{"id":856,"date":"2018-03-20T16:18:15","date_gmt":"2018-03-20T16:18:15","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/?post_type=chapter&p=856"},"modified":"2018-08-08T16:38:23","modified_gmt":"2018-08-08T16:38:23","slug":"10-2-bronsted-lowry-definition-of-acids-and-bases","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/chapter\/10-2-bronsted-lowry-definition-of-acids-and-bases\/","title":{"raw":"10.2 Br\u00f8nsted-Lowry Definition of Acids and Bases","rendered":"10.2 Br\u00f8nsted-Lowry Definition of Acids and Bases"},"content":{"raw":"
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10.2<\/span> Br\u00f8nsted-Lowry Definition of Acids and Bases<\/h2>\r\n
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Learning Objectives<\/h3>\r\n
    \r\n \t
  1. Recognize a compound as a Br\u00f8nsted-Lowry acid or a Br\u00f8nsted-Lowry base.<\/li>\r\n \t
  2. Illustrate the proton transfer process that defines a Br\u00f8nsted-Lowry acid-base reaction.<\/li>\r\n<\/ol>\r\n<\/div>\r\nAmmonia (NH<\/span>3<\/sub>) increases the hydroxide ion concentration in aqueous solution by reacting with water rather than releasing hydroxide ions directly. In fact, the Arrhenius definitions of an acid and a base focus on hydrogen ions and hydroxide ions. Are there more fundamental definitions for acids and bases?<\/span>\r\n\r\n<\/div>\r\n

    In 1923, the Danish scientist Johannes Br\u00f8nsted and the English scientist Thomas Lowry independently proposed new definitions for acids and bases. Rather than considering both hydrogen and hydroxide ions, they focused on the hydrogen ion only. A Br\u00f8nsted-Lowry acid<\/span><\/span>\u00a0is a compound that supplies a hydrogen ion in a reaction. A Br\u00f8nsted-Lowry base<\/span><\/span>, conversely, is a compound that accepts a hydrogen ion in a reaction. Thus, the Br\u00f8nsted-Lowry definitions of an acid and a base focus on the movement of hydrogen ions in a reaction, rather than on the production of hydrogen ions and hydroxide ions in an aqueous solution.<\/p>\r\n

    Let us use the reaction of ammonia in water to demonstrate the Br\u00f8nsted-Lowry definitions of an acid and a base. Ammonia and water molecules are reactants, while the ammonium ion and the hydroxide ion are products:<\/p>\r\nNH3<\/sub>(aq) + H2<\/sub>O(\u2113) \u2192 NH4<\/sub>+<\/sup>(aq) + OH\u2212<\/sup>(aq)<\/span><\/span>\r\n

    What has happened in this reaction is that the original water molecule has donated a hydrogen ion to the original ammonia molecule, which in turn has accepted the hydrogen ion. We can illustrate this as follows:<\/p>\r\n\r\n

    \"image\"<\/div>\r\n

    Because the water molecule donates a hydrogen ion to the ammonia, it is the Br\u00f8nsted-Lowry acid, while the ammonia molecule\u2014which accepts the hydrogen ion\u2014is the Br\u00f8nsted-Lowry base. Thus, ammonia acts as a base in both the Arrhenius sense and the Br\u00f8nsted-Lowry sense.<\/p>\r\n

    Is an Arrhenius acid like hydrochloric acid still an acid in the Br\u00f8nsted-Lowry sense? Yes, but it requires us to understand what really happens when HCl is dissolved in water. Recall that the hydrogen atom<\/em> is a single proton surrounded by a single electron. To make the hydrogen ion<\/em>, we remove the electron, leaving a bare proton. Do we really<\/em> have bare protons floating around in aqueous solution? No, we do not. What really happens is that the H+<\/sup> ion attaches itself to H2<\/sub>O to make H3<\/sub>O+<\/sup>, which is called the hydronium ion<\/em>. For most purposes, H+<\/sup> and H3<\/sub>O+<\/sup> represent the same species, but writing H3<\/sub>O+<\/sup> instead of H+<\/sup> shows that we understand that there are no bare protons floating around in solution. Rather, these protons are actually attached to solvent molecules.<\/p>\r\n\r\n

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    \r\n

    Note<\/h3>\r\n

    A proton in aqueous solution may be surrounded by more than one water molecule, leading to formulas like H5<\/sub>O2<\/sub>+<\/sup> or H9<\/sub>O4<\/sub>+<\/sup> rather than H3<\/sub>O+<\/sup>. It is simpler, however, to use H3<\/sub>O+<\/sup>.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

    With this in mind, how do we define HCl as an acid in the Br\u00f8nsted-Lowry sense? Consider what happens when HCl is dissolved in H2<\/sub>O:<\/p>\r\nHCl + H2<\/sub>O(\u2113) \u2192 H3<\/sub>O+<\/sup>(aq) + Cl\u2212<\/sup>(aq)<\/span><\/span>\r\n

    We can depict this process using Lewis electron dot diagrams:<\/p>\r\n\r\n

    \"image\"<\/div>\r\n

    Now we see that a hydrogen ion is transferred from the HCl molecule to the H2<\/sub>O molecule to make chloride ions and hydronium ions. As the hydrogen ion donor, HCl acts as a Br\u00f8nsted-Lowry acid; as a hydrogen ion acceptor, H2<\/sub>O is a Br\u00f8nsted-Lowry base. So HCl is an acid not just in the Arrhenius sense but also in the Br\u00f8nsted-Lowry sense. Moreover, by the Br\u00f8nsted-Lowry definitions, H2<\/sub>O is a base in the formation of aqueous HCl. So the Br\u00f8nsted-Lowry definitions of an acid and a base classify the dissolving of HCl in water as a reaction between an acid and a base\u2014although the Arrhenius definition would not have labeled H2<\/sub>O a base in this circumstance.<\/p>\r\n\r\n

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    Note<\/h3>\r\n

    All Arrhenius acids and bases are Br\u00f8nsted-Lowry acids and bases as well. But not all Br\u00f8nsted-Lowry acids and bases are Arrhenius acids and bases.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

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    Example 3<\/h3>\r\n

    Aniline (C6<\/sub>H5<\/sub>NH2<\/sub>) is slightly soluble in water. It has a nitrogen atom that can accept a hydrogen ion from a water molecule just like the nitrogen atom in ammonia does. Write the chemical equation for this reaction and identify the Br\u00f8nsted-Lowry acid and base.<\/p>\r\n

    Solution<\/p>\r\n

    C6<\/sub>H5<\/sub>NH2<\/sub> and H2<\/sub>O are the reactants. When C6<\/sub>H5<\/sub>NH2<\/sub> accepts a proton from H2<\/sub>O, it gains an extra H and a positive charge and leaves an OH\u2212<\/sup> ion behind. The reaction is as follows:<\/p>\r\nC6<\/sub>H5<\/sub>NH2<\/sub>(aq) + H2<\/sub>O(\u2113) \u2192 C6<\/sub>H5<\/sub>NH3<\/sub>+<\/sup>(aq) + OH\u2212<\/sup>(aq)<\/span><\/span>\r\n

    Because C6<\/sub>H5<\/sub>NH2<\/sub> accepts a proton, it is the Br\u00f8nsted-Lowry base. The H2<\/sub>O molecule, because it donates a proton, is the Br\u00f8nsted-Lowry acid.<\/p>\r\n\r\n<\/div>\r\n

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    Skill-Building Exercise<\/h3>\r\n
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    1. \r\n
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      Caffeine (C8<\/sub>H10<\/sub>N4<\/sub>O2<\/sub>) is a stimulant found in coffees and teas. When dissolved in water, it can accept a proton from a water molecule. Write the chemical equation for this process and identify the Br\u00f8nsted-Lowry acid and base.<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

      The Br\u00f8nsted-Lowry definitions of an acid and a base can be applied to chemical reactions that occur in solvents other than water. The following example illustrates.<\/p>\r\n\r\n

      \r\n

      Example 4<\/h3>\r\n

      Sodium amide (NaNH2<\/sub>) dissolves in methanol (CH3<\/sub>OH) and separates into sodium ions and amide ions (NH2<\/sub>\u2212<\/sup>). The amide ions react with methanol to make ammonia and the methoxide ion (CH3<\/sub>O\u2212<\/sup>). Write a balanced chemical equation for this process and identify the Br\u00f8nsted-Lowry acid and base.<\/p>\r\n

      Solution<\/p>\r\n

      The equation for the reaction is between NH2<\/sub>\u2212<\/sup> and CH3<\/sub>OH to make NH3<\/sub> and CH3<\/sub>O\u2212<\/sup> is as follows:<\/p>\r\nNH2<\/sub>\u2212<\/sup>(solv) + CH3<\/sub>OH(\u2113) \u2192 NH3<\/sub>(solv) + CH3<\/sub>O\u2212<\/sup>(solv)<\/span><\/span>\r\n

      The label (solv)<\/em> indicates that the species are dissolved in some solvent, in contrast to (aq)<\/em>, which specifies an aqueous (H2<\/sub>O) solution. In this reaction, we see that the NH2<\/sub>\u2212<\/sup> ion accepts a proton from a CH3<\/sub>OH molecule to make an NH3<\/sub> molecule. Thus, as the proton acceptor, NH2<\/sub>\u2212<\/sup> is the Br\u00f8nsted-Lowry base. As the proton donor, CH3<\/sub>OH is the Br\u00f8nsted-Lowry acid.<\/p>\r\n\r\n<\/div>\r\n

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      Skill-Building Exercise<\/h3>\r\n
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      1. \r\n
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        Pyridinium chloride (C5<\/sub>H5<\/sub>NHCl) dissolves in ethanol (C2<\/sub>H5<\/sub>OH) and separates into pyridinium ions (C5<\/sub>H5<\/sub>NH+<\/sup>) and chloride ions. The pyridinium ion can transfer a hydrogen ion to a solvent molecule. Write a balanced chemical equation for this process and identify the Br\u00f8nsted-Lowry acid and base.<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

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        To Your Health: Br\u00f8nsted-Lowry Acid-Base Reactions in Pharmaceuticals<\/h3>\r\n

        There are many interesting applications of Br\u00f8nsted-Lowry acid-base reactions in the pharmaceutical industry. For example, drugs often need to be water soluble for maximum effectiveness. However, many complex organic compounds are not soluble or are only slightly soluble in water. Fortunately, those drugs that contain proton-accepting nitrogen atoms (and there are a lot of them) can be reacted with dilute hydrochloric acid [HCl(aq)]. The nitrogen atoms\u2014acting as Br\u00f8nsted-Lowry bases\u2014accept the hydrogen ions from the acid to make an ion, which is usually much more soluble in water. The modified drug molecules can then be isolated as chloride salts:<\/p>\r\n [latex]RN(sl\\,aq)\\,+\\,H^+(aq)\\,\u2192\\,RNH^+(aq)\\,_{---\\rightarrow}^{Cl^\u2212(aq)}\\,RNHCl(s)[\/latex]\u00a0 <\/span>\r\n

        where RN represents some organic compound containing nitrogen. The label (sl aq)<\/em> means \u201cslightly aqueous,\u201d indicating that the compound RN is only slightly soluble. Drugs that are modified in this way are called hydrochloride salts<\/em>. Examples include the powerful painkiller codeine, which is commonly administered as codeine hydrochloride. Acids other than hydrochloric acid are also used. Hydrobromic acid, for example, gives hydrobromide salts<\/em>. Dextromethorphan, an ingredient in many cough medicines, is dispensed as dextromethorphan hydrobromide. The accompanying figure shows another medication as a hydrochloride salt.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

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        Concept Review Exercise<\/h3>\r\n
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        1. \r\n
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          Give the definitions of a Br\u00f8nsted-Lowry acid and a Br\u00f8nsted-Lowry base.<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n

          \r\n

          Answer<\/h3>\r\n
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            \r\n \t
          1. \r\n
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            A Br\u00f8nsted-Lowry acid is a proton donor, while a Br\u00f8nsted-Lowry base is a proton acceptor.<\/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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            Key Takeaways<\/h3>\r\n
              \r\n \t
            • A Br\u00f8nsted-Lowry acid is a proton donor, and a Br\u00f8nsted-Lowry base is a proton acceptor.<\/li>\r\n \t
            • Br\u00f8nsted-Lowry acid-base reactions are essentially proton transfer reactions.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
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              Exercises<\/h3>\r\n
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              1. \r\n
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                Label each reactant as a Br\u00f8nsted-Lowry acid or a Br\u00f8nsted-Lowry base.<\/p>\r\nHCl(aq) + NH3<\/sub>(aq) \u2192 NH4<\/sub>+<\/sup>(aq) + Cl\u2212<\/sup>(aq)<\/span><\/span>\r\n\r\n<\/div><\/li>\r\n \t

              2. \r\n
                \r\n

                Label each reactant as a Br\u00f8nsted-Lowry acid or a Br\u00f8nsted-Lowry base.<\/p>\r\nH2<\/sub>O(\u2113) + N2<\/sub>H4<\/sub>(aq) \u2192 N2<\/sub>H5<\/sub>+<\/sup>(aq) + OH\u2212<\/sup>(aq)<\/span><\/span>\r\n\r\n<\/div><\/li>\r\n \t

              3. \r\n
                \r\n

                Explain why a Br\u00f8nsted-Lowry acid can be called a proton donor.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

              4. \r\n
                \r\n

                Explain why a Br\u00f8nsted-Lowry base can be called a proton acceptor.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

              5. \r\n
                \r\n

                Write the chemical equation of the reaction of ammonia in water and label the Br\u00f8nsted-Lowry acid and base.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

              6. \r\n
                \r\n

                Write the chemical equation of the reaction of methylamine (CH3<\/sub>NH2<\/sub>) in water and label the Br\u00f8nsted-Lowry acid and base.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

              7. \r\n
                \r\n

                Demonstrate that the dissolution of HNO3<\/sub> in water is actually a Br\u00f8nsted-Lowry acid-base reaction by describing it with a chemical equation and labeling the Br\u00f8nsted-Lowry acid and base.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

              8. \r\n
                \r\n

                Identify the Br\u00f8nsted-Lowry acid and base in the following chemical equation:<\/p>\r\nC3<\/sub>H7<\/sub>NH2<\/sub>(aq) + H3<\/sub>O+<\/sup>(aq) \u2192 C3<\/sub>H7<\/sub>NH3<\/sub>+<\/sup>(aq) + H2<\/sub>O(\u2113)<\/span><\/span>\r\n\r\n<\/div><\/li>\r\n \t

              9. \r\n
                \r\n

                Write the chemical equation for the reaction that occurs when cocaine hydrochloride (C17<\/sub>H22<\/sub>ClNO4<\/sub>) dissolves in water and donates a proton to a water molecule. (When hydrochlorides dissolve in water, they separate into chloride ions and the appropriate cation.)<\/p>\r\n\r\n<\/div><\/li>\r\n \t

              10. \r\n
                \r\n

                If codeine hydrobromide has the formula C18<\/sub>H22<\/sub>BrNO3<\/sub>, what is the formula of the parent compound codeine?<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

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                Answers<\/h3>\r\n<\/div>\r\n
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                1. HCl: Br\u00f8nsted-Lowry acid; NH3<\/sub>: Br\u00f8nsted-Lowry base<\/p>\r\n\r\n<\/div>\r\n

                <\/div>\r\n
                \r\n

                3. A Br\u00f8nsted-Lowry acid gives away an H+<\/sup> ion\u2014nominally, a proton\u2014in an acid-base reaction.<\/p>\r\n\r\n<\/div>\r\n

                <\/div>\r\n
                \r\n

                5. NH3<\/sub> + H2<\/sub>O \u2192 NH4<\/sub>+<\/sup> + OH\u2212<\/sup>; NH3<\/sub>: Br\u00f8nsted-Lowry base; H2<\/sub>O: Br\u00f8nsted-Lowry acid<\/p>\r\n\r\n<\/div>\r\n

                <\/div>\r\n
                \r\n

                7. HNO3<\/sub> + H2<\/sub>O \u2192 H3<\/sub>O+<\/sup> + NO3<\/sub>\u2212<\/sup>; HNO3<\/sub>: Br\u00f8nsted-Lowry acid; H2<\/sub>O: Br\u00f8nsted-Lowry base<\/p>\r\n\r\n<\/div>\r\n

                <\/div>\r\n
                \r\n

                9. C17<\/sub>H22<\/sub>NO4+<\/sub> + H2<\/sub>O \u2192 H3<\/sub>O+<\/sup> + C17<\/sub>H21<\/sub>NO4<\/sub><\/p>\r\n\r\n<\/div>\r\n

                <\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
                <\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>","rendered":"
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                10.2<\/span> Br\u00f8nsted-Lowry Definition of Acids and Bases<\/h2>\n
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                Learning Objectives<\/h3>\n
                  \n
                1. Recognize a compound as a Br\u00f8nsted-Lowry acid or a Br\u00f8nsted-Lowry base.<\/li>\n
                2. Illustrate the proton transfer process that defines a Br\u00f8nsted-Lowry acid-base reaction.<\/li>\n<\/ol>\n<\/div>\n

                  Ammonia (NH<\/span>3<\/sub>) increases the hydroxide ion concentration in aqueous solution by reacting with water rather than releasing hydroxide ions directly. In fact, the Arrhenius definitions of an acid and a base focus on hydrogen ions and hydroxide ions. Are there more fundamental definitions for acids and bases?<\/span><\/p>\n<\/div>\n

                  In 1923, the Danish scientist Johannes Br\u00f8nsted and the English scientist Thomas Lowry independently proposed new definitions for acids and bases. Rather than considering both hydrogen and hydroxide ions, they focused on the hydrogen ion only. A Br\u00f8nsted-Lowry acid<\/span><\/span>\u00a0is a compound that supplies a hydrogen ion in a reaction. A Br\u00f8nsted-Lowry base<\/span><\/span>, conversely, is a compound that accepts a hydrogen ion in a reaction. Thus, the Br\u00f8nsted-Lowry definitions of an acid and a base focus on the movement of hydrogen ions in a reaction, rather than on the production of hydrogen ions and hydroxide ions in an aqueous solution.<\/p>\n

                  Let us use the reaction of ammonia in water to demonstrate the Br\u00f8nsted-Lowry definitions of an acid and a base. Ammonia and water molecules are reactants, while the ammonium ion and the hydroxide ion are products:<\/p>\n

                  NH3<\/sub>(aq) + H2<\/sub>O(\u2113) \u2192 NH4<\/sub>+<\/sup>(aq) + OH\u2212<\/sup>(aq)<\/span><\/span><\/p>\n

                  What has happened in this reaction is that the original water molecule has donated a hydrogen ion to the original ammonia molecule, which in turn has accepted the hydrogen ion. We can illustrate this as follows:<\/p>\n

                  \"image\"<\/div>\n

                  Because the water molecule donates a hydrogen ion to the ammonia, it is the Br\u00f8nsted-Lowry acid, while the ammonia molecule\u2014which accepts the hydrogen ion\u2014is the Br\u00f8nsted-Lowry base. Thus, ammonia acts as a base in both the Arrhenius sense and the Br\u00f8nsted-Lowry sense.<\/p>\n

                  Is an Arrhenius acid like hydrochloric acid still an acid in the Br\u00f8nsted-Lowry sense? Yes, but it requires us to understand what really happens when HCl is dissolved in water. Recall that the hydrogen atom<\/em> is a single proton surrounded by a single electron. To make the hydrogen ion<\/em>, we remove the electron, leaving a bare proton. Do we really<\/em> have bare protons floating around in aqueous solution? No, we do not. What really happens is that the H+<\/sup> ion attaches itself to H2<\/sub>O to make H3<\/sub>O+<\/sup>, which is called the hydronium ion<\/em>. For most purposes, H+<\/sup> and H3<\/sub>O+<\/sup> represent the same species, but writing H3<\/sub>O+<\/sup> instead of H+<\/sup> shows that we understand that there are no bare protons floating around in solution. Rather, these protons are actually attached to solvent molecules.<\/p>\n

                  \n
                  \n

                  Note<\/h3>\n

                  A proton in aqueous solution may be surrounded by more than one water molecule, leading to formulas like H5<\/sub>O2<\/sub>+<\/sup> or H9<\/sub>O4<\/sub>+<\/sup> rather than H3<\/sub>O+<\/sup>. It is simpler, however, to use H3<\/sub>O+<\/sup>.<\/p>\n<\/div>\n<\/div>\n

                  With this in mind, how do we define HCl as an acid in the Br\u00f8nsted-Lowry sense? Consider what happens when HCl is dissolved in H2<\/sub>O:<\/p>\n

                  HCl + H2<\/sub>O(\u2113) \u2192 H3<\/sub>O+<\/sup>(aq) + Cl\u2212<\/sup>(aq)<\/span><\/span><\/p>\n

                  We can depict this process using Lewis electron dot diagrams:<\/p>\n

                  \"image\"<\/div>\n

                  Now we see that a hydrogen ion is transferred from the HCl molecule to the H2<\/sub>O molecule to make chloride ions and hydronium ions. As the hydrogen ion donor, HCl acts as a Br\u00f8nsted-Lowry acid; as a hydrogen ion acceptor, H2<\/sub>O is a Br\u00f8nsted-Lowry base. So HCl is an acid not just in the Arrhenius sense but also in the Br\u00f8nsted-Lowry sense. Moreover, by the Br\u00f8nsted-Lowry definitions, H2<\/sub>O is a base in the formation of aqueous HCl. So the Br\u00f8nsted-Lowry definitions of an acid and a base classify the dissolving of HCl in water as a reaction between an acid and a base\u2014although the Arrhenius definition would not have labeled H2<\/sub>O a base in this circumstance.<\/p>\n

                  \n
                  \n

                  Note<\/h3>\n

                  All Arrhenius acids and bases are Br\u00f8nsted-Lowry acids and bases as well. But not all Br\u00f8nsted-Lowry acids and bases are Arrhenius acids and bases.<\/p>\n<\/div>\n<\/div>\n

                  \n

                  Example 3<\/h3>\n

                  Aniline (C6<\/sub>H5<\/sub>NH2<\/sub>) is slightly soluble in water. It has a nitrogen atom that can accept a hydrogen ion from a water molecule just like the nitrogen atom in ammonia does. Write the chemical equation for this reaction and identify the Br\u00f8nsted-Lowry acid and base.<\/p>\n

                  Solution<\/p>\n

                  C6<\/sub>H5<\/sub>NH2<\/sub> and H2<\/sub>O are the reactants. When C6<\/sub>H5<\/sub>NH2<\/sub> accepts a proton from H2<\/sub>O, it gains an extra H and a positive charge and leaves an OH\u2212<\/sup> ion behind. The reaction is as follows:<\/p>\n

                  C6<\/sub>H5<\/sub>NH2<\/sub>(aq) + H2<\/sub>O(\u2113) \u2192 C6<\/sub>H5<\/sub>NH3<\/sub>+<\/sup>(aq) + OH\u2212<\/sup>(aq)<\/span><\/span><\/p>\n

                  Because C6<\/sub>H5<\/sub>NH2<\/sub> accepts a proton, it is the Br\u00f8nsted-Lowry base. The H2<\/sub>O molecule, because it donates a proton, is the Br\u00f8nsted-Lowry acid.<\/p>\n<\/div>\n

                  \n
                  \n

                  Skill-Building Exercise<\/h3>\n
                    \n
                  1. \n
                    \n

                    Caffeine (C8<\/sub>H10<\/sub>N4<\/sub>O2<\/sub>) is a stimulant found in coffees and teas. When dissolved in water, it can accept a proton from a water molecule. Write the chemical equation for this process and identify the Br\u00f8nsted-Lowry acid and base.<\/p>\n<\/div>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n

                    The Br\u00f8nsted-Lowry definitions of an acid and a base can be applied to chemical reactions that occur in solvents other than water. The following example illustrates.<\/p>\n

                    \n

                    Example 4<\/h3>\n

                    Sodium amide (NaNH2<\/sub>) dissolves in methanol (CH3<\/sub>OH) and separates into sodium ions and amide ions (NH2<\/sub>\u2212<\/sup>). The amide ions react with methanol to make ammonia and the methoxide ion (CH3<\/sub>O\u2212<\/sup>). Write a balanced chemical equation for this process and identify the Br\u00f8nsted-Lowry acid and base.<\/p>\n

                    Solution<\/p>\n

                    The equation for the reaction is between NH2<\/sub>\u2212<\/sup> and CH3<\/sub>OH to make NH3<\/sub> and CH3<\/sub>O\u2212<\/sup> is as follows:<\/p>\n

                    NH2<\/sub>\u2212<\/sup>(solv) + CH3<\/sub>OH(\u2113) \u2192 NH3<\/sub>(solv) + CH3<\/sub>O\u2212<\/sup>(solv)<\/span><\/span><\/p>\n

                    The label (solv)<\/em> indicates that the species are dissolved in some solvent, in contrast to (aq)<\/em>, which specifies an aqueous (H2<\/sub>O) solution. In this reaction, we see that the NH2<\/sub>\u2212<\/sup> ion accepts a proton from a CH3<\/sub>OH molecule to make an NH3<\/sub> molecule. Thus, as the proton acceptor, NH2<\/sub>\u2212<\/sup> is the Br\u00f8nsted-Lowry base. As the proton donor, CH3<\/sub>OH is the Br\u00f8nsted-Lowry acid.<\/p>\n<\/div>\n

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                    Skill-Building Exercise<\/h3>\n
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                      Pyridinium chloride (C5<\/sub>H5<\/sub>NHCl) dissolves in ethanol (C2<\/sub>H5<\/sub>OH) and separates into pyridinium ions (C5<\/sub>H5<\/sub>NH+<\/sup>) and chloride ions. The pyridinium ion can transfer a hydrogen ion to a solvent molecule. Write a balanced chemical equation for this process and identify the Br\u00f8nsted-Lowry acid and base.<\/p>\n<\/div>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n

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                      To Your Health: Br\u00f8nsted-Lowry Acid-Base Reactions in Pharmaceuticals<\/h3>\n

                      There are many interesting applications of Br\u00f8nsted-Lowry acid-base reactions in the pharmaceutical industry. For example, drugs often need to be water soluble for maximum effectiveness. However, many complex organic compounds are not soluble or are only slightly soluble in water. Fortunately, those drugs that contain proton-accepting nitrogen atoms (and there are a lot of them) can be reacted with dilute hydrochloric acid [HCl(aq)]. The nitrogen atoms\u2014acting as Br\u00f8nsted-Lowry bases\u2014accept the hydrogen ions from the acid to make an ion, which is usually much more soluble in water. The modified drug molecules can then be isolated as chloride salts:<\/p>\n

                      [latex]RN(sl\\,aq)\\,+\\,H^+(aq)\\,\u2192\\,RNH^+(aq)\\,_{---\\rightarrow}^{Cl^\u2212(aq)}\\,RNHCl(s)[\/latex]\u00a0 <\/span><\/p>\n

                      where RN represents some organic compound containing nitrogen. The label (sl aq)<\/em> means \u201cslightly aqueous,\u201d indicating that the compound RN is only slightly soluble. Drugs that are modified in this way are called hydrochloride salts<\/em>. Examples include the powerful painkiller codeine, which is commonly administered as codeine hydrochloride. Acids other than hydrochloric acid are also used. Hydrobromic acid, for example, gives hydrobromide salts<\/em>. Dextromethorphan, an ingredient in many cough medicines, is dispensed as dextromethorphan hydrobromide. The accompanying figure shows another medication as a hydrochloride salt.<\/p>\n<\/div>\n<\/div>\n

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                      Concept Review Exercise<\/h3>\n
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                      1. \n
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                        Give the definitions of a Br\u00f8nsted-Lowry acid and a Br\u00f8nsted-Lowry base.<\/p>\n<\/div>\n<\/li>\n<\/ol>\n<\/div>\n

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                        Answer<\/h3>\n
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                          A Br\u00f8nsted-Lowry acid is a proton donor, while a Br\u00f8nsted-Lowry base is a proton acceptor.<\/p>\n<\/div>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n

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                          Key Takeaways<\/h3>\n