How could i know that any element is a strong or weak acid/base/salts if given in examinations.
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
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
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
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 |
