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A "strong" acid simply means that when added to water, the acid will dissociate completely, therefore a strong acid is strong because it dissociates completely in aqueous solution while a weak acid is weak because it does not dissociate completely in aqueous solution. There are only 7 common inorganic strong acids so I would simply memorize them. As far as why they are strong acids i.e. why they dissociate completely while other acids do not is a little more complicated and to be honest I am probably not the person to ask. I haven't looked into inorganic acids too extensively, though organic acids you must look at the stability of the conjugate base. A more stable conjugate base means the acid will more easily donate a proton. I do know for a fact that the conjugate base of sulfuric acid exhibits resonance which is a major stabilizing factor. However, based on the structure of sulfurous acid I would expect it to also exhibit resonance, though I could be wrong. The acid dissociation constant Ka is determined experimentally. Strong acids have a Ka that is >> 1 while weak acids have a Ka that is << 1. Hopefully I answered your question, but if not I'm sure someone with a little more inorganic chemistry experience will fill in the holes.
Learning Objectives
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 → H+(aq) + Cl−(aq) (100%) 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: HC2H3O2→ H+(aq) + C2H3O2−(aq) (~5%) Because this reaction does not go 100% to completion, it is more appropriate to write it as an equilibrium: HC2H3O2 ⇄ H+(aq) + C2H3O2−(aq) As it turns out, there are very few strong acids, which are given in Table 12.2 “Strong Acids and Bases”. 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. 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 (see Table 12.2 “Strong Acids and Bases”); 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.
Table 12.2 Strong Acids and Bases
Identify each acid or base as strong or weak. Solution
Test Yourself Identify each acid or base as strong or weak. Answers
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 12.2 “Strong Acids and Bases”, this reaction proceeds 100% to products. Test Yourself 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: NaCl → Na+(aq) + Cl−(aq) Will the Na+(aq) ion hydrolyze? If it does, it will interact with the OH− ion to make NaOH: Na+(aq) + H2O → NaOH + H+(aq) However, NaOH is a strong base, which means that it is 100% ionized in solution: NaOH → Na+(aq) + OH−(aq) The free OH−(aq) ion reacts with the H+(aq) ion to remake a water molecule: 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: Cl−(aq) + H2O → HCl + OH− However, HCl is a strong acid, which means that it is 100% ionized in solution: HCl → H+(aq) + Cl−(aq) The free H+(aq) ion reacts with the OH−(aq) ion to remake a water molecule: 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 salt. 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: C2H3O2−(aq) + H2O → HC2H3O2 + OH−(aq) Does this happen? Yes, it does. Why? Because HC2H3O2is 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: NH4+(aq) + H2O → NH3(aq) + H3O+(aq) Recall from Section 12.1 “Arrhenius Acids and Bases” 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.
Identify each salt as acidic, basic, or neutral. Solution
Test Yourself Identify each salt as acidic, basic, or neutral. Answers 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.
Key Takeaways
a) HF b) HCl c) HC2O4
4. Identify each as a strong base or a weak base. Assume aqueous solutions. a) NaOH b) Al(OH)3 c) C4H9NH2
5. Write a chemical equation for the ionization of each acid and indicate whether it proceeds 100% to products or not. a) HNO3 b) HNO2 c) HI3
6. Write a chemical equation for the ionization of each base and indicate whether it proceeds 100% to products or not. a) NH3 b) (CH3)3N c) Mg(OH)2
7. Write the balanced chemical equation for the reaction of each acid and base pair. a) HCl + C5H5N b) H2C2O4 + NH3 c) HNO2 + C7H9N
8. Write the balanced chemical equation for the reaction of each acid and base pair. a) H3C5H5O7 + Mg(OH)2 b) HC3H3O3 + (CH3)3N c) HBr + Fe(OH)3
9. Identify each salt as neutral, acidic, or basic. a) NaBr b) Fe(NO3)2 c) Fe(NO3)3
10. Identify each salt as neutral, acidic, or basic. a) NH4I b) C2H5NH3Cl c) KI
11. Identify each salt as neutral, acidic, or basic. a) NaNO2 b) NaNO3 c) NH4NO3
12. Identify each salt as neutral, acidic, or basic. a) KC2H3O2 b) KHSO4 c) KClO3
13. Write the hydrolysis reaction that occurs, if any, when each salt dissolves in water. a) K2SO3 b) KI c) NH4ClO3
14. Write the hydrolysis reaction that occurs, if any, when each salt dissolves in water. a) NaNO3 b) CaC2O4 c) C5H5NHCl
15. When NH4NO2 dissolves in H2O, both ions hydrolyze. Write chemical equations for both reactions. Can you tell if the solution will be acidic or basic overall?
16. When pyridinium acetate (C5H5NHC2H3O2) dissolves in H2O, both ions hydrolyze. Write chemical equations for both reactions. Can you tell if the solution will be acidic or basic overall?
17. A lab technician mixes a solution of 0.015 M Mg(OH)2. Is the resulting OH− concentration greater than, equal to, or less than 0.015 M? Explain your answer.
18. A lab technician mixes a solution of 0.55 M HNO3. Is the resulting H+ concentration greater than, equal to, or less than 0.55 M? Explain your answer. Answers 1. A strong acid is 100% ionized in aqueous solution, whereas a weak acid is not 100% ionized. 3. a) weak acid b) strong acid c) weak acid 5. a) HNO3(aq) → H+(aq) + NO3−(aq); proceeds 100% b) HNO2(aq) → H+(aq) + NO2−(aq); does not proceed 100% c) HI3(aq) → H+(aq) + I3−(aq); does not proceed 100% 9. a) HCl + C5H5N → Cl− + C5H5NH+ b) H2C2O4 + 2NH3 → C2O42− + 2NH4+ c) HNO2 + C7H9N → NO2− + C7H9NH+11. a) neutral b) acidic c) acidic 13. a) basic b) neutral c) acidic 15. a) SO32− + H2O → HSO3− + OH− b) no reaction c) NH4+ + H2O → NH3 + H3O+17. NH4+ + H2O → NH3 + H3O+; NO2− + H2O → HNO2 + OH−; it is not possible to determine whether the solution will be acidic or basic. 19. greater than 0.015 M because there are two OH− ions per formula unit of Mg(OH)2 |