AP® Chemistry · Unit 8 · Topic 8.3
Weak Base Equilibria
A weak base raises pH not by containing hydroxide but by pulling a proton off water, leaving hydroxide behind; Kb measures how far that reaction goes, and it only partly proceeds, unlike a strong base that dumps in OH- directly.
Ammonia, NH3, has no hydroxide in its formula, yet a solution of it is basic. That's because a Bronsted-Lowry base is defined by what it does, not what it contains: it accepts a proton. Ammonia's nitrogen has a lone pair that grabs an H+ off a water molecule. Water splits apart, NH3 becomes NH4+, and the leftover piece is OH-. The hydroxide is a product of the reaction, not an ingredient of ammonia itself. That single mechanism is the whole story behind every weak base you'll meet.
Because this proton transfer is an equilibrium, it doesn't go to completion. Only a small fraction of NH3 molecules react at any moment, which is why ammonia is a weak base and why Kb is small. You find [OH-] the same way you'd find [H+] for a weak acid: set up an ICE table, let x be the amount reacted, and because x is tiny compared to the starting concentration, approximate Kb as x2/c. Solve for x to get [OH-], take -log to get pOH, then subtract from 14 to get pH.
Ka and Kb are linked through the conjugate pair: for NH4+ and NH3, Ka times Kb equals Kw, 1.0 x 10−14, at 25 C. That's because adding the two reactions, the acid donating a proton and the base accepting one, regenerates water on both sides, and multiplying their equilibrium expressions cancels everything except Kw. This relationship lets you jump between acid and base strength of a conjugate pair without a second experiment, and it sets up buffers, where both members of that pair sit together in solution and resist pH change.
Key terms
- Weak base
- A substance that only partially accepts protons from water, establishing an equilibrium rather than reacting completely.
- Kb
- The equilibrium constant for a base reacting with water to form its conjugate acid and hydroxide.
- Conjugate acid
- The species formed when a base accepts a proton, such as NH4+ from NH3.
- Small-x approximation
- Treating the amount of base that reacts as negligible compared to its initial concentration to simplify the equilibrium math.
- Kw
- The ion product of water, 1.0 x 10−14 at 25 C, equal to Ka times Kb for any conjugate acid-base pair.
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End of topic.