Di- and Polybasic Acids: Stepwise Ionization and Factors Affecting Acid Strength

Acids that can donate more than one proton per molecule — dibasic acids such as

$\text{H}_2\text{CO}_3$ and $\text{H}_2\text{SO}_4$, or polybasic (or polyprotic) acids such as

$\text{H}_3\text{PO}_4$ — do not release all of their protons in a single step. Instead, ionization

proceeds through a series of distinct equilibria, one for each proton, each with its own ionization

constant. For carbonic acid,

A striking and completely general pattern emerges: each successive ionization constant is dramatically

smaller than the one before it ($K_{a_1} \gg K_{a_2} \gg K_{a_3} \dots$), typically by four to six

orders of magnitude. Two reasons account for this. First, it becomes progressively harder to remove a

positively charged proton, $\text{H}^+$, from a species that is already negatively charged — the

electrostatic attraction between the departing proton and the increasingly negative conjugate base

grows stronger with each step, directly opposing further ionization. Second, once the first proton has

been removed, the resulting species is a weaker acid in its own right, since removing the second

proton requires breaking a bond in a more electron-rich, more stable anion. A practical, useful

consequence of this large gap between successive constants is that, for calculating the pH of a

dibasic or polybasic acid solution, the first ionization step alone is usually sufficient — the

contribution of the second (and any further) ionization to the total $[\text{H}^+]$ is negligible by

comparison.

Beyond the number of ionization steps, the overall strength of an acid — how large its $K_a$ is —

is governed chiefly by how well the conjugate base can stabilize the negative charge left behind after

the proton departs; anything that stabilizes this negative charge makes the parent acid stronger. For

the oxoacids of a given element, such as the chlorine oxoacids $\text{HClO}$, $\text{HClO}_2$,

$\text{HClO}_3$ and $\text{HClO}_4$, acid strength increases sharply as the number of oxygen atoms

attached to the central chlorine increases: $\text{HClO} < \text{HClO}_2 < \text{HClO}_3 < \text{HClO}_4$.

Each additional highly electronegative oxygen atom withdraws more electron density from the O–H bond …