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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">A fellow student sent a question about acid strength based on structure and the strength of the conj. bases.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">“I thought that when looking at the differences between acid strength in a group (especially in halides), bond<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">strength was the most important factor, not electronegativity. Thus, HBrO would be a stronger acid (weaker<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">bonds) than HClO. I also thought this because my TA taught me that acid strength increases as you go down<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">a group (with HI being the strongest acid), so wouldn't HBrO be a stronger acid than HClO, and ClO- would be<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">the strongest base?”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">This is conflating two different things about acid structure and strength. This is taking something for binary<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">acids and apply it to ternary acids (oxyacids).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">A binary acid has a H atom and one other atom and the acidic H atom is attached to the other atom. The<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">acidity of a binary acid depends on two things, the polarity of the H-X bond and it’s bond strength. Within<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">a group (like HF, HCl, HBr and HI) the bond strength is the primary factor. Going down the group the H-X<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">bond gets longer and weaker and is more easily broken (takes less energy to break the bond) so the H atom<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">comes off more easily as an H+. The acid strength inc. going down the group. You can also think about this<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">in terms of the conj. bases. As the X- gets larger it allows the negative charge to spread out. When it spreads<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">out it’s not as available to react with an H+. That makes the X- a weaker base and thus it’s conj. acid is<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">stronger.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Within a row the size of the X atoms doesn’t change much. X gets smaller going from left to right in a row<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">but the size doesn’t change much, certainly not like it does going down a column. However, electroneg.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">of X changes a lot. You can see the EN of the atoms in Table in Fig 8.8. The EN changes a lot as you got from<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">left to right in a row, especially in row 2, ending with F with an EN of 4.0. This really changes the bond<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">polarity, which increases a lot as you go from left to right in a row. The bond strength doesn’t inc. much<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">going left to right as it gets shorter because the size of the X doesn’t change much. So in a row the polarity<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">of the bond is the more impt factor. The polarity of the H-X bond inc. going left to right so acid strength<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">inc. from left to right in a row.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">So for BINARY acids ONLY there is a periodic trend:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"> Acid strength inc. left to right and top to bottom. I have this in the notes.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">*************************<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">For oxyacids you’re considering the acidity of an OH bond with the O connected to another atom (such as<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">in H2SO4, HNO3, HClO3, etc.). Tthere are two things you need to consider, both depend on the polarity of<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">the OH bond and what affects it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">1) Same structure, different central atom.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"> Examples: HIO2 < HBrO2 < HClO2<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"> H2TeO4 < H2SeO4 < H2SO4<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Notice for these examples there’s the same number of H and O atoms but the central atom is different.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Note also, the central atoms are from the same group. This is why they have the same structure.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">For these the only difference is the central atom. The central atom draws electron density away from the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">OH bonds. The more it can draw density away the more polar the OH bond and the easier it loses the H<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">atom as an H+, stronger acid. This depends on the EN of the central atom. The more electroneg. the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">central atom the more polar it makes the OH bond and the stronger the acid. Thus, the acid strength<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">inc. going UP the group, opposite the behavior of binary acids.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">2) Same central atom but a different # of O atoms.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"> Examples: HClO < HClO2 < HClO3 < HClO4<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">The only difference here is the # of O atoms on the central atom. The more O atoms you have<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">(technically O atoms w/o a H atom) the more electron density is pulled away from the OH groups on<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">the central atom. So more O atoms the stronger the acid.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">You can NOT compare binary acids to oxyacids based on structure.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">*************************<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">Then there’s carboxylic acids. You can’t compare them to the above based on structure.<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">These have an -COOH (-CO2H) group,<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"> OH<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 can be a H atom (formic or methanoic acid) or a carbon group. We often use acetic acid<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">(ethanoic acid), CH3-CO2H, as a weak acid example. Vinegar is about a 5% acetic acid solution.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">This formula is often written as HC2H3O2, with the acidic H written to the left even though it’s on<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">an O atom (like in H2SO4, HNO3, 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">The H atom of the OH bond is acidic in carboxylic acids (Ka values on the order of 10^-3 to about<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">10^-6 for many of them). The OH bond in an alcohol is not, like CH3CH2OH (Ka of about 10^-16).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">The reason the OH is acidic in carboxylic acids can be explained in two ways:<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"> 1) The C=O functional group (carbonyl group) is a very polar group and draws electron<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> density away from the OH bond making it more polar and making the H atom more<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> acidic (easier for the OH bond to break and the H to leave as an 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"> 2) The conj. base, X-CO2^- is resonance stabilized. Two resonance structures can be<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> drawn for the -CO2^- group. This spreads the neg charge out over the three atoms<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> in the -CO2^- group. That means the charge is not as readily available to react with<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> an H+. That means it’s a weaker base and thus it’s conj. acid is stronger. The<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> conj base of an alcohol, like CH3CH2O- for ethanol, can not spread out the neg. chg.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> The conj. bases of alcohols are strong bases (alcohols do not act as acids in water,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> the soln remains neutral).<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"> 3) Also, any electron withdrawing groups on a alpha carbon can inc. the acidity of the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> H atom of the OH bond in the CO2 group. The alpha carbon is the C atom attached<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> to the C=O carbon atom (carbonyl carbon atom). In acetic acid,CH3CO2H, the alpha<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> C is the CH3 carbon atom. The electron withdrawing groups on the alpha C atom<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> pull electron density away from the OH group making it a stronger acid. The more<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> EN the electron withdrawing group the stronger the acid. The more electron<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> withdrawing groups on the alpha carbon the stronger the 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">I would suggest if you’re still struggling with this you re-watch either the pre-recorded or Zoom<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">lecture.<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 with this.<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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