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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I got the following question from a student in a previous semester:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">“When figuring out conjugate acids and bases for molecules such as HCOOH, is there a general<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">rule for which hydrogen to remove? I know taking one over the other would most likely effect<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">it in some way but I'm not sure what exactly that effect would be.”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">The simplest answer is the acidic H is the one that comes off. Okay, which one is that?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">HCOOH if formic acid, a carboxylic acid. Acetic acid, what’s in vinegar, is CH3COOH. This is<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">often the “favorite” weak acid you see used quite often. It can also be written as CH3CO2H<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">or more commonly as HC2H3O2. Note this last designation. The acidic H atom for carboxylic<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">acids is the one connect to the O atom of the -CO2H group but in the formula, HC2H3O2, is the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">far left-hand one. This is actually addressed in section 4.3.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Formic acid is the simplest carboxylic acid.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> O-H<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> |<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> X – C = O<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">The X is a H atom (formic acid) or a carbon group (such as CH3, CH3CH2, etc.). <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Hopefully my structure above didn’t lose it’s formatting.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Even though the acidic H atom is connected to an O atom, as stated above, it is often<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">written first in the condensed formula for an acid, HC2H3O2. H atoms connected to a<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">C atom are not going to be “acidic” H atoms and won’t come off to form H+ in solution.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">This is often the case for polyatomic acids (oxyacids) like H2SO4, H3PO4, HNO3, etc.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">The H atoms written first in the formulas are the acidic H atoms and are connected to<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">the O atoms. This is discussed in section 2.8 and 4.3. See Figure 4.6 in section 4.3.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Binary acids are easier. These have a H atom and one other atom and the H atom is<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">connect to the other atom. Examples are HCl, H2S, H2O, H3O+, etc.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">In all cases, the acidic H atom comes from a polar bond. H atoms as part of a nonpolar
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">are not going to be acidic and won’t come off as an H+, at least not in water. Even some<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">H atoms which are part of a polar bond won’t come off as an H+. I discussed this when<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I covered section 16.10.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I hope this helps those of you with this question.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Dr. Zellmer<o:p></o:p></span></p>
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