Strong and Weak Acids and Bases

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 [latex]\ce{HCl}{(aq)}[/latex]:

When [latex]\ce{HCl}[/latex] is dissolved in [latex]\ce{H_{2}O}[/latex], it completely dissociates into [latex]\ce{H^{+}}{(aq)}[/latex] and [latex]\ce{Cl^{-}}{(aq)}[/latex] ions; all the [latex]\ce{HCl}[/latex] molecules become ions, as can be seen below:

[latex]\ce{HCl} → \ce{H^{+}}{(aq)} + \ce{Cl^{-}}{(aq)}[/latex]

Any acid that dissociates 100% into ions is called a strong acid. If it does not dissociate 100%, it is a weak acid. [latex]\ce{HC_{2}H_{2}O_{2}}[/latex] is an example of a weak acid and its dissociation is shown below:

[latex]\ce{HC_{2}H_{3}O_{2}} → \ce{H^{+}}{(aq)} + \ce{C_{2}H_{3}O_{2}^{-}}{(aq) }[/latex]

Because this reaction does not go 100% to completion, it is more appropriate to write it as an equilibrium, as shown below:

[latex]\ce{HC_{2}H_{3}O_{2}} ⇄ \ce{H^{+}}{(aq)} + \ce{C_{2}H_{3}O_{2}^{-}}{(aq)}[/latex]

As it turns out, there are very few strong acids, which are given in Table 6.4.1 “Strong Acids and Bases”. If an acid is not listed here, it is a weak acid. It may be 1% ionised or 99% ionised, but it is still classified as a weak acid.

This concept applies to the bases as well: a strong base is a base that is 100% ionised in solution. If it is less than 100% ionised in solution, it is a weak base. As previously mentioned, there are not many strong bases, and in fact, any base not listed in Table 6.4.1 is considered a weak base. All strong bases are [latex]\ce{OH^{-}}[/latex] compounds. So a base based on some other mechanism, such as [latex]\ce{NH_{3}}[/latex] (which does not contain [latex]\ce{OH^{-}}[/latex] ions as part of its formula), will be a weak base.

Acidity and Basicity of Salt Solutions

Certain salts will also affect the acidity or basicity of aqueous solutions because some of the ions will undergo hydrolysis, just like [latex]\ce{NH_{3}}[/latex] does, to make a basic solution. The general rule is that salts with ions that are part of strong acids or bases will not hydrolyse, while salts with ions that are part of weak acids or bases will hydrolyse.

• Consider [latex]\ce{NaCl}[/latex]. When it dissolves in an aqueous solution, it separates into [latex]\ce{Na^{+}}[/latex] ions and [latex]\ce{Cl^{-}}[/latex] ions, as shown below:

[latex]\ce{NaCl} → \ce{Na^{+}}{(aq)} + \ce{Cl^{-}}{(aq)}[/latex]

Will the [latex]\ce{Na^{+}}{(aq)}[/latex]  ion hydrolyse? If it does, it will interact with the [latex]\ce{OH^{-}}[/latex] ion to make [latex]\ce{NaOH}[/latex]:

[latex]\ce{Na^{+}}{(aq)} + \ce{H_{2}O} → \ce{NaOH} + \ce{H^{+}}{(aq)}[/latex]

However, [latex]\ce{NaOH}[/latex] is a strong base, which means that it is 100% ionised in solution:

[latex]\ce{NaOH} → \ce{Na^{+}}{(aq)} + \ce{OH^{-}}{(aq)}[/latex]

The free [latex]\ce{OH^{-}}{(aq)}[/latex] ion reacts with the [latex]\ce{H^{+}}{(aq)}[/latex] ion to remake a water molecule:

[latex]\ce{H^{+}}{(aq)} + \ce{OH^{-}}{(aq)} → \ce{H_{2}O}[/latex]

What is the net result? There is no change, so there is no effect on the acidity or basicity of the solution from the [latex]\ce{Na^{+}}{(aq)}[/latex] ion. What about the [latex]\ce{Cl^{-}}[/latex] ion? Will it hydrolyse? If it does, it will take an [latex]\ce{H^{+}}[/latex] ion from a water molecule, as shown below:

[latex]\ce{Cl^{-}}{(aq)} + \ce{H_{2}O} → \ce{HCl} + \ce{OH^{-}}[/latex]

However, [latex]\ce{HCl}[/latex] is a strong acid, which means that it is 100% ionised in solution:

[latex]\ce{HCl} → \ce{H^{+}}{(aq)} + \ce{Cl^{-}}{(aq)}[/latex]

The free [latex]\ce{H^{+}}{(aq)}[/latex] ion reacts with the [latex]\ce{OH^{-}}{(aq)}[/latex] ion to remake a water molecule:

[latex]\ce{H^{+}}{(aq)} + \ce{OH^{-}}{(aq)} → \ce{H_{2}O}[/latex]

What is the net result? There is no change, so there is no effect on the acidity or basicity of the solution from the [latex]\ce{Cl^{-}}{(aq)}[/latex] ion. Because neither ion in [latex]\ce{NaCl}[/latex] affects the acidity or basicity of the solution, [latex]\ce{NaCl}[/latex] is an example of a neutral salt.

• Let’s consider a salt like [latex]\ce{NaC_{2}H_{3}O_{2}}[/latex]. We already know that the [latex]\ce{Na^{+}}[/latex] ion won’t affect the acidity of the solution. What about the acetate ion? If it hydrolyses, it will take an [latex]\ce{H^{+}}[/latex] from a water molecule, as shown in the following equation:

[latex]\ce{C_{2}H_{3}O_{2}^{-}}{(aq)} + \ce{H_{2}O} → \ce{HC_{2}H_{3}O_{2}} + \ce{OH^{-}}{(aq}[/latex]

Does this happen? Yes, it does. Why? Because [latex]\ce{HC_{2}H_{3}O_{2}}[/latex] is a weak acid. Any chance a weak acid has to form, it will (the same with a weak base). As some [latex]\ce{C_{2}H_{3}O_{2}^{-}}[/latex] ions hydrolyse with [latex]\ce{H_{2}O}[/latex] to make the molecular weak acid, [latex]\ce{OH^{-}}[/latex] ions are produced. [latex]\ce{OH^{-}}[/latex] ions make solutions basic. Thus, [latex]\ce{NaC_{2}H_{3}O_{2}}[/latex] solutions are slightly basic, so such a salt is called a basic salt.

• There are also salts whose aqueous solutions are slightly acidic. [latex]\ce{NH_{4}Cl}[/latex] is an example. When [latex]\ce{NH_{4}Cl}[/latex] is dissolved in water, it separates into [latex]\ce{NH_{4}^{+}}[/latex] ions and [latex]\ce{Cl^{-}}[/latex] ions. We have already seen that the [latex]\ce{Cl^{-}}[/latex] ion does not hydrolyse. However, the [latex]\ce{NH_{4}^{+}}[/latex] ion will, as can be seen in the following equation:

[latex]\ce{NH_{4}^{+}}{(aq)} + \ce{H_{2}O} → \ce{NH_{3}}{(aq)} + \ce{H_{3}O^{+}}{(aq)}[/latex]

The [latex]\ce{H_{3}O^{+}}[/latex] ion is the hydronium ion, the more chemically proper way to represent the [latex]\ce{H^{+}}[/latex] ion. This is the classic acid species in solution, so a solution of [latex]\ce{NH_{4}^{+}}{(aq)}[/latex] ions is slightly acidic. [latex]\ce{NH_{4}Cl}[/latex] is an example of an acid salt. The molecule [latex]\ce{NH_{3}}[/latex]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.

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 in this book.