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 \(\ce{HCl(aq)}\). When \(\ce{HCl}\) is dissolved in \(\ce{H2O}\), it completely dissociates into \(\ce{H^{+}(aq)}\) and \(\ce{Cl^{−}(aq)}\) ions; all the \(\ce{HCl}\) molecules become ions: \[\ce{HCl\overset{100\%}{\rightarrow}H^{+}(aq)+Cl^{-}(aq)}\nonumber \] Any acid that dissociates 100% into ions is called a strong acid . If it does not dissociate 100%, it is a weak acid . Acetic acid (\(\ce{HC2H3O2}\)) is an example of a weak acid: \[\ce{HC2H3O2 \overset{\sim 5\%}{\longrightarrow} H^{+}(aq) + C2H3O2^{-}(aq)} \nonumber \] Because this reaction does not go 100% to completion, it is more appropriate to write it as a reversible reaction: \[\ce{HC2H3O2 \rightleftharpoons H^{+}(aq) + C2H3O2^{-}(aq)} \nonumber \] 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. Acids Bases HCl LiOH HBr NaOH HI KOH HNO3 RbOH H2SO4 CsOH HClO3 Mg(OH)2 HClO4 Ca(OH)2 NaN Sr(OH)2 NaN 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 dependent on some other mechanism, such as NH 3 (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. HCl Mg(OH) 2 C 5 H 5 N 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 C 5 H 5 N 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. RbOH HNO 2 Answer a strong base Answer b 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 Ca 2 + ions and OH − ions. When an ionic compound dissolves, it separates into its constituent ions: \[\ce{Ca(OH)2 → Ca^{2+}(aq) + 2OH^{-}(aq)} \nonumber \nonumber \] 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 (HN 3 ) and indicate whether it proceeds 100% to products or not. Answer The reaction is as follows: \[\ce{HN3 → H+(aq) + N3^{-}(aq)} \nonumber \nonumber \] 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 NH 3 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: \[\ce{NaCl → Na+(aq) + Cl^{−}(aq)}\nonumber \] Will the Na + (aq) ion hydrolyze? If it does, it will interact with the OH − ion to make NaOH: \[\ce{Na+(aq) + H2O → NaOH + H+(aq)}\nonumber \] However, NaOH is a strong base, which means that it is 100% ionized in solution: \[\ce{NaOH → Na^+(aq) + OH^{−}(aq)}\nonumber \] The free OH − (aq) ion reacts with the H + (aq) ion to remake a water molecule: \[\ce{H+(aq) + OH^{−}(aq) → H2O}\nonumber \] 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: \[\ce{Cl^{−}(aq) + H2O → HCl + OH^{−}}\nonumber \] However, HCl is a strong acid, which means that it is 100% ionized in solution: \[\ce{HCl → H+(aq) + Cl^{−}(aq)}\nonumber \] The free H + (aq) ion reacts with the OH − (aq) ion to remake a water molecule: \[\ce{H+(aq) + OH^{−}(aq) → H2O}\nonumber \] 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 salt. A neutral salt is any ionic compound that does not affect the acidity or basicity of its aqueous solution. Things change, however, when we consider a salt like NaC 2 H 3 O 2 . 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: \[\ce{C2H3O2^{−}(aq) + H2O \rightleftharpoons HC2H3O2 + OH−(aq)}\nonumber \] Does this happen? Yes, it does. Why? Because HC 2 H 3 O 2 is a weak acid. Any chance a weak acid has to form, it will (the same with a weak base). As some C 2 H 3 O 2 − ions hydrolyze with H 2 O to make the molecular weak acid, OH − ions are produced. OH − ions make solutions basic. Thus NaC 2 H 3 O 2 solutions are slightly basic, so such a salt is called a basic salt . There are also salts whose aqueous solutions are slightly acidic. NH 4 Cl is an example. When NH 4 Cl is dissolved in H 2 O, it separates into NH 4 + ions and Cl − ions. We have already seen that the Cl − ion does not hydrolyze. However, the NH 4 + ion will: \[\ce{NH4+(aq) + H2O \rightleftharpoons NH3(aq) + H3O+(aq)}\nonumber \] Recall from Section 12.2, that H 3 O + 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 NH 4 + (aq) ions is slightly acidic. NH 4 Cl is an example of an acid salt . The molecule NH 3 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: If an ion derives from a strong acid or base, it will not affect the acidity of the solution. 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 \(\PageIndex{3}\) Identify each salt as acidic, basic, or neutral. KCl KNO 2 NH 4 Br Solution 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. Although the K + ion derives from a strong base (KOH), the NO 2 − ion derives from a weak acid (HNO 2 ). Therefore the solution will be basic, and KNO 2 is a basic salt. Although the Br − ions derive from a strong acid (HBr), the NH 4 + ion derives from a weak base (NH 3 ), so the solution will be acidic, and NH 4 Br is an acidic salt. Exercise \(\PageIndex{3}\) Identify each salt as acidic, basic, or neutral. (C 5 H 5 NH)Cl Na 2 SO 3 Answer a acidic Answer b 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.