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:

"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, which one is that?
HCOOH if formic acid, a carboxylic acid.  Acetic acid, what's in vinegar, is CH3COOH.  This is
often the "favorite" weak acid you see used quite often.  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 connect to the O atom of the -CO2H group but in the formula, HC2H3O2, is the
far left-hand one.   This is actually addressed in section 4.3.


Formic acid 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 it's 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 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.

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
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+.  I discussed this when
I covered section 16.10.

I hope this helps those of you with this question.

Dr. Zellmer
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