KEY2CHEM

Net Direction of Acid-Base Reactions

Reactions between acids and bases are proton transfer reactions. The acid donates a proton to form its conjugate base, and the base accepts a proton to form its conjugate acid. As such, there is an acid and a base on each side in proton transfer reaction. The net direction of the reaction (which side is favored at equilibrium) is determined by the relative strength of the components.

The stronger acid is more able to donate a proton, and the stronger base is better able to accept a proton. As such, the net direction of the reaction favors the reaction of the stronger acid with the stronger base to generate the weaker acid and the weaker base.

$$\text{stronger acid + stronger base}\require{mhchem}\ce{ <=>} \text{weaker acid + weaker base} \;\;\; K > 1$$

Note that there is an inverse relationship between acid and conjugate base strength, so the stronger acid will always have the weaker conjugate base.

Example 1.

What is the value of K for the reaction below? The $$K_a$$ of $$\require{mhchem}\ce{HNO2}$$ is $$4.5 \times 10 ^{-4}$$, and the $$K_a$$ of $$\require{mhchem}\ce{NH4+}$$ is $$5.6 \times 10 ^{-10}$$.

$$\require{mhchem}\ce{HNO2(aq) + NH3(aq) <=> NO2^{-}(aq) + NH4+(aq)}$$

A. $$K > 1$$

B. $$K = 1$$

C. $$K < 1$$

Solution

A. $$K > 1$$

The net direction of the reaction favors the reaction of the stronger acid with the stronger base to form the weaker acid and the weaker base. $$\require{mhchem}\ce{HNO2}$$ has a larger $$K_a$$ value and is the stronger acid (better proton donor). Since $$\require{mhchem}\ce{HNO2}$$ is a stronger acid, its conjugate base ($$\require{mhchem}\ce{NO2-}$$) is a weaker base. $$\require{mhchem}\ce{NH4+}$$ has a smaller $$K_a$$ value and is the weaker acid (less able to donate a proton). Since $$\require{mhchem}\ce{NH4+}$$ is a weaker acid, its conjugate base ($$\require{mhchem}\ce{NH3}$$) is a stronger base. As such the products (the weaker side) are favored, so $$K > 1$$.

Example 2.

What is the conjugate base formed when aqueous solutions of $$\require{mhchem}\ce{CH3COOH}$$ and$$\require{mhchem}\ce{F-}$$ react?

A. $$\require{mhchem}\ce{HF}$$

B. $$\require{mhchem}\ce{CH3COO-}$$

C. $$\require{mhchem}\ce{HF}$$

Solution

B. $$\require{mhchem}\ce{CH3COO-}$$

$$\require{mhchem}\ce{CH3COOH}$$ is the acid (proton donor), and its conjugate base is $$\require{mhchem}\ce{CH3COO-}$$.

Example 3.

In a proton transfer reaction, the net direction of the reaction will favor the ______ acid and base.

A. weaker

B. stronger

C. neutral

Solution

A. weaker

A proton transfer reaction occurs to a greater extent to form the weaker acid and weaker base from the reaction of a stronger acid with a stronger base.