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 $$\require{mhchem}\ce{OH-}$$.