Question

In my book (Bruice) it is specified how it is possible to compare the pKa of two substances to understand who acts as an acid and who as a base. Here's the example in the book: NH₃ + H₂O.

pKₐ(NH₃) = 36
pKₐ (H₂O) = 15.7
Now it is explained that, being the pKₐ of water lower than that of ammonia, water will be the substance that will behave as an acid, so NH₄⁺ and OH⁻ will be the products. But this reasoning, in my opinion, does not make much sense, as the pKₐ of two different reactions are compared: the acid hydrolysis reaction of ammonia, which generates NH₂⁻ and H₃O⁺ and whose pKₐ is actually 36, and the autoprotolysis reaction of water, which generates H₃O⁺ and OH⁻ and whose pKₐ is actually 15.7. So, I wanted to understand if it actually makes sense to compare the pKₐ of two different equilibria to predict who the acid is.
Furthermore, also NH₃, and not necessarily H₂O, can act as an acid. Indeed, ammonia can give both acid hydrolysis and basic hydrolysis to which it is possible to associate a pKₐ and a pK_b.

Reference (1) Paula Yurkanis Bruice, Organic Chemistry, 8th ed.; Pearson Education, Inc., 2011, pp. 58.

Answer 1

To determine the acid-base behavior in a reaction involving NH₃ and H₂O, we compare pKa values, which show that water acts as the acid and NH₃ as the base, leading to the formation of NH₄⁺ and OH⁻ ions.

Step-by-step explanation:

When comparing the pKa values of different substances to determine their behavior as acids or bases, it's important to consider the context of the reactions.

The specified pKa of NH₃ (36) and H₂O (15.7) indicates that water is more likely to donate a proton than ammonia, making water the stronger acid in the reaction where NH₃ acts as a base.

In the given reaction, NH₃ + H₂O ⇒ NH₄⁺ + OH⁻, water donates a proton to ammonia, resulting in the formation of ammonium ions (NH₄⁺) and hydroxide ions (OH⁻).

This reaction is favored because the conjugate acid of NH₃, the NH₄⁺ ion, has a measurable pKa value and the ammonium ion is a weaker acid compared to water.

Additionally, the autoionization of water (2H₂O(l) ⇒ H₃O⁺(aq) + OH⁻(aq)) has a set equilibrium constant (Kw) at a given temperature, reflecting water's amphiprotic nature.