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<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">I got the following question from someone about the attractive forces (AF) e-mail I sent a couple of<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">days ago.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">"How can we tell what attractive forces are stronger in two different molecules if one has hydrogen<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">bonding but the other is bigger? In other words, which aspect, size or hydrogen bonding, should we<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">consider first when determining which molecule has the stronger intermolecular force"<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">Here's my answer:<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">There's no hard and fast rule for this (sorry). For instance, lets look at the following molecules<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"> H2O C8H18 CCl4<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">MW 18 114 154<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">b.p. 100 125 77 (all in degC)<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">If you look at the Clausius-Clapeyron Eqn. in the book and my notes and the associated graph, H2O<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">has a bigger slope than CCl4 and thus a higher H_vap (proportional to slope). It has a higher b.p. This<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">means it has stronger attractive forces (AF) than CCl4, even though CCl4 is much larger. The LF for<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">CCl4 are much larger than the LF between H2O molecules. However, H2O is polar and can form HB<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">between H2O molecules as well. Water's higher b.p. is mainly due to the HB between H2O molecules.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">What about octane (C8H18, more specifically, n-octane the straight-chain isomer)? It's nonpolar and<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">has only LF, like CCl4. It's smaller in size (MW) than CCl4. Yet, it has a higher b.p., indicating stronger<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">AF (LF) between the molecules in the liquid state. Why? CCl4 has a rather spherical charge distribution<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">while C8H18 is long and cylindrical. That means C8H18 has more points of contact (more surface<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">contact) with another C8H18. This maximizes the LF. The CCl4 molecules have less surface contact and<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">can't maximize the LF which it would have based on its size if it wasn't spherical.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">In fact the C8H18 is big enough with lots of points of contact that it's AF are stronger than the AF<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">between the H2O molecules, even though water has DD and HB in addition to it's small LF. n-Heptane<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">(C7H16) which is only slightly smaller than octane has a b.p. of 98 degC, slightly below that of H2O.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">There in lies the problem. There's no hard and fast rule for how much larger a molecule has to be than<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">H2O to have stronger AF than H2O. Remember, as discussed above, shape also plays a rule. Even so,<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">water has a remarkably high b.p. relative to its size due to HB. Nonpolar molecules have to be<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">"significantly" larger (The MW of 118 for octane is 6.3 times bigger than 18 for H2O).<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">Propanol (CH3CH2CH2-OH) and butanol (CH3CH2CH2-OH) have LF, DD and HB, like H2O. Propanol has<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">a b.p. of 97 degC and butanol has a b.p. of 118 degC. Propanol, butanol and H2O all have LF, DD and HB.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">H2O is more polar and can form more HB per molecule. This fact is enough to outweigh the extra size and<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">larger LF of C3H7-OH (which can form only up to 3 HB per molecule). However, adding another CH2 group<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">to get butanol (C4H9-OH) makes it big enough so its overall AF consisting of DD (weaker than H2O) and HB<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">(fewer and weaker than H2O) and larger LF are enough to outweigh the stronger and more frequent HB in<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">H2O. Butanol's molar mass is about 4 times larger than that of water.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">Furthermore, keep in mind, the strength of a single HB between two molecules increases in the following<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">order, N < O < F. This is because F is the most electronegative of the three atoms and a single HB between<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">two HF molecules is stronger than a single HB between two H2O molecules. Water has a higher b.p. than<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">HF because water can form more HB per molecule (up to 4 for H2O compared to up to 2 for HF). More HB<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">AF have to be broken to separate the H2O molecules than to separate HF molecules so more energy has to<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">be added to separate (boil) the H2O molecules. So the number of HB a molecule can form plays a role.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">NH3 is similar to HF in that it forms only up to 2 HB per molecule (HF has 3 lone pair electrons and 1 H and<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">NH3 has 1 lone pair electrons and 3 H atoms, H2O has a balance of 2 H atoms and 2 lone pairs).<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">Generally, when we give questions about AF and how they affect properties of different substances we try<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">to give molecules which are relatively close in size or very different in size. Certainly if I gave you H2O and<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">C20H42 you would expect C20H42 to have much stronger AF than H2O even though C20H42 has only LF.<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">It's really large. A molecule that large is most likely to be a solid at room temp (in fact, this is icosane or<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">eicosane and the m.p. of its straight-chain isomer is ~ 37 degC and it's b.p. is 343 degC).<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">I hope this answers the question. Perhaps not as satisfying as it would be if there was a hard and fast cutoff<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">but that's the way it is sometimes when you're speaking in generalities. We have to often speak generally<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">because there's a whole lot of different substances and sometimes there's exceptions to these general<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">statements. The general statements help and when we find those interesting exceptions we try to explain<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">them. Like the fact ice (solid H2O) floats on liquid water. That's very unusual. It's due to the fact the water<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">molecules actually move further apart as they form the many HB between the H2O molecules (each H2O in<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">the interior of ice has 4 HB between it and 4 other H2O molecules).<o:p></o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span style="font-family:"Arial",sans-serif">Dr. Zellmer<o:p></o:p></span></p>
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