Strong Acids and Bases

Learning Goals

You will learn a little about the method of ranking of acids and bases.

Synopsis

Looking at the listing of strong acids we see that HClO₄ is ranked above HCl and that HCl is ranked above H₃O⁺. We have previously stated that the stronger acid is the one that more completely donates a proton. But in water solution both HClO₄ and HCl are completely dissociated. Water is a strong enough base to accept protons from either acid and so water has a leveling effect on the strength of these very strong acids. In a water solution we cannot differentiate between the strengths of these two acids. So how do we differentiate between these acids? We need to use a solvent that is a weaker base than water so that it will more readily accept protons from the stronger acid than from the weaker acid. Such a solvent is called a differentiating solvent. Diethyl ether, C₂H₅OC₂H₅, is a weaker base than water, and in diethyl ether HCl is only partially dissociated, but HClO₄ is still totally dissociated. So we can then state that HClO4 is a stronger acid than HCl. However, we will be mainly concerned with acid-base activity in aqueous solutions and in such solutions all acids at and above H₃O⁺ are equally strong. We can make the same statement regarding bases at and above OH^- in a water solution.

On-Line Activity

Do a web search for differentiating solvents and see what more you can learn about this method of differentiating between acids of equal strength in a water solution.