KEY2CHEM

Proton Transfer Reactions

A Bronsted-Lowry acid is a proton donor, and a Bronsted-Lowry base is a proton acceptor. In a proton transfer reaction, the acid donates a proton to the base, forming the conjugate acid of the base and the conjugate base of the acid.


Example 1.

 

What is the conjugate base of \(\require{mhchem}\ce{HCO3-}\)?

A. \(\require{mhchem}\ce{CO3^{2-}}\)

B. \(\require{mhchem}\ce{H2CO3}\)

C. \(\require{mhchem}\ce{H2O}\)

 

 

 

 

 

 

 

 

Solution 

A. \(\require{mhchem}\ce{CO3^{2-}}\)

The conjugate base of an acid has one fewer \(\require{mhchem}\ce{H}\) and one additional negative charge.

 


Example 2.

 

What is the conjugate acid of \(\require{mhchem}\ce{NH3}\)?

A. \(\require{mhchem}\ce{NH2-}\)

B. \(\require{mhchem}\ce{NH4+}\)

C. \(\require{mhchem}\ce{NH3}\)

 

 

 

 

Solution

 

B. \(\require{mhchem}\ce{NH4+}\)

The conjugate acid has one more \(\require{mhchem}\ce{H}\) and one fewer negative charge relative to the base.

 


Example 3.

 

In the proton transfer reaction, what conjugate acid is formed as a product?

\(\require{mhchem}\ce{HSO4^{-}(aq) + O^{2-}(aq)<=> OH^{-}(aq) + SO4^{2-}(aq) }\)

A. \(\require{mhchem}\ce{H2SO4}\)

B. \(\require{mhchem}\ce{OH-}\)

C. \(\require{mhchem}\ce{SO4^{2-}}\)

 

 

 

 

Solution

B. \(\require{mhchem}\ce{OH-}\)

\(\require{mhchem}\ce{O^{2-}}\) is the base, and its conjugate acid is \(\require{mhchem}\ce{OH-}\).