all acids most acids encountered in everyday life are aqueous solutions 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 bases due to the lone pairs of electrons on their oxygen and nitrogen atoms arrhenius acids svante arrhenius in 1884 svante arrhenius attributed the properties of acidity to hydrogen cations h later described as protons or hydrons an arrhenius acid is a substance that when added to water increases the concentration of h ions in the water 4 5 chemists often write h aq and refer to the hydrogen cation when describing acid base reactions but the free hydrogen nucleus a proton does not exist alone in water it exists as the hydronium ion h 3 o or other forms h 5 o 2 h 9 o 4 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 on the other hand is a substance that increases the concentration of hydroxide oh ions when dissolved in water this decreases the concentration of hydronium because the ions react to form h 2 o molecules h 3 o aq oh aq h 2 o liq h 2 o liq 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 main article brønsted lowry acid base theory acetic acid a weak acid donates a proton hydrogen ion highlighted in green to water in an equilibrium reaction to give the acetate ion and the hydronium ion red oxygen black carbon white hydrogen while the arrhenius concept is useful for describing many reactions it is also quite limited in its scope in 1923 chemists johannes nicolaus brønsted and 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 5 brønsted lowry acid base theory has several advantages over arrhenius theory consider the following reactions of acetic acid ch 3 cooh the organic acid that gives vinegar its characteristic taste ch 3 cooh h 2 o ch 3 coo h 3 o ch 3 cooh nh 3 ch 3 coo nh 4 both theories easily describe the first reaction ch 3 cooh acts as an arrhenius acid because it acts as a source of h 3 o when dissolved in water and it acts as a brønsted acid by donating a proton to water in the second example ch 3 cooh undergoes the same transformation in this case donating a proton to ammonia nh 3 but does not relate to the arrhenius definition of an acid because the reaction does not produce hydronium nevertheless ch 3 cooh is both an arrhenius and a brønsted lowry acid brønsted lowry theory can be used to describe reactions of molecular compounds in nonaqueous solution or the gas phase hydrogen chloride hcl and ammonia combine under several different conditions to form ammonium chloride nh 4 cl in aqueous solution hcl behaves as hydrochloric acid and exists as hydronium and chloride ions the following reactions illustrate the limitations of arrhenius s definition h 3 o aq cl aq nh 3 cl aq nh 4 aq h 2 o hcl benzene nh 3 benzene nh 4 cl s hcl g nh 3 g nh 4 cl 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 react to form solid ammonium chloride in a benzene solvent and in the third gaseous hcl and nh 3 combine to form the solid lewis acids main article lewis acids and bases 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 5 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 to boron trifluoride to form the product tetrafluoroborate fluoride loses a pair of valence electrons because the electrons shared in the b f bond are located in the region of space between the two atomic nuclei and are therefore more distant from the fluoride nucleus than they are in the lone fluoride ion bf 3 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 h 3 o 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 or an electrophile organic brønsted acids such as acetic citric or oxalic acid are not lewis acids 4 they dissociate in water to produce a lewis acid h but at the same time they 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 this reaction is referred to as protolysis the protonated form ha of an acid is also sometimes referred to as the free acid 6 acid base conjugate pairs differ by one proton and can be interconverted by the addition or removal of a proton protonation and deprotonation respectively 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 between the acid and its conjugate base the equilibrium constant k is an expression of the equilibrium concentrations of the molecules or the ions in solution brackets indicate concentration such that h 2 o means the concentration of h 2 o the acid dissociation constant k a is generally used in the context of acid base reactions the numerical value of k a is equal to the product multiplication 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 h a ha displaystyle k_ a frac ce h a ce ha the stronger of two acids will have a higher k a than the weaker acid the ratio of hydrogen cations 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 log 10 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 anions 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 as its anion so the hydro prefix is used and the ide suffix makes the name take the form hydrochloric acid classical naming system anion prefix anion suffix acid prefix acid suffix example per ate per ic acid perchloric acid hclo 4 chloric acid hclo 3 ite ous acid chlorous acid hclo 2 hypo ite hypo ous acid hypochlorous acid hclo ide hydro ic acid hydrochloric acid hcl in the iupac 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 main article 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 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 acids are hydrochloric acid hcl hydroiodic acid hi hydrobromic acid hbr perchloric acid hclo 4 nitric acid hno 3 and sulfuric acid h 2 so 4 in water each of these essentially ionizes 100 the stronger an acid is the more easily it lose...