Definition of a Feeble acid?

1. Jan 31, 2014

chemnoob

Definition of a "Feeble" acid?

I'm currently trying to re-familiarize myself with chemistry, after a very long absence of any study in this field. At the moment I'm working my way through the text "Fundamentals of Chemistry" by "David E Goldberg" and I notice the author introduces the concept of "feeble" acids (and bases).

I was familiar with the concept of weak (partially dissociated) acids versus strong (near 100% dissociated) acids. However the distinction between a weak acid and a feeble acid is not something I've come across before.

It's clear from the text that a "feeble" acid is weaker than a "weak" acid, but the author doesn't make it all that clear where exactly "weak" ends and "feeble" begins. Googling for the term "feeble acid" doesn't seem to reveal many relevant hits, so I'm not even sure how widely this distinction between weak/feeble is actually used.

Just wondering if anyone can shed a bit more light on this for me. Is there a particular value of dissociation constant (say 10^-8 for example) below which one can say that an acid is feeble?

2. Jan 31, 2014

Staff: Mentor

First time I hear this term. And while I definitely don't know everything I have spent substantial amount of time studying the subject.

3. Jan 31, 2014

Staff: Mentor

In his textbook, did Goldberg name any feeble acids?

4. Jan 31, 2014

chemnoob

Yes, he gives Boric acid (HBH2O3) and Phenol (HOC6H5) as examples of "feeble" acids. I looked these up and they both have dissociation constants order of magnitude around 10^-10. By contrast the examples of weak acids he gives tend to have dissociation constants around 10^-4 and 10^-5.

5. Jan 31, 2014

chemnoob

Ok thanks. That tends to confirm my suspicion that this distinction is not so widely used. Perhaps it's one of the authors own "thangs".

BTW. He introduces this terminology while discussing the concept of conjugate acids and bases. He explains how the stronger the acid the weaker it's conjugate base and vise versa. However he then claims that the conjugate of a strong acid is NOT a weak base, but rather a "feeble" base. And similarly that the conjugate of a strong base is not a weak acid, but rather a "feeble" acid.

He defines these conjugate pair relationships as follows.

acid/base <=> conjugate base/acid
-----------------------------------
strong <=> feeble
weak <=> weak
feeble <=> strong

Last edited: Jan 31, 2014
6. Jan 31, 2014

Staff: Mentor

Never seen it. As far as I know there is no difference between weak and "feeble" conjugate acids/bases - other that quantitative (ie. very small Ka/Kb).