KEY2CHEM

Relationship between Structure and Acid/Base Strength

An acid is a compound that donates a proton (according to the Bronsted-Lowry definition). The stronger the acid, the more it dissociates to donate the proton ($$\require{mhchem}\ce{HA + H2O <=> H3O+ + A-}$$). The equilibrium constant for a stronger acid will be larger than the equilibrium constant for a weaker acid, since the equilibrium will favor formation of more products via dissociation. Stability of the conjugate base formed also increases acid strength, since the dissociation of the proton leaves behind a stable molecule. Similar trends are observed for bases (proton acceptors, based on the Bronsted-Lowry definition).

Example 1.

Which acid is the weakest?

A. $$\require{mhchem}\ce{HBr}$$

B. $$\require{mhchem}\ce{HCl}$$

C. $$\require{mhchem}\ce{HF}$$

Solution

C. $$\require{mhchem}\ce{HF}$$

$$\require{mhchem}\ce{F}$$ is the smallest, most highly electronegative of these atoms. The $$\require{mhchem}\ce{H-F}$$ bond is therefore stronger and less easily broken. This makes it more difficult for $$\require{mhchem}\ce{H+}$$ to dissociate and means $$\require{mhchem}\ce{HF}$$ is the weakest of these acids.

Example 2.

A strong base will have a $$\require{mhchem}\ce{K_{b}}$$ (base dissociation constant) _____ $$1$$.

A. $$=$$

B. $$>$$

C. $$<$$

Solution

B. $$>$$

A strong base is one whose equilibrium favors the products, leading to $$\text{K}_b > 1$$.

Example 3.

Which oxoacid is the strongest acid?

A. $$\require{mhchem}\ce{HClO4}$$

B. $$\require{mhchem}\ce{HClO3}$$

C. $$\require{mhchem}\ce{HClO2}$$

Solution

A. $$\require{mhchem}\ce{HClO4}$$

Increasing the number of electronegative oxygen atoms stabilizes the negative charge on the conjugate base that is formed, leading to a stronger acid.