14.7: Strong and Weak Acids and Bases
- Page ID
- 25856
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- 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:
\[HCl\overset{100\%}{\rightarrow}H^{+}(aq)+Cl^{-}(aq) \nonumber \]
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:
\[HC_{2}H_{3}O_{2}\overset{\sim 5\%}{\longrightarrow}H^{+}(aq)+C_{2}H_{3}O_{2}^{-}(aq) \nonumber \]
Because this reaction does not go 100% to completion, it is more appropriate to write it as a reversible reaction:
\[HC_{2}H_{3}O_{2}\rightleftharpoons H^{+}(aq)+C_{2}H_{3}O_{2}^{-}(aq) \nonumber \]
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.
| 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.
Identify each acid or base as strong or weak.
- HCl
- Mg(OH)2
- C5H5N
Solution
- Because HCl is listed in Table \(\PageIndex{1}\), it is a strong acid.
- Because Mg(OH)2 is listed in Table \(\PageIndex{1}\), it is a strong base.
- 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.
Identify each acid or base as strong or weak.
- \(\ce{RbOH}\)
- \(\ce{HNO_2}\)
- Answer a
- strong base
- Answer b
- weak acid
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.
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.
Key Takeaways
- 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.