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14.7: Strong and Weak Acids and Bases

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  • Page ID
    98104
  • SKILLS TO DEVELOP

    • 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.

    Strong and Weak Acids

    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.

    Any acid that dissociates 100% into ions is called a strong acid. If it does not dissociate 100%, it is 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

    Strong and Weak Bases

    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 \(\PageIndex{1}\): Identifying Strong and Weak Acids and Bases

    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. \(\ce{RbOH}\)
    2. \(\ce{HNO_2}\)
    Answer a
    strong base
    Answer b
    weak acid

    Example \(\PageIndex{2}\): Characterizing Base IOnization

    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:

    \[\ce{Ca(OH)2 → Ca^{2+}(aq) + 2OH^{−}(aq)} \nonumber\]

    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 a
    The reaction is as follows:
    \[\ce{HN3 → H^{+}(aq) + N3^{−}(aq)} \nonumber\]
    It does not proceed 100% to products because hydrazoic acid is not a strong acid.

    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.

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