Question

Explain why the hydronium ion, \(\mathrm{H}_{3} \mathrm{O}^{+}\), is the strongest acid that can exist in aqueous solution. What is the strongest base that can exist in aqueous solution?

Short answer

Answer: The hydronium ion, H3O+, is the strongest acid that can exist in an aqueous solution because it can donate a proton to any other species in the solution. In contrast, the strongest base in an aqueous solution is the hydroxide ion, OH-, as it can accept a proton from any other species in the solution. Both ions act as the limit for the strength of acids and bases in an aqueous solution, as stronger acids or bases would not be stable and would immediately react with any available protons.

Step-by-step solution

Understanding Acids and Bases in Aqueous Solution

Acids are substances that release hydrogen ions (H+) or protons when they dissolve in water. They can donate protons to water molecules, forming hydronium ions, \(\mathrm{H}_{3}\mathrm{O}^{+}\). Bases, on the other hand, release hydroxide ions (OH-) when they dissolve in water; they can accept protons from water molecules. The strength of an acid or base depends on its ability to donate or accept protons, respectively.

Dissociation of Water

In an aqueous solution, a small number of water molecules naturally dissociate into hydronium ions and hydroxide ions through a process called auto-ionization: \[2\mathrm{H}_{2}\mathrm{O} \leftrightarrow \mathrm{H}_{3}\mathrm{O}^{+} + \mathrm{OH}^{-}\] The concentration of hydronium ions equals the concentration of hydroxide ions in pure water. The acidity or basicity of an aqueous solution is determined by the relative concentrations of these ions.

Hydronium Ion as the Strongest Acid

When an acid dissolves in water, it donates a proton to a water molecule, forming a hydronium ion: \[\mathrm{HA} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{H}_{3}\mathrm{O}^{+} + \mathrm{A}^{-}\] In this reaction, the acid (HA) loses a proton to water and forms a conjugate base (A-). Since the hydronium ion, \(\mathrm{H}_{3}\mathrm{O}^{+}\), can donate a proton to any other species in the solution, it is the strongest acid that can exist in water. An even stronger acid would completely dissociate in water and lose all its protons.

Strongest Base in Aqueous Solution

The strongest base that can exist in aqueous solution is the hydroxide ion, \(\mathrm{OH}^{-}\). When a base dissolves in water, it accepts a proton from a water molecule and turns it into its conjugate acid: \[\mathrm{B} + \mathrm{H}_{2}\mathrm{O} \leftrightarrow \mathrm{HB}^{+} + \mathrm{OH}^{-}\] In this reaction, the base (B) gains a proton from water, forming a conjugate acid (HB+). Since the hydroxide ion can accept a proton from any other species in the solution, it is the strongest base that can exist in an aqueous solution. A stronger base would not be stable and would immediately react with any protons available.