Acids and Bases: Molecular Structure and Behavior
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Introduction
Molecular Structure
Ionization Constants
Salts
Buffers
Lewis Theory

Polyprotic Acids and Bases

The Lewis structure of oxalic acid is shown below. As you can see, there are two carboxyl groups, which means there are two acidic hydrogens. Molecules containing more than one acidic hydrogen are called polyprotic acids . Similarily, if a molecule only has one acidic hydrogen, it is called a monoprotic acid .

Oxalic acid, H2C2O4, is a weak acid. How will the acidity of HC2O4- compare to the acidity of H2C2O4?

Stronger acid Weaker acid Same strength

The product of the first ionization, HC2O4- has a negative charge. The negative charge will make it harder for HC2O4- to act as an acid since the positively charged proton feels an attraction to the negative charge.

Good! Since the positively charged proton feels an attraction to the negative charge, HC2O4- is weaker than H2C2O4.










Below are molecular representations of several polyprotic acids. Which best represents an aqueous solution of oxalic acid?

NOTE: To simplify these images, the water molecules have been omitted.

Oxalic acid is a weak acid and will ionize in an aqueous solution.

Oxalic acid is a weak acid and will only partially ionize in an aqueous solution.

There are two acidic protons in oxalic acid. The first ionization produces HC2O4-, which is also a weak acid and will also ionize.

Good! It is important to realize the ionization of oxalic acid occurs in two steps. In the first step, a small amount of the weak acid H2C2O4 ionizes to form HC2O4-. Since HC2O4- is also a weak acid, a small amount of it then ionizes to form C2O42-. Since HC2O4- is a weaker acid than H2C2O4, less HC2O4- will ionize than H2C2O4.

Note: The above representations are not to scale. The second ionization constant of most polyprotic acids is much smaller than the first ionization constant. If there were only seven molecules (as shown in this representation), there would be no fully deprotonated species present since the second ionization constant is so small.

Just as there are polyprotic acids, there are polyprotic bases . The structure of hydrazine is shown below. As you can see, there are two nitrogens, each with a lone pair of electrons. This allows hydrazine to accept two protons.