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12.5: Strong and Weak Acids and Bases and their Salts

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  • Page ID
    64083
  • Learning Objectives

    • Define a strong and a weak acid and base.
    • Recognize an acid or a base as strong or weak.
    • Determine if a salt produces an acidic or a basic solution.

    Except 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 H2O, it completely dissociates into H+(aq) and Cl(aq) ions; all the HCl molecules become ions:

    \[HCl\overset{100\%}{\rightarrow}H^{+}(aq)+Cl^{-}(aq)\]

    Any acid that dissociates 100% into ions is called a strong acid. If it does not dissociate 100%, it is a weak acid. HC2H3O2 is an example of a weak acid:

    \[HC_{2}H_{3}O_{2}\overset{\sim 5\%}{\longrightarrow}H^{+}(aq)+C_{2}H_{3}O_{2}^{-}(aq)\]

    Because this reaction does not go 100% to completion, it is more appropriate to write it as a reversible reaction:

    \[HC_{2}H_{3}O_{2}\rightleftharpoons H^{+}(aq)+C_{2}H_{3}O_{2}^{-}(aq)\]

    As it turns out, there are very few strong acids, which are given in Table \(\PageIndex{1}\). 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.

    Table \(\PageIndex{1}\): Strong Acids and Bases
    Acids Bases
    HCl LiOH
    HBr  NaOH
    HI KOH
    HNO3 RbOH
    H2SO4 CsOH
    HClO3 Mg(OH)2
    HClO4 Ca(OH)2
      Sr(OH)2
      Ba(OH)2

    The issue is similar with bases: a strong base is a base that is 100% ionized in solution. If it is less than 100% ionized in solution, it is a weak base. There are very few strong bases (Table \(\PageIndex{1}\)); any base not listed is a weak base. All strong bases are OH compounds. So a base based on some other mechanism, such as NH3 (which does not contain OH ions as part of its formula), will be a weak base. 

    Example \(\PageIndex{1}\):

    Identify each acid or base as strong or weak.

    1. HCl
    2. Mg(OH)2
    3. C5H5N

    Solution

    1. Because HCl is listed in Table \(\PageIndex{1}\), it is a strong acid.
    2. Because Mg(OH)2 is listed in Table \(\PageIndex{1}\), it is a strong base.
    3. The nitrogen in C5H5N 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.

    Exercise \(\PageIndex{1}\)

    Identify each acid or base as strong or weak.

    1. RbOH
    2. HNO2

    Answers

    1. strong base
    2. weak acid

    Example \(\PageIndex{2}\):

    Write the balanced chemical equation for the dissociation of Ca(OH)2 and indicate whether it proceeds 100% to products or not.

    Solution

    This is an ionic compound of Ca2+ ions and OH ions. When an ionic compound dissolves, it separates into its constituent ions:

    Ca(OH)2 → Ca2+(aq) + 2OH(aq)

    Because Ca(OH)2 is listed in Table \(\PageIndex{1}\), this reaction proceeds 100% to products.

    Exercise \(\PageIndex{2}\)

    Write the balanced chemical equation for the dissociation of hydrazoic acid (HN3) and indicate whether it proceeds 100% to products or not.

    Answer

    The reaction is as follows:

    HN3 → H+(aq) + N3(aq)

    It does not proceed 100% to products because hydrazoic acid is not a strong acid.

    Certain salts will also affect the acidity or basicity of aqueous solutions because some of the ions will undergo hydrolysis, just like NH3 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.

    Consider NaCl. When it dissolves in an aqueous solution, it separates into Na+ ions and Cl ions:

    \[\ce{NaCl → Na+(aq) + Cl^{−}(aq)}\]

    Will the Na+(aq) ion hydrolyze? If it does, it will interact with the OH ion to make NaOH:

    \[\ce{Na+(aq) + H2O → NaOH + H+(aq)}\]

    However, NaOH is a strong base, which means that it is 100% ionized in solution:

    \[\ce{NaOH → Na^+(aq) + OH^{−}(aq)}\]

    The free OH(aq) ion reacts with the H+(aq) ion to remake a water molecule:

    \[\ce{H+(aq) + OH^{−}(aq) → H2O}\]

    The net result? There is no change, so there is no effect on the acidity or basicity of the solution from the Na+(aq) ion. What about the Cl ion? Will it hydrolyze? If it does, it will take an H+ ion from a water molecule:

    \[\ce{Cl^{−}(aq) + H2O → HCl + OH^{−}}\]

    However, HCl is a strong acid, which means that it is 100% ionized in solution:

    \[\ce{HCl → H+(aq) + Cl^{−}(aq)}\]

    The free H+(aq) ion reacts with the OH(aq) ion to remake a water molecule:

    \[\ce{H+(aq) + OH^{−}(aq) → H2O}\]

    The net result? There is no change, so there is no effect on the acidity or basicity of the solution from the Cl(aq) ion. Because neither ion in NaCl affects the acidity or basicity of the solution, NaCl is an example of a neutral saltAn ionic compound that does not affect the acidity of its aqueous solution..

    Things change, however, when we consider a salt like NaC2H3O2. We already know that the Na+ ion won’t affect the acidity of the solution. What about the acetate ion? If it hydrolyzes, it will take an H+ from a water molecule:

    \[\ce{C2H3O2^{−}(aq) + H2O \rightleftharpoons HC2H3O2 + OH−(aq)}\]

    Does this happen? Yes, it does. Why? Because HC2H3O2 is a weak acid. Any chance a weak acid has to form, it will (the same with a weak base). As some C2H3O2 ions hydrolyze with H2O to make the molecular weak acid, OH ions are produced. OH ions make solutions basic. Thus NaC2H3O2 solutions are slightly basic, so such a salt is called a basic salt.

    There are also salts whose aqueous solutions are slightly acidic. NH4Cl is an example. When NH4Cl is dissolved in H2O, it separates into NH4+ ions and Cl ions. We have already seen that the Cl ion does not hydrolyze. However, the NH4+ ion will:

    \[\ce{NH4+(aq) + H2O \rightleftharpoons NH3(aq) + H3O+(aq)}\]

    Recall from Section 12.2, that H3O+ ion is the hydronium ion, the more chemically proper way to represent the H+ ion. This is the classic acid species in solution, so a solution of NH4+(aq) ions is slightly acidic. NH4Cl is an example of an acid salt. The molecule NH3 is a weak base, and it will form when it can, just like a weak acid will form when it can.

    So there are two general rules:

    1. If an ion derives from a strong acid or base, it will not affect the acidity of the solution.
    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.

    Example \(\PageIndex{13}\)

    Identify each salt as acidic, basic, or neutral.

    1. KCl
    2. KNO2
    3. NH4Br

    Solution

    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.
    2. Although the K+ ion derives from a strong base (KOH), the NO2 ion derives from a weak acid (HNO2). Therefore the solution will be basic, and KNO2 is a basic salt.
    3. Although the Br ions derive from a strong acid (HBr), the NH4+ ion derives from a weak base (NH3), so the solution will be acidic, and NH4Br is an acidic salt.

    Exercise \(\PageIndex{1}\)

    Identify each salt as acidic, basic, or neutral.

    1. (C5H5NH)Cl
    2. Na2SO3

    Answers

    1. acidic
    2. basic

    Some 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.

    Summary

    Strong acids and bases are 100% ionized in aqueous solution.Weak acids and bases are less than 100% ionized in aqueous solution.Salts of weak acids or bases can affect the acidity or basicity of their aqueous solutions.