Question from student: "Which H atom gets removed from an acid?"

[Zellmer, Robert]

I got the following question from a student in a previous semester and a similar one yesterday:

"When figuring out conjugate acids and bases for molecules such as HCOOH, is there a general
rule for which hydrogen to remove? I know taking one over the other would most likely effect
it in some way but I'm not sure what exactly that effect would be."

The simplest answer is the acidic H is the one that comes off.  Okay, duh!  Which one is that?

HCOOH is formic acid, a carboxylic acid.  Acetic acid, what's in vinegar, is CH3COOH.  This is
often the "favorite" weak acid you see as an example.  It can also be written as CH3CO2H
or more commonly as HC2H3O2.  Note this last designation.  The acidic H atom for carboxylic
acids is the one connected to the O atom of the -CO2H group but in the formula, HC2H3O2, it
is the far left-hand one, just like for H2SO4, HNO3, H3PO4, etc.

Formic acid, HCOOH , is the simplest carboxylic acid.

                    O-H
                     |
              X - C = O

The X is a H atom (formic acid) or a carbon group (such as CH3, CH3CH2, etc.).
Hopefully, my structure above didn't lose its formatting.

Even though the acidic H atom is connected to an O atom, as stated above, it is often
written first in the condensed formula for an acid, HC2H3O2.  H atoms connected to a
C atom are not going to be "acidic" H atoms and won't come off to form H+ in an
aqueous solution.

This is often the case for polyatomic acids (oxyacids) like H2SO4, H3PO4, HNO3, etc.
The H atoms written first in the formulas are the acidic H atoms and are connected to
the O atoms.  This is discussed in section 2.8 and 4.3.  See Figure 4.6 in section 4.3.
One exception is H3PO3.  One of the H atoms is on the P atom and is barely acidic
so H3PO3 is often considered a diprotic acid.

Binary acids are easier.  These have a H atom and one other atom, and the H atom is
connect to the other atom.  Examples are HCl, H2S, H2O, H3O+, etc.

In all cases, the acidic H atom comes from a polar bond.  H atoms as part of a nonpolar
molecule (like CH4, CH3CH3, C6H6, etc.) are not going to be acidic and won't come
off as an H+, at least not in water.  Even some H atoms which are part of a polar bond
won't come off as an H+.  This is the case for most alcohols (such as CH3OH,
CH3CH2OH, etc.).  I discussed this when I covered section 16.10.  Phenols (an OH
on a benzene ring, C6H5-OH) are different and are slightly acidic.  I went over why at
the end of Section 16.10 in lecture (it's not in my posted notes).  It has to do with
resonance stabilization of the conjugate base.

I hope this helps those of you with this question.

Dr. Zellmer
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