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⚙️Learning Objectives
- Define a strong and a weak acid and base.
- Recognize an acid or a base asstrong or weak.
Electrolytes were previously described as substances that yield ions when dissolved in water, which meansthat aqueous solutions of electrolytes are able to conduct electricity. It should be clear that soluble ionic compounds areelectrolytes. While it may seemthat molecular compounds should all benonelectrolytes, it turns out that this isn't always the case. Molecular compounds that are classified as acids or bases are electrolytes – some are strong electrolytes; most, however, are weak electrolytes.
An acid that is a strong electrolyte is called a strong acid, while an acid that is a weak electrolyte is a weak acid. A base that is a strong electrolyte is called a strong base, while a base that is a weak electrolyte is called a weak base. Classifying acids or bases as strong or weak has nothing to do with their concentration. It is possible to have a dilute solution of a strong acid or base and it is also possible to have a concentrated solution of a weak acid or base.
Weak acids and bases may be just as dangerous as their stronger counterpartswhenconcentrated. For example, vinegar is an aqueous solution of acetic acid, HC2H3O2, usually with a concentration of 5%. Vinegar is used to provide flavor and preserve foods. One might think of acetic acid as being quite harmless due to its widespread presence in foods. However, acetic acid in its pure form, called glacial acetic acid (seeFigure \(\PageIndex{1}\)), can cause severe skin burns in its concentrated form. One of the authors of this section has had such an experience back in his days in college when working with glacial acetic acid as a student in lab.
Strong Acids and Strong Bases
The terms strong acidand strong baseare used to indicate that these substances are strong electrolytes when dissolved in water. The hydroxides of elements in Group IA and Group IIA on the periodic table areall strong electrolytes [except for Mg(OH)2and Be(OH)2since they are mostly insoluble in water]. As soluble ionic compounds, they dissociate completely to yield the metal ion and the hydroxide ion. Consider an aqueous solution of NaOH:
\[\mathrm{NaOH}\;(aq)\;\xrightarrow{100\%}\;\mathrm{Na}^+\;(aq)\;+\;\mathrm{OH}^-\;(aq)\]
NaOH is a base, due to the formation of OH–in water and it is a strong electrolyte, since it dissociates completely in water. We say it dissociates, because NaOH is already an ionic compound and the ions simply dissociate, or separate, from each other when NaOH is dissolved in water.Because NaOHis a base and a strong electrolyte, NaOH isclassified as a strong base.
When HCl is dissolved in water, it yields a solution that is strongly conducting. This is an indication that it is completely converted into ions. Since HCl is a molecular compound, we can't say it dissociates into its ions, but rather, we say that it ionizes completely in the presence of water. The term ionize means that ions are formed from something that was not originally ionic. To explain the existence of ions, we can show that HClreacts with water to yieldH3O+(aq) and Cl−(aq) ions:
\[\mathrm{HCl}\;(aq)\;+\;{\mathrm H}_2\mathrm O\;(l)\;\xrightarrow{100\%}\;{\mathrm H}_3\mathrm O^+\;(aq)\;+\;\mathrm{Cl}^-\;(aq)\]
Since HClionizes completely to form ions, it is a strong electrolyte. HCl is classified as an acid due to the formation of H3O+ions in water. Consequently, HCl is classified as a strong acid.
Weak Acids and Weak Bases
The terms weakacidand weakbaseare used to indicate that these substances areweakelectrolytes when dissolved in water. Most acids and bases are weak electrolytes and are also molecular compounds. As molecular compounds, this means they must ionize (react with water) to be an electrolyte, but that they do not ionize completely.
For example, when acetic acid is placed in water, approximately 1% of the molecules ionize. The remaining 99% of HC2H3O2molecules remain intact and unreacted. Since most of the molecules do not ionize, the solution is only weakly conducting. This reaction may be visualized in equation form as:
\[{\mathrm{HC}}_2{\mathrm H}_3{{\mathrm O}_2\;(aq)\;+\;{\mathrm H}_2\mathrm O\;(l)\;}\overset{\sim1\%}\longrightarrow\;{\mathrm H}_3\mathrm O^+(aq)\;+\;{\mathrm C}_2{\mathrm H}_3\mathrm O_2^-\;(aq)\]
Since this reaction does not go 100% to completion, it is more appropriate to write it as a reversible reactionto indicate that most of the reaction mixture remains as reactants:
\[{\mathrm{HC}}_2{\mathrm H}_3{{\mathrm O}_2\;(aq)\;+\;{\mathrm H}_2\mathrm O\;(l)\;}\rightleftharpoons\;{\mathrm H}_3\mathrm O^+(aq)\;+\;{\mathrm C}_2{\mathrm H}_3\mathrm O_2^-\;(aq)\]
Since HC2H3O2ionizes only partiallyto form ions, it is a weakelectrolyte. HC2H3O2 is classified as an acid due to the formation of H3O+ions in water. Consequently, HC2H3O2 is classified as a weakacid.
When NH3is dissolved in water, it ionizes to yielda solution that is weaklyconducting. Therefore, we show its reaction with wateras a reversible reactionto indicate that most of the reaction mixture remains as reactants and not as ions:
\[{\mathrm{NH}}_3{\;(aq)\;+\;{\mathrm H}_2\mathrm O\;(l)\;}\rightleftharpoons\;\mathrm{NH}_4^+(aq)\;+\;\mathrm{OH}^-\;(aq)\]
NH3 is a base, due to the formation of OH–in water and it is a weakelectrolyte,since it ionizes only partiallyin water.Because NH3is a base and aweakelectrolyte, NH3isclassified as a weakbase.
Classification of Acids and Bases as Strong or Weak
As it turns out, there are very few strong acids and very few strong bases. Those classified as strong acids and strong basesare listedin Table \(\PageIndex{1}\). If an acid is not listed here, it is a weak acid.If abaseis not listed here, it is a weak base.
*Mg(OH)2is sometimes classified as a strong base, since it ionizes completely. However, because it is relatively insoluble, little dissolves and relatively few OH–ions are formed in solution. Therefore, is is also sometimes classified as a weak base. It will be considered a weak base in this text.
As mentioned above,Table \(\PageIndex{1}\) is an all-inclusive list of strong acids and a strong bases.If an acid or base does not appear in on the list in Table \(\PageIndex{1}\), it is classified as weak. Table \(\PageIndex{2}\) below shows a few common weak acids and weak bases:
| Weak Acids | Weak Bases |
|---|---|
| HC2H3O2 | Mg(OH)2 |
| H3PO4 | NH3 |
| HF | |
| H2CO3 |
✅Example \(\PageIndex{1}\): Identifying Strong and Weak Acids and Bases
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 \(\PageIndex{2}\), it is a weak base.
- Since it is implied this must be an acid or a base, it must be a base since it does not have H out at the front of the formula. It can be assumed that C5H5N would act as a proton acceptor and therefore act asa base. Sinceit does not contain aOH–ion, it cannot be considered a strong base; it is a weak base.
✏️Exercise \(\PageIndex{1}\)
Identify each acid or base as strong or weak.
- LiOH
- HNO2
- Answer A
- strong base
- Answer B
- weak acid
✅Example \(\PageIndex{2}\): Characterizing Base Ionization
Write the balanced chemical equation for the dissociation of Ba(OH)2 and indicate whether it proceeds 100% to products or not.
Solution
This is an ionic compound of Ba2+ ions and OH− ions. When an ionic compound dissolves, it separates into its constituent ions:
\(\mathrm{Ba}{(\mathrm{OH})}_2\;(aq)\;\xrightarrow{100\%}\;\mathrm{Ba}^{2+}\;(aq)\;+\;2\;\mathrm{OH}^-\;(aq)\)
Ba(OH)2 is listed in Table \(\PageIndex{1}\), so this reaction proceeds 100% to products.
✏️Exercise \(\PageIndex{2}\)
Write the balanced chemical equation for the ionizationof nitrousacid (HNO2) and indicate whether it proceeds 100% to products or not.
- Answer
- The reaction is as follows:
\[{\mathrm{HNO}}_2\;(aq)\;+\;{\mathrm H}_2\mathrm O\;(l)\;\rightleftharpoons{\mathrm H}_3\mathrm O^+\;(aq)\;+\;\mathrm{NO}_2^-\;(aq) \nonumber\]
It does not proceed 100% to products because nitrousacid is a weakacid.
Key Takeaways
- Strong acids and bases are 100% ionized in aqueous solution.
- Weak acids and bases are less than 100% ionized in aqueous solution.
This pageis shared under aCC BY-NC-SA 4.0license and was authored, remixed, and/or curated byMarisa Alviar-Agnew,Henry Agnew and Lance S. Lund (Anoka-Ramsey Community College).
