{"id":870,"date":"2018-03-20T16:23:04","date_gmt":"2018-03-20T16:23:04","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-orgbiochemistry\/?post_type=chapter&p=870"},"modified":"2018-10-03T14:19:22","modified_gmt":"2018-10-03T14:19:22","slug":"10-4-the-strengths-of-acids-and-bases","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-monroecc-orgbiochemistry\/chapter\/10-4-the-strengths-of-acids-and-bases\/","title":{"raw":"10.4 The Strengths of Acids and Bases","rendered":"10.4 The Strengths of Acids and Bases"},"content":{"raw":"
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Learning Objectives<\/h3>\r\n
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  1. Describe the difference between strong and weak acids and bases.<\/li>\r\n \t
  2. Describe how a chemical reaction reaches chemical equilibrium.<\/li>\r\n \t
  3. Define the pH scale and use it to describe acids and bases.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

    Acids and bases do not all demonstrate the same degree of activity in solution. Different acids and bases have different strengths.\u00a0 As with strong and weak electrolytes, the difference lies with how completely the acid or base ionizes or dissociates in water.\u00a0 Strong acids and bases completely dissociate into component ions, represented in an equation by using a single-headed arrow\u00a0\u2192.\u00a0 Weak acids and bases have some ionization, but at equilibrium, most of the acid or base remains in the form of molecules, represented in an equation using a double-headed arrow\u00a0\u2194.<\/p>\r\n\r\n

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    Strong and Weak Acids<\/h2>\r\n

    A small number of strong acids ionize completely in aqueous solution. For example, when HCl dissolves in water, every molecule of HCl separates into a hydronium ion and a chloride ion:<\/p>\r\n

    HCl\u00a0+\u00a0H2<\/sub>O(\u2113)\u00a0\u2192H3<\/sub>O+<\/sup>(aq)\u00a0+\u00a0Cl\u2212<\/sup>(aq) <\/span><\/p>\r\n

    HCl(aq) is one example of a strong acid<\/span><\/strong><\/span>, which is a compound that is essentially 100% ionized in aqueous solution. There are very few strong acids. The important ones are listed in Table 10.2 \"Strong Acids and Bases (All in Aqueous Solution)\"<\/a>.<\/p>\r\n\r\n

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    Table 10.2<\/span> Strong Acids and Bases (All in Aqueous Solution)<\/strong><\/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
    Strong Acids<\/th>\r\nStrong Bases<\/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\nMg(OH)2<\/sub><\/td>\r\n<\/tr>\r\n
    H2<\/sub>SO4<\/sub><\/td>\r\nCa(OH)2<\/sub><\/td>\r\n<\/tr>\r\n
    HClO4<\/sub><\/td>\r\n<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

    By analogy, a strong base<\/span><\/strong><\/span>\u00a0is a compound that is essentially 100% ionized in aqueous solution. As with acids, there are only a few strong bases, which are also listed in Table 10.2 \"Strong Acids and Bases (All in Aqueous Solution)\"<\/a>.<\/p>\r\n

    If an acid is not listed in Table 10.2 \"Strong Acids and Bases (All in Aqueous Solution)\"<\/a>, it is likely a weak acid <\/span><\/strong>that is far less than 100% ionized in aqueous solution.<\/span><\/span> Similarly, a weak base<\/span><\/span><\/strong>\u00a0is a compound that is not 100% ionized in aqueous solution. For example, acetic acid (HC2<\/sub>H3<\/sub>O2<\/sub>) is a weak acid. The ionization reaction for acetic acid is as follows:<\/p>\r\n

    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)<\/span><\/span><\/p>\r\n

    Depending on the concentration of HC2<\/sub>H3<\/sub>O2<\/sub>, only for 1%\u20135% of the acetic acid molecules may be ionized.<\/p>\r\n\r\n

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    Looking Closer: Household Acids and Bases<\/h3>\r\n

    Many household products are acids or bases. For example, the owner of a swimming pool may use muriatic acid to clean the pool. Muriatic acid is another name for hydrochloric acid HCl(aq). Vinegar is a dilute solution of acetic acid HC2<\/sub>H3<\/sub>O2<\/sub>(aq). In a medicine chest, one may find a bottle of vitamin C tablets; the chemical name of vitamin C is ascorbic acid HC6<\/sub>H7<\/sub>O6<\/sub>.<\/p>\r\n \r\n

    One of the more familiar household bases is ammonia NH3<\/sub>, which is found in numerous cleaning products. Many soaps are also slightly basic because they contain compounds that act as Br\u00f8nsted-Lowry bases, accepting protons from water and forming excess hydroxide ions.\u00a0 Perhaps the most dangerous household chemical is the lye-based drain cleaner. Lye is a common name for sodium hydroxide, although it is also used as a synonym for potassium hydroxide. Lye is an extremely caustic chemical that can react with grease, hair, food particles, and other substances that may build up and form a clog in a pipe. Unfortunately, lye can also attack tissues and other substances in our bodies. When using lye-based drain cleaners, safe practice would be to wear gloves and protective safety glasses!. Safer, non-lye drain cleaners use peroxide compounds to react on the materials in the clog and clear the drain.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n

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    Chemical Equilibrium<\/h2>\r\n

    The behavior of weak acids and bases illustrates a key concept in chemistry. Does the chemical reaction describing the ionization of a weak acid or base just stop when the acid or base is done ionizing? Actually, no. Rather, the reverse process\u2014the reformation of the molecular form of the acid or base\u2014occurs as well. For example, the ionization of the weak acid HC2<\/sub>H3<\/sub>O2<\/sub> (aq) is as follows:<\/p>\r\n

    HC2<\/sub>H3<\/sub>O2<\/sub>(aq) + H2<\/sub>O(\u2113) \u2192 H3<\/sub>O+<\/sup>(aq) + C2<\/sub>H3<\/sub>O2<\/sub>\u2212<\/sup>(aq)<\/span><\/span><\/p>\r\n

    The reverse process also begins to occur:<\/p>\r\n

    H3<\/sub>O+<\/sup>(aq) + C2<\/sub>H3<\/sub>O2<\/sub>\u2212<\/sup>(aq) \u2192 HC2<\/sub>H3<\/sub>O2<\/sub>(aq) + H2<\/sub>O(\u2113)<\/span><\/span><\/p>\r\n

    Eventually, there is a balance between the two opposing processes, and no additional change occurs. The chemical reaction is better represented at this point with a double arrow:<\/p>\r\n

    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)<\/span><\/span><\/p>\r\n

    The \u21c6 implies that both the forward and reverse reactions are occurring, When there is no net change in concentrations, the reaction is considered to be at chemical equilibrium (or equilibrium)<\/span><\/strong><\/span>. It is important to note that the processes do not stop. They balance out each other so that there is no further net change; that is, chemical equilibrium is a dynamic equilibrium<\/em>.<\/p>\r\n\r\n

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    Example 6<\/h3>\r\n

    Write the equilibrium chemical equation for the partial ionization of each weak acid or base.<\/p>\r\n\r\n

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    1. HNO2<\/sub>(aq)<\/li>\r\n \t
    2. C5<\/sub>H5<\/sub>N(aq)<\/li>\r\n<\/ol>\r\n

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

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      1. HNO2<\/sub>(aq) + H2<\/sub>O(\u2113) \u21c6 NO2<\/sub>\u2212<\/sup>(aq) + H3<\/sub>O+<\/sup>(aq)<\/li>\r\n \t
      2. C5<\/sub>H5<\/sub>N(aq) + H2<\/sub>O(\u2113) \u21c6 C5<\/sub>H5<\/sub>NH+<\/sup>(aq) + OH\u2212<\/sup>(aq)<\/li>\r\n<\/ol>\r\n<\/div>\r\n
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        Skill-Building Exercise<\/h3>\r\n

        Write the equilibrium chemical equation for the partial ionization of each weak acid or base.<\/p>\r\n\r\n

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        1. \r\n
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          HF(aq)<\/p>\r\n\r\n<\/div><\/li>\r\n \t

        2. \r\n
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          AgOH(aq)<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n \r\n

          Furthermore, the autoionization of water is actually an equilibrium process, so it is properly written with the double arrow:<\/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\r\n<\/div>\r\n

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          The pH Scale<\/h2>\r\n

          One qualitative measure of the strength of an acid or a base solution is the pH scale<\/span><\/strong><\/span>, which is based on the concentration of the hydronium (or hydrogen) ion in aqueous solution. A neutral (neither acidic nor basic) solution, one that has the same concentration of hydrogen and hydroxide ions, has a pH of 7. A pH below 7 means that a solution is acidic, with lower values of pH corresponding to increasingly acidic solutions. A pH greater than 7 indicates a basic solution, with higher values of pH corresponding to increasingly basic solutions. Thus, given the pH of several solutions, you can state which ones are acidic, which ones are basic, and which are more acidic or basic than others. Table 10.3 \"The pH Values of Some Common Solutions\"<\/a> lists the pH of several common solutions, including some biological fluids.<\/p>\r\n\r\n

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          Table 10.3<\/span> The pH Values of Some Common Solutions<\/strong><\/h5>\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
          Solution<\/th>\r\npH<\/th>\r\n[H3<\/sub>O+<\/sup>]<\/td>\r\n<\/tr>\r\n<\/thead>\r\n
          battery acid<\/td>\r\n0.3<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          stomach acid<\/td>\r\n1\u20132<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          lemon or lime juice<\/td>\r\n2.1<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          vinegar<\/td>\r\n2.8\u20133.0<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          Coca-Cola<\/td>\r\n3.0<\/td>\r\n1 x 10-3<\/sup> M<\/td>\r\n<\/tr>\r\n
          wine<\/td>\r\n2.8\u20133.8<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          beer<\/td>\r\n4\u20135<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          coffee<\/td>\r\n5.0<\/td>\r\n1 x 10-5<\/sup> M<\/td>\r\n<\/tr>\r\n
          milk<\/td>\r\n6.0<\/td>\r\n1 x 10-6<\/sup> M<\/td>\r\n<\/tr>\r\n
          urine<\/td>\r\n6.0<\/td>\r\n1 x 10-6<\/sup> M<\/td>\r\n<\/tr>\r\n
          pure H2<\/sub>O<\/td>\r\n7.000<\/td>\r\n1.00 x 10-7<\/sup> M<\/td>\r\n<\/tr>\r\n
          (human) blood<\/td>\r\n7.3\u20137.5<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          sea water<\/td>\r\n8.0<\/td>\r\n1 x 10-8<\/sup> M<\/td>\r\n<\/tr>\r\n
          antacid (milk of magnesia)<\/td>\r\n10.5<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          NH3<\/sub> (1 M)<\/td>\r\n11.6<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          bleach<\/td>\r\n12.6<\/td>\r\n<\/td>\r\n<\/tr>\r\n
          NaOH (1 M)<\/td>\r\n14.0<\/td>\r\n<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

          Weak acids and bases are relatively common. You may notice from Table 10.3 \"The pH Values of Some Common Solutions\"<\/a> that many food products are slightly acidic. They are acidic because they contain solutions of weak acids. If the acid components of these foods were strong acids, the food would likely be inedible.<\/p>\r\n\r\n<\/div>\r\n

          Calculating pH, given molarity of H3<\/sub>O+<\/sup>\u00a0 [H3<\/sub>O+<\/sup>]<\/h2>\r\n

          The molarity of\u00a0H3<\/sub>O+<\/sup> symbolized by [H3<\/sub>O+<\/sup>] can vary over 14 orders of magnitude, that is, from 0.00000000000001 M to 1 M.\u00a0 Because of this wide range, the pH scale is defined on a logarithmic basis, where a difference of one pH unit reflects a 10-fold difference in [H3<\/sub>O+<\/sup>].\u00a0 Mathematically, pH = -log [H3<\/sub>O+<\/sup>]<\/strong>.\u00a0 When [H3<\/sub>O+<\/sup>] is 1 x 10-x<\/sup>, the pH is x.\u00a0 See entries for Coca-Cola\u00ae<\/sup>, coffee, milk, urine, pure water, and sea water in Table 10.3 above.\u00a0 But if the coefficient is other than 1, it is necessary to use a calculator.\u00a0 Different brands of calculators use different steps, and it is up to the student to determine the steps for their own calculator.\u00a0 pH is a unitless value.<\/p>\r\n\r\n

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          Example 7<\/h3>\r\n

          Calculate the pH for a solution that has the given value for [H3<\/sub>O+<\/sup>].<\/p>\r\n\r\n

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          1. [H3<\/sub>O+<\/sup>]= 4.56 x 10-4 <\/sup>M<\/li>\r\n \t
          2. [H3<\/sub>O+<\/sup>]= 8.2 x 10-9<\/sup> M<\/li>\r\n<\/ol>\r\n

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

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            1. pH = -log [H3<\/sub>O+<\/sup>]= -log(4.56 x 10-4\u00a0<\/sup>) = 3.341035 (calculator answer).\u00a0 Logs have their own sig fig rules.\u00a0 The only sig figs in the [H3<\/sub>O+<\/sup>] are the coefficient, not the power of ten.\u00a0 The only sig figs in the pH are the digits after the decimal because the digits before the decimal point represent the power of ten.\u00a0 So with 3 digits in the coefficient, the pH should be rounded to have 3 sig figs after the decimal point, 3.34.<\/li>\r\n \t
            2. pH = -log [H3<\/sub>O+<\/sup>]= -log(8.2 x 10-9 <\/sup>) = 8.086186 (calculator answer), properly rounded for sig figs, 8.09.\r\n

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

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              Calculating [H3<\/sub>O+<\/sup>], given pH<\/h2>\r\n[H3<\/sub>O+<\/sup>] = 10-pH<\/sup><\/strong>\u00a0 This is the inverse of the equation to determine pH. If pH is a whole number x, then the [H3<\/sub>O+] value is 1 x 10-x<\/sup> M.\u00a0 Again, see entries for Coca-Cola\u00ae<\/sup>, coffee, milk, urine, pure water, and sea water in Table 10.3 above.\u00a0 Most pH meters report pHs with one or two decimal places though, so again it is necessary to use a calculator, and each student must know the steps for their own calculator. [H3<\/sub>O+]\u00a0 values must be reported in standard scientific notation, not left as 10 to non-whole number value.\r\n\r\n<\/div>\r\n
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              Example 8<\/h3>\r\n

              Calculate the [H3<\/sub>O+], given the pH.<\/p>\r\n\r\n

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              1. pH is 7.00<\/li>\r\n \t
              2. pH is 11.351<\/li>\r\n<\/ol>\r\n

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

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                1. [H3<\/sub>O+<\/sup>] = 10-pH<\/sup> = 10-7.00<\/sup>= 1.0 x 10-7 <\/sup>M.\u00a0 pH had 2 zeros after the decimal point, indicating 2 sig figs, so the coefficient has 2 sig figs total.<\/li>\r\n \t
                2. [H3<\/sub>O+<\/sup>] = 10-pH<\/sup> = 10-11.351<\/sup>= 4.47 x 10-12 <\/sup>M.\u00a0 pH had 3 zeros after the decimal point, indicating 3 sig figs, so the coefficient has 3 sig figs total.<\/li>\r\n<\/ol>\r\n<\/div>\r\n
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                  Concept Review Exercises<\/h3>\r\n
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                    Explain the difference between a strong acid or base and a weak acid or base.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

                  2. \r\n
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                    Explain what is occurring when a chemical reaction reaches equilibrium.<\/p>\r\n\r\n<\/div><\/li>\r\n \t

                  3. \r\n
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                    Define pH<\/em>.<\/p>\r\n\r\n<\/div><\/li>\r\n<\/ol>\r\n<\/div>\r\n

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                    Answers<\/h3>\r\n[reveal-answer q=\"497506\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"497506\"]\r\n\r\n1. A strong acid or base is 100% ionized in aqueous solution; a weak acid or base is less than 100% ionized.\r\n\r\n2. The overall reaction progress stops because the reverse process balances out the forward process.\r\n\r\n3. pH is a measure of the hydrogen ion concentration.\u00a0[\/hidden-answer]\r\n
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                    Key Takeaways<\/h3>\r\n<\/div>\r\n
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