🧪 Topic Summary
Polyprotic acids are acids that can donate more than one proton (hydrogen ion) per molecule. Each ionization step has its own equilibrium constant, denoted as $K_{a1}$, $K_{a2}$, $K_{a3}$, and so on. The first ionization is generally the strongest ($K_{a1}$ is the largest), and subsequent ionizations are weaker ($K_{a2} < K_{a1}$, $K_{a3} < K_{a2}$, etc.). When solving problems involving polyprotic acids, it's often sufficient to consider only the first ionization step, especially if the $K_a$ values are significantly different. However, for more accurate calculations or when dealing with higher concentrations, subsequent ionization steps may need to be considered. Let's dive into some practice problems to see how this works!
🧠 Part A: Vocabulary
Match the term with its definition:
| Term |
Definition |
| 1. Polyprotic Acid |
A. The negative logarithm of the acid dissociation constant ($K_a$). |
| 2. Monoprotic Acid |
B. An acid that can donate only one proton per molecule. |
| 3. $K_a$ |
C. The equilibrium constant for the dissociation of an acid. |
| 4. $pK_a$ |
D. An acid that can donate more than one proton per molecule. |
| 5. Ionization |
E. The process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often producing ions in solution. |
✍️ Part B: Fill in the Blanks
Complete the following paragraph:
A polyprotic acid, such as ________ acid ($H_2SO_4$), can donate more than one ________. Each donation has its own ________ constant, labeled $K_{a1}$, $K_{a2}$, and so on. Generally, $K_{a1}$ is ________ than $K_{a2}$ because it is easier to remove a proton from a ________ molecule.
🤔 Part C: Critical Thinking
Explain why the $K_{a1}$ value is generally greater than the $K_{a2}$ value for a diprotic acid. Use principles of electrostatics and molecular stability in your explanation.