Stepwise Ionization and Equilibrium of Polyprotic Acids and Bases

All flashcards

What is a monoprotic acid?

An acid that can donate one proton per molecule in an acid–base reaction.

Why is the total number of hydrogen atoms not always the acid’s proton count?

Some hydrogen atoms are not ionizable in the acid–base reaction. Classification counts only the protons the substance can donate.

What is a diprotic acid?

An acid that can donate two protons per molecule, typically through two successive ionization reactions.

How does a polyprotic acid ionize?

It donates protons in separate steps. The product of one step can act as the acid in the next step.

What is the usual trend in successive $K_a$ values?

They decrease from one ionization step to the next, indicating that each successive proton is donated less extensively.

Quiz questions

1. Which species donates a proton in the second ionization of carbonic acid?Answer: The bicarbonate ion, $\mathrm{HCO_3^-}$.
2. How does sulfuric acid's first ionization in water compare with its second?Answer: The first is essentially complete; the second is not.
3. For a weak diprotic acid, when is it generally reasonable to calculate the first ionization separately?Answer: When $K_{a1}$ is at least about 20 times larger than $K_{a2}$.
4. For a weak acid with initial concentration $C$, what approximation is used when the ionized amount $x$ is small?Answer: Approximate $C-x$ as $C$ in the equilibrium expression.
5. Why is carbonate, $\mathrm{CO_3^{2-}}$, classified as a diprotic base?Answer: It can accept two protons in successive reactions with water.