{"id":7623,"date":"2021-06-08T21:57:04","date_gmt":"2021-06-08T21:57:04","guid":{"rendered":"https:\/\/opentextbc.ca\/introductorychemistry\/chapter\/strong-and-weak-acids-and-bases-and-their-salts\/"},"modified":"2021-10-06T21:11:02","modified_gmt":"2021-10-06T21:11:02","slug":"strong-and-weak-acids-and-bases-and-their-salts","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/introductorychemistry\/chapter\/strong-and-weak-acids-and-bases-and-their-salts\/","title":{"raw":"Strong and Weak Acids and Bases and Their Salts","rendered":"Strong and Weak Acids and Bases and Their Salts"},"content":{"raw":"
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Learning Objectives<\/p>\r\n\r\n<\/header>\r\n

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    \r\n \t
  1. Define a strong and a weak acid and base.<\/li>\r\n \t
  2. Recognize an acid or a base as strong or weak.<\/li>\r\n \t
  3. Determine if a salt produces an acidic or a basic solution.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\nExcept for their names and formulas, so far we have treated all acids as equals, especially in a chemical reaction. However, acids can be very different in a very important way. Consider HCl(aq). When HCl is dissolved in H2<\/sub>O, it completely dissociates into H+<\/sup>(aq) and Cl\u2212<\/sup>(aq) ions; all the HCl molecules become ions:\r\n

    HCl\u00a0\u2192 H+<\/sup>(aq)\u00a0+\u00a0Cl\u2212<\/sup>(aq) (100%)<\/p>\r\nAny acid that dissociates 100% into ions is called a strong acid<\/strong>. If it does not dissociate 100%, it is a weak acid<\/strong>. HC2<\/sub>H3<\/sub>O2<\/sub> is an example of a weak acid:\r\n

    HC2<\/sub>H3<\/sub>O2<\/sub>\u2192 H+<\/sup>(aq)\u00a0+\u00a0C2<\/sub>H3<\/sub>O2<\/sub>\u2212<\/sup>(aq) (~5%)<\/p>\r\nBecause this reaction does not go 100% to completion, it is more appropriate to write it as an equilibrium:\r\n

    HC2<\/sub>H3<\/sub>O2\u00a0<\/sub>\u21c4 H+<\/sup>(aq)\u00a0+\u00a0C2<\/sub>H3<\/sub>O2<\/sub>\u2212<\/sup>(aq)<\/p>\r\nAs it turns out, there are very few strong acids, which are given in Table 12.1 \"Strong Acids and Bases\". If an acid is not listed here, it is a weak acid. It may be 1% ionized or 99% ionized, but it is still classified as a weak acid.\r\n\r\nThe issue is similar with bases: a strong base<\/strong>\u00a0is a base that is 100% ionized in solution. If it is less than 100% ionized in solution, it is a weak base<\/strong>. There are very few strong bases (see Table 12.1); any base not listed is a weak base. All strong bases are OH\u2013<\/sup> compounds. So a base based on some other mechanism, such as NH3<\/sub> (which does not contain OH\u2212<\/sup> ions as part of its formula), will be a weak base.\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
    Table 12.1 Strong Acids and Bases<\/caption>\r\n
    Acids<\/th>\r\nBases<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
    HCl<\/td>\r\nLiOH<\/td>\r\n<\/tr>\r\n
    HBr<\/td>\r\nNaOH<\/td>\r\n<\/tr>\r\n
    HI<\/td>\r\nKOH<\/td>\r\n<\/tr>\r\n
    HNO3<\/sub><\/td>\r\nRbOH<\/td>\r\n<\/tr>\r\n
    H2<\/sub>SO4<\/sub><\/td>\r\nCsOH<\/td>\r\n<\/tr>\r\n
    HClO3<\/sub><\/td>\r\nMg(OH)2<\/sub><\/td>\r\n<\/tr>\r\n
    HClO4<\/sub><\/td>\r\nCa(OH)2<\/sub><\/td>\r\n<\/tr>\r\n
    <\/td>\r\nSr(OH)2<\/sub><\/td>\r\n<\/tr>\r\n
    <\/td>\r\nBa(OH)2<\/sub><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
    \r\n

    Example 12.2<\/p>\r\n\r\n<\/header>\r\n

    \r\n\r\nIdentify each acid or base as strong or weak.\r\n
      \r\n \t
    1. HCl<\/li>\r\n \t
    2. Mg(OH)2<\/sub><\/li>\r\n \t
    3. C5<\/sub>H5<\/sub>N<\/li>\r\n<\/ol>\r\nSolution<\/em>\r\n
        \r\n \t
      1. Because HCl is listed in Table 12.1, it is a strong acid.<\/li>\r\n \t
      2. Because Mg(OH)2<\/sub> is listed in Table 12.1, it is a strong base.<\/li>\r\n \t
      3. The nitrogen in C5<\/sub>H5<\/sub>N would act as a proton acceptor and therefore can be considered a base, but because it does not contain an OH compound, it cannot be considered a strong base; it is a weak base.<\/li>\r\n<\/ol>\r\nTest Yourself<\/em>\r\nIdentify each acid or base as strong or weak.\r\n
          \r\n \t
        1. RbOH<\/li>\r\n \t
        2. HNO2<\/sub><\/li>\r\n<\/ol>\r\nAnswers<\/em>\r\n
            \r\n \t
          1. strong base<\/li>\r\n \t
          2. weak acid<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
            \r\n

            Example 12.3<\/p>\r\n\r\n<\/header>\r\n

            \r\n\r\nWrite the balanced chemical equation for the dissociation of Ca(OH)2<\/sub> and indicate whether it proceeds 100% to products or not.\r\n\r\nSolution<\/em>\r\nThis is an ionic compound of Ca2+<\/sup> ions and OH\u2212<\/sup> ions. When an ionic compound dissolves, it separates into its constituent ions:\r\n

            Ca(OH)2<\/sub> \u2192 Ca2+<\/sup>(aq) + 2OH\u2212<\/sup>(aq)<\/p>\r\nBecause Ca(OH)2<\/sub> is listed in Table 12.1, this reaction proceeds 100% to products.\r\n\r\nTest Yourself<\/em>\r\nWrite the balanced chemical equation for the dissociation of hydrazoic acid (HN3<\/sub>) and indicate whether it proceeds 100% to products or not.\r\n\r\nAnswer<\/em>\r\nThe reaction is as follows:\r\n

            HN3<\/sub> \u2192 H+<\/sup>(aq) + N3<\/sub>\u2212<\/sup>(aq)<\/p>\r\nIt does not proceed 100% to products because hydrazoic acid is not a strong acid.\r\n\r\n<\/div>\r\n<\/div>\r\nCertain salts will also affect the acidity or basicity of aqueous solutions because some of the ions will undergo hydrolysis, just like NH3<\/sub> does to make a basic solution. The general rule is that salts with ions that are part of strong acids or bases will not hydrolyze, while salts with ions that are part of weak acids or bases will hydrolyze.\r\n\r\nConsider NaCl. When it dissolves in an aqueous solution, it separates into Na+<\/sup> ions and Cl\u2212<\/sup> ions:\r\n

            NaCl \u2192\u00a0Na+<\/sup>(aq) +\u00a0Cl\u2212<\/sup>(aq)<\/p>\r\nWill the Na+<\/sup>(aq) ion hydrolyze? If it does, it will interact with the OH\u2212<\/sup> ion to make NaOH:\r\n

            Na+<\/sup>(aq) +\u00a0H2<\/sub>O \u2192\u00a0NaOH +\u00a0H+<\/sup>(aq)<\/p>\r\nHowever, NaOH is a strong base, which means that it is 100% ionized in solution:\r\n

            NaOH \u2192\u00a0Na+<\/sup>(aq) +\u00a0OH\u2212<\/sup>(aq)<\/p>\r\nThe free OH\u2212<\/sup>(aq) ion reacts with the H+<\/sup>(aq) ion to remake a water molecule:\r\n

            H+<\/sup>(aq) +\u00a0OH\u2212<\/sup>(aq) \u2192\u00a0H2<\/sub>O<\/p>\r\nThe net result? There is no change, so there is no effect on the acidity or basicity of the solution from the Na+<\/sup>(aq) ion. What about the Cl\u2212<\/sup> ion? Will it hydrolyze? If it does, it will take an H+<\/sup> ion from a water molecule:\r\n

            Cl\u2212<\/sup>(aq) +\u00a0H2<\/sub>O \u2192\u00a0HCl +\u00a0OH\u2212<\/sup><\/p>\r\nHowever, HCl is a strong acid, which means that it is 100% ionized in solution:\r\n

            HCl \u2192\u00a0H+<\/sup>(aq) +\u00a0Cl\u2212<\/sup>(aq)<\/p>\r\nThe free H+<\/sup>(aq) ion reacts with the OH\u2212<\/sup>(aq) ion to remake a water molecule:\r\n

            H+<\/sup>(aq) +\u00a0OH\u2212<\/sup>(aq) \u2192\u00a0H2<\/sub>O<\/p>\r\nThe net result? There is no change, so there is no effect on the acidity or basicity of the solution from the Cl\u2212<\/sup>(aq) ion. Because neither ion in NaCl affects the acidity or basicity of the solution, NaCl is an example of a neutral salt<\/strong>.\r\n\r\nThings change, however, when we consider a salt like NaC2<\/sub>H3<\/sub>O2<\/sub>. We already know that the Na+<\/sup> ion won\u2019t affect the acidity of the solution. What about the acetate ion? If it hydrolyzes, it will take an H+<\/sup> from a water molecule:\r\n

            C2<\/sub>H3<\/sub>O2<\/sub>\u2212<\/sup>(aq) +\u00a0H2<\/sub>O \u2192\u00a0HC2<\/sub>H3<\/sub>O2<\/sub> +\u00a0OH\u2212<\/sup>(aq)<\/p>\r\nDoes this happen? Yes, it does. Why? Because HC2<\/sub>H3<\/sub>O2<\/sub> is a weak acid.<\/em> Any chance a weak acid has to form, it will (the same with a weak base). As some C2<\/sub>H3<\/sub>O2<\/sub>\u2212<\/sup> ions hydrolyze with H2<\/sub>O to make the molecular weak acid, OH\u2212<\/sup> ions are produced. OH\u2212<\/sup> ions make solutions basic. Thus NaC2<\/sub>H3<\/sub>O2<\/sub> solutions are slightly basic, so such a salt is called a basic salt<\/strong>.\r\n\r\nThere are also salts whose aqueous solutions are slightly acidic. NH4<\/sub>Cl is an example. When NH4<\/sub>Cl is dissolved in H2<\/sub>O, it separates into NH4<\/sub>+<\/sup> ions and Cl\u2212<\/sup> ions. We have already seen that the Cl\u2212<\/sup> ion does not hydrolyze. However, the NH4<\/sub>+<\/sup> ion will:\r\n

            NH4<\/sub>+<\/sup>(aq) +\u00a0H2<\/sub>O \u2192\u00a0NH3<\/sub>(aq) +\u00a0H3<\/sub>O+<\/sup>(aq)<\/p>\r\nRecall from the section \"Arrhenius Acids and Bases\"<\/a> that H3<\/sub>O+<\/sup> ion is the hydronium ion, the more chemically proper way to represent the H+<\/sup> ion. This is the classic acid species in solution, so a solution of NH4<\/sub>+<\/sup>(aq) ions is slightly acidic. NH4<\/sub>Cl is an example of an acid salt<\/strong>. The molecule NH3<\/sub> is a weak base, and it will form when it can, just like a weak acid will form when it can.\r\n\r\nSo there are two general rules:\r\n

              \r\n \t
            1. If an ion derives from a strong acid or base, it will not affect the acidity of the solution.<\/li>\r\n \t
            2. If an ion derives from a weak acid, it will make the solution basic; if an ion derives from a weak base, it will make the solution acidic.<\/li>\r\n<\/ol>\r\n
              \r\n

              Example 12.4<\/p>\r\n\r\n<\/header>\r\n

              \r\n\r\nIdentify each salt as acidic, basic, or neutral.\r\n
                \r\n \t
              1. KCl<\/li>\r\n \t
              2. KNO2<\/sub><\/li>\r\n \t
              3. NH4<\/sub>Br<\/li>\r\n<\/ol>\r\nSolution<\/em>\r\n
                  \r\n \t
                1. The ions from KCl derive from a strong acid (HCl) and a strong base (KOH). Therefore, neither ion will affect the acidity of the solution, so KCl is a neutral salt.<\/li>\r\n \t
                2. Although the K+<\/sup> ion derives from a strong base (KOH), the NO2<\/sub>\u2212<\/sup> ion derives from a weak acid (HNO2<\/sub>). Therefore the solution will be basic, and KNO2<\/sub> is a basic salt.<\/li>\r\n \t
                3. Although the Br\u2212<\/sup> ions derive from a strong acid (HBr), the NH4<\/sub>+<\/sup> ion derives from a weak base (NH3<\/sub>), so the solution will be acidic, and NH4<\/sub>Br is an acidic salt.<\/li>\r\n<\/ol>\r\nTest Yourself<\/em>\r\nIdentify each salt as acidic, basic, or neutral.\r\n
                    \r\n \t
                  1. (C5<\/sub>H5<\/sub>NH)Cl<\/li>\r\n \t
                  2. Na2<\/sub>SO3<\/sub><\/li>\r\n<\/ol>\r\nAnswers<\/em>\r\n
                      \r\n \t
                    1. acidic<\/li>\r\n \t
                    2. basic<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\nSome salts are composed of ions that come from both weak acids and weak bases. The overall effect on an aqueous solution depends on which ion exerts more influence on the overall acidity. We will not consider such salts here.\r\n
                      \r\n

                      Key Takeaways<\/p>\r\n\r\n<\/header>\r\n

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