<p>I'm having a little trouble with some chemistry concepts, and I was wondering if you guys could clear me up :) </p>
<p>1) What is the exact requirements for a hydrogen bond? I know it is a hydrogen bonded to N,O, or F, but does there have to be a lone pair of electrons somewhere? Also, what is a hydrogen bond acceptor and donor</p>
<p>2) When there are dipole-dipole forces, are there also dispersion forces? Is this true for any other forces?</p>
<p>3) When you have molecules that all have similar bonds (dispersion, ion, dipole-dipole), do you then use the formula weight of the molecules to determine the strength of the bond, or do you use the electronegativity difference in the atoms in the molecules?</p>
<p>1) If an H atom is located near an N, O, or F atom then there is most likely hydrogen bonding. Note that the H is not actually bonded in the usual sense of a covalent or ionic bond.</p>
<p>2) There are always London dispersion forces.</p>
<p>3) You use electronegativity for dipole-dipole forces, and covalent and ionic bonds. I don't know if there's a way to tell how strong the dispersion forces are.</p>
<p>Remember to make the distinction between intermolecular forces and intramolecular forces. Dispersion, dipole-dipole, and hydrogen bonding are intermolecular forces while covalent and ionic bonds are intramolecular forces.</p>
<p>Weasel basically covered everything, but I think that he could make one point clearer: there are hydrogen bonds that exist intermolecularly and those which exist intramolecularly. Intramolecular hydrogen bonds have acceptors/ donors. I'm not too sure about this part, because I only read about it briefly, but I think that the hydrogen-bond donor is an electronegative atom that is bonded to H. This then bonds again with the acceptor, which is yet another electronegative atom:</p>
<p>D-H bonds with A:
D-H .... A (where .... is a hydrogen bond)</p>
<p>D and A are negative, while H is positive.</p>
<p>Like I said, we didn't really address this in my Chem class, so you might want to ask for a second opinion =)</p>
<p>Like dissolves like is the way to go. Try phrasing it like this for your teacher: "Because sodium chloride is ionic, it is polar. Water is also polar. As a result, the salt crystals will dissolve in water." Your teacher may feel that "like dissolves like" is too simplistic, so phrasing it as I did should let him/her know that you understand what is going on and are not just memorizing a phrase.</p>
<p>I second like dissolves like. You can say that the heat of hydration plus heat of solvent separation is negative or a very small positive number to be more technical. </p>
<p>Also, H bonds are just a H bonded to a very electronegative element usually N,O,F. The small size of H and unequal sharing results in extremely large dipole moments for nonsymmetric molecules containing the H bonds. Thus they are the strongest type of dipole bonds. </p>
<p>About your other ques. if there is no lone pair on the electroneg. atom, then the molecule would be symmetric, so no hydrogen bonding occurs since it would be symmetric, eliminating the dipole moment.</p>
<p>so the molecule has to be asymetric in order to undergo H bonding? </p>
<p>I thought for the solubility thing, that things aren't soluble (things with only dispersion forces) because it would take more energy to break the H bonds in H2O than the dispersion forces, so its not likely to be soluble... How does that relate to like disolves like</p>
<p>Isn't the fact that "The forces of attraction between the solute and solvent must be stronger than those between the solute and itself for a substance to be soluble." better than "like dissolves like" when determining solubility?</p>
<p>"I second like dissolves like. You can say that the heat of hydration plus heat of solvent separation is negative or a very small positive number to be more technical."</p>
<p>The two are basically stating the same idea. 'Like dissolves like' is just the incredibly simplified form of what you want to say.</p>
<ol>
<li><p>F, O, or N bonded to H creates H-bonding intermolecular forces. Nothing else really to know other than the bond must be polar covalent. So, (NH4)Cl has no H-bonding since it is ionic.</p></li>
<li><p>Always london dispersion forces.</p></li>
<li><p>Give an example please. Your question doesn't make much sense. Give me 2 molecules and I'll tell you which one has greater intermolecular forces and why. When 2 molecules only have london dispersion, then compare molecule length and mass. So a long, linear hydrocarbon (i.e. CH3CH2Ch2CH2Ch2Ch3) has high london dispersion forces for its high mass and and "long" length. By length I mean like a chain.</p></li>
</ol>
<p>"About your other ques. if there is no lone pair on the electroneg. atom, then the molecule would be symmetric, so no hydrogen bonding occurs since it would be symmetric, eliminating the dipole moment."</p>
<p>This is definitely false. Counter-example: NH3. It has 3 bonded pairs on the N and no unbonded pairs and has H-bonding. This is why ammonia is a liquid.</p>
<p>bah, my chemsitry teacher sucks! HE cannot teach well at all! No one knows about anything thats going to be on this stupid intermolecular forces test tomorrow...</p>
<p>Oh , my bad. I was thinking of N in my head as the 3A column. My bad. So what's an example(other than previously mentioned ionic) where H-bonding does not occur but the atom has N, O, or F bonded to H?</p>
<p>BTW, NH3 only forms NH4+ in water. I was referring to pure ammonia which is liquid.</p>