{"id":863,"date":"2018-03-20T16:20:39","date_gmt":"2018-03-20T16:20:39","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/?post_type=chapter&p=863"},"modified":"2018-10-02T20:10:58","modified_gmt":"2018-10-02T20:10:58","slug":"10-3-water-both-an-acid-and-a-base","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-monroecc-orgbiochemistry\/chapter\/10-3-water-both-an-acid-and-a-base\/","title":{"raw":"10.3 Water: Both an Acid and a Base","rendered":"10.3 Water: Both an Acid and a Base"},"content":{"raw":"
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10.3<\/span> Water: Both an Acid and a Base<\/h2>\r\n
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\u00a0Learning Objective<\/h3>\r\n
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  1. Write chemical equations for water acting as an acid and as a base.<\/li>\r\n<\/ol>\r\n<\/div>\r\nWater (H<\/span>2<\/sub>O) is an interesting compound in many respects, including its ability to behave as an acid or a base.<\/span>\r\n\r\n<\/div>\r\n

    In some circumstances, a water molecule will accept a proton and thus act as a Br\u00f8nsted-Lowry base, which happens when HCl dissolves in H2<\/sub>O:<\/p>\r\n

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

    In other circumstances, a water molecule can donate a proton and thus act as a Br\u00f8nsted-Lowry acid. For example, in the presence of the amide ion (see Example 4 in Section 10.2 \"Br\u00f8nsted-Lowry Definition of Acids and Bases\"<\/a>), a water molecule donates a proton, making ammonia as a product:<\/p>\r\n

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

    So, depending on the circumstances, H2<\/sub>O can act as either a Br\u00f8nsted-Lowry acid or a Br\u00f8nsted-Lowry base. Water is not the only substance that can react as an acid in some cases or a base in others, but it is certainly the most common example\u2014and the most important one. A substance that can either donate or accept a proton, depending on the circumstances, is called an amphiprotic<\/span><\/span>\u00a0compound.<\/p>\r\n

    A water molecule can act as an acid or a base even in a sample of pure water. About 2 in every 555 million\u00a0 water molecules undergo the following reaction:<\/p>\r\n

    H2<\/sub>O(\u2113) + H2<\/sub>O(\u2113) \u21c6 H3<\/sub>O+<\/sup>(aq) + OH\u2212<\/sup>(aq)<\/span><\/span><\/p>\r\n

    This process is called the autoionization of water<\/span><\/span>\u00a0(Figure 10.2 \"Autoionization\"<\/a>) and occurs in every sample of water, whether it is pure or part of a solution.<\/p>\r\n\r\n

    \r\n\r\n \r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1325\"]\"image\" Figure 10.2 Autoionization\u00a0 <\/em>Note: Each H+<\/sup>ion in the diagram above could be viewed as attaching to an H2<\/sub>O molecule, forming H3<\/sub>O+<\/sup>.\u00a0 Also, it would require 555 million water molecules to have only one each of H3<\/sub>O+<\/sup> and OH-<\/sup>, so the picture vastly over-represents the amount of autoionization of water.
    <\/em>[\/caption]\r\n

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

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    Example 5<\/h3>\r\n

    Identify water as either a Br\u00f8nsted-Lowry acid or a Br\u00f8nsted-Lowry base.<\/p>\r\n\r\n

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    1. H2<\/sub>O(\u2113) + NO2<\/sub>\u2212<\/sup>(aq) \u2192 HNO2<\/sub>(aq) + OH\u2212<\/sup>(aq)<\/li>\r\n \t
    2. HC2<\/sub>H3<\/sub>O2<\/sub>(aq) + H2<\/sub>O(\u2113) \u21c6 H3<\/sub>O+<\/sup>(aq) + C2<\/sub>H3<\/sub>O2<\/sub>\u2212<\/sup>(aq)<\/li>\r\n<\/ol>\r\n

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

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      1. In this reaction, the water molecule donates a proton to the NO2<\/sub>\u2212<\/sup> ion, making OH\u2212<\/sup>(aq). As the proton donor, H2<\/sub>O acts as a Br\u00f8nsted-Lowry acid.<\/li>\r\n \t
      2. In this reaction, the water molecule accepts a proton from HC2<\/sub>H3<\/sub>O2<\/sub>, becoming H3<\/sub>O+<\/sup>(aq). As the proton acceptor, H2<\/sub>O is a Br\u00f8nsted-Lowry base.<\/li>\r\n<\/ol>\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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          HCOOH(aq) + H2<\/sub>O(\u2113) \u21c6 H3<\/sub>O+<\/sup>(aq) + HCOO\u2212<\/sup>(aq)<\/p>\r\n\r\n<\/div><\/li>\r\n \t

        2. \r\n
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          H2<\/sub>O(\u2113) + PO4<\/sub>3\u2212<\/sup>(aq) \u2192 OH\u2212<\/sup>(aq) + HPO4<\/sub>2\u2212<\/sup>(aq)<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

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          Concept Review Exercises<\/h3>\r\n
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          1. \r\n
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            Explain how water can act as an acid.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

          2. \r\n
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            Explain how water can act as a base.<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n

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            answers<\/h3>\r\n
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            1. \r\n
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              Under the right conditions, H2<\/sub>O can donate a proton, making it a Br\u00f8nsted-Lowry acid.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

            2. \r\n
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              Under the right conditions, H2<\/sub>O can accept a proton, making it a Br\u00f8nsted-Lowry base.<\/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 Takeaway<\/h3>\r\n