2

Generally, the conjugate base of a weak acid is strong. NH4+ is considered a weak acid. Is its conjugate base NH3 therefore strong?

I have some ideas about this but would like to read the thoughts of other Merspi users.

flag
Is there no way to include subscript and superscript in question titles? – Hugh Feb 21 at 22:50
@Hugh: Not that I know of, no, unfortunately! – Collin Li♦ Feb 21 at 23:22

3 Answers

2

I wouldn't be too hung up by the weak/strong semantics. When we say weak or strong, we ought to have a reference point (how strong? how weak?). For practical reasons, we use pKa/pKb to determine the acidity/basicity of the substance. NH4+ has a pKa of 9.25 while NH3 has a pKb of 4.75. If you remember the rule:Ka/Kb high = pKa/pKb low = strong acid/base. Thus, NH4+ is weakly acidic while NH3 is quite a strong base in comparison. But, NH4+ isn't the weakest acid and NH3 isn't the strongest base in absolute terms (which mean 100% protonation or depronotation in solution, if we use Lowry-Bronsted theory).

link|flag
1

Strong bases have a weak conjugate acid.

Let's consider the relationship between the strength of the ammonium (NH4+) and its conjugate base, ammonia (NH3). The NH4+ ion is a weak acid because ammonia is a reasonably good base.

(Source)

link|flag
This from chemguide.co.uk/physical/acidbaseeqia/bases.html Ammonia is a typical weak base. Ammonia reacts with water to produce ammonium ions and hydroxide ions. However, the reaction is reversible, and at any one time about 99% of the ammonia is still present as ammonia molecules. Only about 1% has actually produced hydroxide ions. A weak base is one which doesn't convert fully into hydroxide ions in solution. – Hugh Feb 22 at 20:56
-1

The words weak and strong are proxy terms for the fundamental concept of (thermodynamic) stability.

Something that is weak is stable, and hence is reluctant to change. It has a relatively low energy level, whereas something that is strong is unstable and wishes to change from its current state. It has a relatively high energy level, and it wishes to give it up in order to achieve a lower energy level (i.e.: more stable, and more weak).

Hence, a weak acid has a relatively low energy level compared to its conjugate base (relatively high energy level), and so the base tends to be strong.

I'd say your statement is true. The more weak the acid (hence the more stable), the greater energy difference there is in between the acid and the conjugate base, and hence the conjugate base is extremely high in energy, and wishes to change into the stable acid form. Hence, this makes the conjugate base extremely reactive and strong.

link|flag
Colin, thanks for the response but I have voted against your answer as ammonia is considered a weak base. I was looking for more insight into this peculiar situation where both ammonium and ammonia are both considered weak. Perhaps they are intermediate in strength, but compared to strong acids and bases they are weak? – Hugh Feb 22 at 3:41
@Hugh: I'd say they are intermediate in strength. Sorry to not answer your question directly, but I did try to "discuss" the issues of strong and weak. – Collin Li♦ Feb 22 at 6:57
No problem Colin. The reason I asked the question was because we are taught that the conjugate base of a weak acid is strong, yet we find that both ammonia and ammonium are weak. I am hoping that someone can shed some light as to why this is the case... – Hugh Feb 22 at 11:51

Your Answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.