LEARNING objective recognize acids or bases Introduction In 1884, the Swedish chemist Svante Arrhenius proposed two specific classifications of compounds, termed acids and bases. When dissolved in an aqueous solution, certain ions were released into the solution. As defined by Arrhenius, acid-base reactions are characterized by acids, which dissociate in aqueous solution to form hydrogen ions (H + ) and bases, which form hydroxide (OH − ) ions. Arrhenius received the lowest passing score for his doctoral thesis with these innovative ideas about acids and bases. Ten years later he was awarded the Nobel Prize for his insights. Acids are defined as a compound or element that releases hydrogen (H + ) ions into the solution (mainly water). \[ \ce{NHO_3 (aq) + H_2O(l) \rightarrow H_3O^+ + NO_3^- (aq)}\] In this reaction nitric acid (HNO 3 ) disassociates into hydrogen (H + ) and nitrate (NO 3 - ) ions when dissolved in water. Bases are defined as a compound or element that releases hydroxide (OH - ) ions into the solution. \[ \ce{LiOH(s) ->[\ce{H2O}] Li^{+}(aq) + OH^{-}(aq)}\] In this reaction lithium hydroxide (\(\ce{LiOH}\)) dissociates into lithium (\(\ce{Li^{+}}\)) and hydroxide (\(\ce{OH^{-}}\)) ions when dissolved in water. One way to define a class of compounds is by describing the various characteristics its members have in common. In the case of the compounds known as acids, the common characteristics include a sour taste, the ability to change the color of the vegetable dye litmus to red, and the ability to dissolve certain metals and simultaneously produce hydrogen gas. For the compounds called bases, the common characteristics are a slippery texture, a bitter taste, and the ability to change the color of litmus to blue. Acids and bases also react with each other to form compounds generally known as salts. Although we include their tastes among the common characteristics of acids and bases, we never advocate tasting an unknown chemical! Chemists prefer, however, to have definitions for acids and bases in chemical terms. The Swedish chemist Svante Arrhenius developed the first chemical definitions of acids and bases in the late 1800s. Arrhenius defined an acid as a compound that increases the concentration of hydrogen ion (H + ) in aqueous solution. Many acids are simple compounds that release a hydrogen cation into solution when they dissolve. Similarly, Arrhenius defined a base as a compound that increases the concentration of hydroxide ion (OH − ) in aqueous solution. Many bases are ionic compounds that have the hydroxide ion as their anion, which is released when the base dissolves in water. Acids Acids.1 Bases Bases.1 Formula Name Formula Name HCl(aq) hydrochloric acid NaOH(aq) sodium hydroxide HBr(aq) hydrobromic acid KOH(aq) potassium hydroxide HI(aq) hydriodic acid Mg(OH)2(aq) magnesium hydroxide H2S(aq) hydrosulfuric acid Ca(OH)2(aq) calcium hydroxide HC2H3O2(aq) acetic acid NH3(aq) ammonia HNO3(aq) nitric acid NaN NaN HNO2(aq) nitrous acid NaN NaN H2SO4(aq) sulfuric acid NaN NaN H2SO3(aq) sulfurous acid NaN NaN HClO3(aq) chloric acid NaN NaN HClO4(aq) perchloric acid NaN NaN HClO2(aq) chlorous acid NaN NaN H3PO4(aq) phosphoric acid NaN NaN H3PO3(aq) phosphorous acid NaN NaN Many bases and their aqueous solutions are named using the normal rules of ionic compounds that were presented previously; that is, they are named as hydroxide compounds. For example, the base sodium hydroxide (NaOH) is both an ionic compound and an aqueous solution. However, aqueous solutions of acids have their own naming rules. The names of binary acids (compounds with hydrogen and one other element in their formula) are based on the root of the name of the other element preceded by the prefix hydro - and followed by the suffix - ic acid . Thus, an aqueous solution of HCl [designated “HCl(aq)”] is called hydrochloric acid, H 2 S(aq) is called hydrosulfuric acid, and so forth. Acids composed of more than two elements (typically hydrogen and oxygen and some other element) have names based on the name of the other element, followed by the suffix - ic acid or -ous acid , depending on the number of oxygen atoms in the acid’s formula. Other prefixes, like per- and hypo- , also appear in the names for some acids. Unfortunately, there is no strict rule for the number of oxygen atoms that are associated with the - ic acid suffix; the names of these acids are best memorized. Table \(\PageIndex{1}\) lists some acids and bases and their names. Note that acids have hydrogen written first, as if it were the cation, while most bases have the negative hydroxide ion, if it appears in the formula, written last. The name oxygen comes from the Latin meaning “acid producer” because its discoverer, Antoine Lavoisier, thought it was the essential element in acids. Lavoisier was wrong, but it is too late to change the name now. Example \(\PageIndex{1}\) Name each substance. HF(aq) Sr(OH) 2 (aq) Solution This acid has only two elements in its formula, so its name includes the hydro - prefix. The stem of the other element’s name, fluorine, is fluor , and we must also include the - ic acid ending. Its name is hydrofluoric acid. This base is named as an ionic compound between the strontium ion and the hydroxide ion: strontium hydroxide. Exercise \(\PageIndex{1}\) Name each substance. H 2 Se(aq) Ba(OH) 2 (aq) Notice that one base listed in Table \(\PageIndex{1}\)—ammonia—does not have hydroxide as part of its formula. How does this compound increase the amount of hydroxide ion in aqueous solution? Instead of dissociating into hydroxide ions, ammonia molecules react with water molecules by taking a hydrogen ion from the water molecule to produce an ammonium ion and a hydroxide ion: \[NH_{3(aq)} + H_2O_{(ℓ)} \rightarrow NH^+_{4(aq)} + OH^−_{(aq)} \label{Eq1}\] Because this reaction of ammonia with water causes an increase in the concentration of hydroxide ions in solution, ammonia satisfies the Arrhenius definition of a base. Many other nitrogen-containing compounds are bases because they too react with water to produce hydroxide ions in aqueous solution.