{"id":848,"date":"2018-03-20T16:16:37","date_gmt":"2018-03-20T16:16:37","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/?post_type=chapter&p=848"},"modified":"2018-10-03T13:52:53","modified_gmt":"2018-10-03T13:52:53","slug":"10-1-arrhenius-definition-of-acids-and-bases","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-monroecc-orgbiochemistry\/chapter\/10-1-arrhenius-definition-of-acids-and-bases\/","title":{"raw":"10.1 Arrhenius Definition of Acids and Bases","rendered":"10.1 Arrhenius Definition of Acids and Bases"},"content":{"raw":"
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10.1<\/span> Arrhenius Definition of Acids and Bases<\/h2>\r\n
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Learning Objective<\/h3>\r\n
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  1. Recognize a compound as an Arrhenius acid or an Arrhenius base.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

    One way to define a class of compounds is by describing the various characteristics its members have in common. In the case of acids, the common characteristics include a sour taste, the ability to change the color of litmus<\/em> to red, and the ability to react with certain metals<\/strong>, eroding the metal and producing hydrogen gas and an ionic compound. For bases, the common characteristics are a slippery texture, a bitter taste, and the ability to change the color of litmus to blue.<\/strong>\u00a0 Acids and bases also react with each other in neutralization reactions, forming water and ionic compounds, also known as salts.<\/p>\r\nIndicators<\/strong> are compounds that exhibit different colors when placed in solutions with different acid\/base (pH) balances.\u00a0 As described in the previous paragraph, litmus, a compound obtained from lichens, is an indicator because it is blue when in a basic solutions and red when in an acidic solution. Juice from purple cabbage is another natural indicator, and it exhibits a wide range of colors based on the pH it experiences as seen in Figure 10.1 below.\u00a0 Other indicators include phenolphthalein and\u00a0 bromothymol blue.\u00a0 Indicaotrs are often used in titration<\/strong>, a measured form of\u00a0 neutralization reaction, to visually indicate that the moles of acid and moles of base are balanced at the equivalence point.\r\n

    \"\"Figure 10.1 Purple Cabbage Juice Indicator at Various pH Levels.\u00a0 Attribution: By Indikator-Blaukraut.JPG: Supermartlderivative work: Haltopub (Indikator-Blaukraut.JPG) [CC BY-SA 3.0 (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) or GFDL (http:\/\/www.gnu.org\/copyleft\/fdl.html)], via Wikimedia Commons<\/div>\r\n
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    Note<\/h3>\r\n

    Although tastes are included among the common characteristics of acids and bases, never taste an unknown chemical!<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

    Chemists define acids and bases in terms of their chemical properties, but the definitions differ in models developed at different times.\u00a0 The Swedish chemist Svante Arrhenius developed the first chemical definitions of acids and bases in the late 1800s. Arrhenius<\/strong> defined an acid<\/span><\/span>\u00a0as a compound that increases the concentration of hydrogen ion (H+<\/sup>) in aqueous solution<\/strong>. Many acids are simple compounds that release a hydrogen cation into solution when they dissolve. Similarly, Arrhenius defined a base<\/span><\/span>\u00a0as a compound that increases the concentration of hydroxide ion (OH\u2212<\/sup>) in aqueous solution.<\/strong> Many bases are ionic compounds that have the hydroxide ion as their anion, which is released when the base dissolves in water.<\/p>\r\n

    Many bases and their aqueous solutions are named using the normal rules of ionic compounds that were presented in Chapter 3 \"Ionic Bonding and Simple Ionic Compounds\"<\/a>, Section 3.4 \"Ionic Nomenclature\".\u00a0<\/a> For example, the name sodium hydroxide (NaOH) is used as the name of the ionic compound and<\/em> for an aqueous solution of the compound.<\/p>\r\n

    However, aqueous solutions of binary acids,\u00a0compounds with hydrogen and one other element in their formula, have one name for the compound alone and a different name when the compound is dissolved in water. Such a compound alone is named hydrogen\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/span>ide<\/em> with the blank being filled with the element's root name.\u00a0 For example, HCl not dissolved in water is hydrogen chloride.But when hydrogen chloride dissolves in water, its name changes to hydrochloric acid, following the pattern\u00a0hydro \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 <\/span>ic acid, <\/em> again with the blank being replaced by the element's root name.<\/p>\r\n

    Acids composed of more than two elements, typically one to three hydrogen atoms with a polyatomic ion. The name of an oxyacid is based on the name of the polyatomic ion.\u00a0 If the polyatomic ion name ends with -ate<\/em>, name of its acid will end in -ic acid<\/em>.\u00a0 If the polyatomic ion mane ends in -ite<\/em>, the name of the acid ends in -ous acid. <\/em>\u00a0 For example HNO3<\/sub> is nitric acid because it contains the nitrate polyatomic ion.\u00a0 HNO2<\/sub> is nitrous acid because it contains the polyatomic ion nitrite.<\/p>\r\n

    . Table 10.1 \"Formulas and Names for Some Acids and Bases\"<\/a> lists some acids and bases and their names. Note that acids have hydrogen written first, as if it were the cation, while bases that have the negative hydroxide ion have it written last.<\/p>\r\n\r\n

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    Note<\/h3>\r\n

    The name oxygen<\/em> comes from the Latin meaning \u201cacid producer\u201d 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.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

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    Table 10.1<\/span> Formulas and Names for Some Acids and Bases<\/p>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
    Formula<\/th>\r\nName<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
    Acids<\/strong><\/td>\r\n<\/tr>\r\n
    HCl(aq)<\/td>\r\nhydrochloric acid<\/td>\r\n<\/tr>\r\n
    HBr(aq)<\/td>\r\nhydrobromic acid<\/td>\r\n<\/tr>\r\n
    HI(aq)<\/td>\r\nhydriodic acid<\/td>\r\n<\/tr>\r\n
    H2<\/sub>S(aq)<\/td>\r\nhydrosulfuric acid<\/td>\r\n<\/tr>\r\n
    HC2<\/sub>H3<\/sub>O2<\/sub>(aq)<\/td>\r\nacetic acid<\/td>\r\n<\/tr>\r\n
    HNO3<\/sub>(aq)<\/td>\r\nnitric acid<\/td>\r\n<\/tr>\r\n
    HNO2<\/sub>(aq)<\/td>\r\nnitrous acid<\/td>\r\n<\/tr>\r\n
    H2<\/sub>SO4<\/sub>(aq)<\/td>\r\nsulfuric acid<\/td>\r\n<\/tr>\r\n
    H2<\/sub>SO3<\/sub>(aq)<\/td>\r\nsulfurous acid<\/td>\r\n<\/tr>\r\n
    HClO3<\/sub>(aq)<\/td>\r\nchloric acid<\/td>\r\n<\/tr>\r\n
    HClO4<\/sub>(aq)<\/td>\r\nperchloric acid<\/td>\r\n<\/tr>\r\n
    HClO2<\/sub>(aq)<\/td>\r\nchlorous acid<\/td>\r\n<\/tr>\r\n
    H3<\/sub>PO4<\/sub>(aq)<\/td>\r\nphosphoric acid<\/td>\r\n<\/tr>\r\n
    H3<\/sub>PO3<\/sub>(aq)<\/td>\r\nphosphorous acid<\/td>\r\n<\/tr>\r\n
    Bases<\/strong><\/td>\r\n<\/tr>\r\n
    NaOH(aq)<\/td>\r\nsodium hydroxide<\/td>\r\n<\/tr>\r\n
    KOH(aq)<\/td>\r\npotassium hydroxide<\/td>\r\n<\/tr>\r\n
    Mg(OH)2<\/sub>(aq)<\/td>\r\nmagnesium hydroxide<\/td>\r\n<\/tr>\r\n
    Ca(OH)2<\/sub>(aq)<\/td>\r\ncalcium hydroxide<\/td>\r\n<\/tr>\r\n
    NH3<\/sub>(aq)<\/td>\r\nammonia<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n
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    Example 1<\/h3>\r\n

    Name each substance.<\/p>\r\n\r\n

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    1. HF(aq)<\/li>\r\n \t
    2. Sr(OH)2<\/sub>(aq)<\/li>\r\n<\/ol>\r\n

      Solution<\/p>\r\n\r\n

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      1. This acid has only two elements in its formula, so its name includes the hydro<\/em>- prefix. The stem of the other element\u2019s name, fluorine, is fluor<\/em>, followed by\u00a0 the -ic acid<\/em> ending. Its name is hydrofluoric acid.<\/li>\r\n \t
      2. This base is named as an ionic compound between the strontium ion and the hydroxide ion: strontium hydroxide.<\/li>\r\n<\/ol>\r\n<\/div>\r\n
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        Skill-Building Exercise<\/h3>\r\n
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          H2<\/sub>Se(aq)<\/p>\r\n\r\n<\/div><\/li>\r\n \t

        2. \r\n
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          Ba(OH)2<\/sub>(aq)<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

          Notice that one base listed in Table 10.1 \"Formulas and Names for Some Acids and Bases\"<\/a>\u2014ammonia\u2014does 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:<\/p>\r\n

          NH3<\/sub>(aq) + H2<\/sub>O(\u2113) \u21c6 NH4<\/sub>+<\/sup>(aq) + OH\u2212<\/sup>(aq)<\/span><\/span><\/p>\r\n

          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.<\/p>\r\n

          As noted previously, acids and bases react chemically with each other to form salts<\/em>. A salt is a general chemical term for any ionic compound formed from an acid and a base. In reactions where the acid is a hydrogen ion containing compound and the base is a hydroxide ion containing compound, water is also a product. The general reaction is as follows:<\/p>\r\n

          acid + base \u2192 water + salt<\/span><\/span><\/p>\r\n

          The reaction of acid and base to make water and a salt is called neutralization<\/span><\/span>. Like any chemical equation, a neutralization chemical equation must be properly balanced. For example, the neutralization reaction between sodium hydroxide and hydrochloric acid is as follows:<\/p>\r\n

          NaOH(aq) + HCl(aq) \u2192 NaCl(aq) + H2<\/sub>O(\u2113)<\/span><\/span><\/p>\r\n

          with coefficients all understood to be one. The neutralization reaction between sodium hydroxide and sulfuric acid is as follows:<\/p>\r\n

          2NaOH(aq) + H2<\/sub>SO4<\/sub>(aq) \u2192 Na2<\/sub>SO4<\/sub>(aq) + 2H2<\/sub>O(\u2113)<\/span><\/span><\/p>\r\n

          Once a neutralization reaction is properly balanced, it can be used to perform stoichiometry calculations, such as the ones\u00a0 in Chapter 5 \"Introduction to Chemical Reactions\"<\/a> and Chapter 6 \"Quantities in Chemical Reactions\"<\/a>.<\/p>\r\n\r\n

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          Example 2<\/h3>\r\n

          Nitric acid, HNO3<\/sub>(aq), can be neutralized by calcium hydroxide Ca(OH)2<\/sub>(aq).<\/p>\r\n\r\n

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          1. Write a balanced chemical equation for the reaction between these two compounds and identify the salt it produces.<\/li>\r\n \t
          2. For one reaction, 16.8 g of HNO3<\/sub> is present initially. How many grams of Ca(OH)2<\/sub> are needed to neutralize that much HNO3<\/sub>?<\/li>\r\n \t
          3. In a second reaction, 805 mL of 0.672 M Ca(OH)2<\/sub> is present initially. What volume of 0.432 M HNO3<\/sub> solution is necessary to neutralize the Ca(OH)2<\/sub> solution?<\/li>\r\n<\/ol>\r\n

            Solution<\/p>\r\n\r\n

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            1. \r\n

              Because there are two OH\u2212<\/sup> ions in the formula for Ca(OH)2<\/sub>, we need two moles of HNO3<\/sub> to provide enough H+<\/sup> ions. The balanced chemical equation is as follows:<\/p>\r\n

              Ca(OH)2<\/sub>(aq) + 2HNO3<\/sub>(aq) \u2192 Ca(NO3<\/sub>)2<\/sub>(aq) + 2H2<\/sub>O(\u2113)<\/span><\/span><\/p>\r\n

              The salt formed is calcium nitrate.<\/p>\r\n<\/li>\r\n \t

            2. \r\n

              This calculation is much like the calculations in Chapter 6 \"Quantities in Chemical Reactions\"<\/a>. First convert the mass of HNO3<\/sub> to moles using its molar mass of 1.01 + 14.00 + 3(16.00) = 63.01 g\/mol; then use the balanced chemical equation to determine the related number of moles of Ca(OH)2<\/sub> needed to neutralize it; and then convert that number of moles of Ca(OH)2<\/sub> to the mass of Ca(OH)2<\/sub> using its molar mass of 40.08 + 2(1.01) + 2(16.00) = 74.10 g\/mol.<\/p>\r\n [latex]16.8\\text{ g HNO}_3\\times{\\frac{1\\text{ mol HNO}_3}{63.01\\text{ gHNO}_3}}\\times\\frac{{1\\text{ mol Ca(OH)}_2}}{2\\text{ mol HNO}_3}\\times{\\frac{74.10\\text{ g Ca(OH)}_2}{1\\text{ mol Ca(OH)}_2}}=9.88\\text{ g Ca(OH)}_2[\/latex]\r\n<\/span><\/li>\r\n \t

            3. \r\n

              Having concentration information allows use of skills from\u00a0 Chapter 9 \"Solutions\"<\/a>. First, use the concentration and volume data to determine the number of moles of Ca(OH)2<\/sub> present. Recognizing that 805 mL = 0.805 L,<\/p>\r\n[latex]0.805\\text{ L Ca(OH)}_2\\text{ solution}\\times{\\frac{0.672\\text{ mol Ca(OH)}_2}{1\\text{ L Ca(OH)}_2\\text{ solution}}}\\times{\\frac{2\\text{ mol HNO}_3}{1\\text{ mol Ca(OH)}_2}}\\times{\\frac{1\\text{ L HNO}_3\\text{ solution}}{0.432\\text{ mol HNO}_3}}=2.50\\text{ L}[\/latex] \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/span>\r\n\r\nIf the volume is required in mL, 2.50 x 103<\/sup> mL expresses the correct value to the correct number of sig figs.<\/span>\r\n<\/span><\/li>\r\n<\/ol>\r\n<\/div>\r\n

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              Skill-Building Exercise<\/h3>\r\n
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                Write a balanced chemical equation for the reaction between these two compounds and identify the salt it produces.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

              2. \r\n
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                For one reaction, 37.5 g of HCN is present initially. How many grams of KOH are needed to neutralize that much HCN?<\/p>\r\n\r\n<\/div><\/li>\r\n \t

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                In a second reaction, 43.0 mL of 0.0663 M KOH is present initially. What volume of 0.107 M HCN solution is necessary to neutralize the KOH solution?<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

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                Note<\/h3>\r\n

                Hydrocyanic acid (HCN) is one exception to the acid-naming rules that specify using the prefix hydro-<\/em> for binary acids (acids composed of hydrogen and only one other element).<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

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                Concept Review Exercises<\/h3>\r\n<\/div>\r\n
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                  Give the Arrhenius definitions of an acid and a base.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

                2. \r\n
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                  What is neutralization?<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n

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                  Answers<\/h3>\r\n
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                    Arrhenius acid: a compound that increases the concentration of hydrogen ion (H+<\/sup>) in aqueous solution; Arrhenius base: a compound that increases the concentration of hydroxide ion (OH\u2212<\/sup>) in aqueous solution.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

                  2. \r\n
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                    the reaction of an acid and a base<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

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                    Key Takeaway<\/h3>\r\n