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Magnitude of Intermolecular forces!


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Ok i got a question(again)!


What are the forces that hold the molecule together to form a liquid?

Look up examples of the magnitude of these forces and give value in kj/molecule!


Thats the part i am stuck in, how can i possibly know the magnitude or hydrogen bonds, london forces etc.


Can anyone help!

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it's all about what you search for exactly. Remember the actual search phrase doesnt have to make any sense. Just put in anything you want to see "kJ" was the clue for this one, since many websites have qualitative mention of the relative strengths but not many had an actual value in kJ

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There are a couple different "types" of intermolecular forces which have very specific effects on the general 'nature' of a substance, particularly how that substances "phase" (solid, liquid, gas) is effected by temperature changes.

The strongest intermolecular force is the "ion". An ion occurs when electrons are transferred from one atom to another. This typically occurs between an atom which is a strong electron donator and an atom which is higly electronegative. What does this mean exactly?

Take, for example, two atoms on opposite sides of the period table: Lets take NA (sodium) and CL (Chlorine). NA has 11 protons in its nucleus (core), and each of these protons is exerting a positive force which keeps the negatively charged electrons "trapped" in orbits around its nucleus. These 11 electrons, which all carry identical negative charges, are simultaneously drawn toward the positively charged core and repulsed by the other negatively charged electrons. This results in electrons circling the nucleus in a manner which minimizes electron-electron repulsion: the electrons occupy different "layers" and "shells". Think about this: since the protons are exerting a positive pull outward, any electrons which occur between the nucleus and the outer-shell electrons will "absorb" some of the positive pull so to speak, shielding the valence (outer-shell) electrons from the magnetism. Na happens to only have a single electron in its outermost shell, so this electron is weakly held. Therefore, it is easy to give up, and if it is given up, the sodium ion reaches a very stable Na+ form. This is known as "ionizing".

Now look at chlorine. Chlorine is in the same "row" as sodium, therefore it has the same outer layer. However, chlorine has 17 protons, therefore the amount of magnetism exerted from the core on to the electrons is much greater. Since the bulk of its electrons all occur in the outer shell, they don't serve to "shield" eachother from the inner pull. Furthermore, if chlorine gains a single electron, its outer shell achieves "noble gas configuration" - a highly stable state where all of the valence shells are filled. (note that sodium can reach this same configuration by giving up an electron). That is why, when these atoms encounter one another, they "ionize" - sodium gives up an electron to chlorine, forming Na+ Cl- ---> table salt!! This intermolecular force is the most powerful, and this is why salt melts at 900 degrees celsius (or something like that).


The next signifigant intermolecular force is known as "hydrogen bonding". This occurs when you have covalent bonds between hydrogen and sulfur, oxygen, or nitrogen. "Covalent" is not the same as ionizing. As I described above, ionizing involves the exchange of electrons. Covalent, rather, involves the sharing of electrons. Covalency still obeys the same rules, however - atoms will share bonds in such a way as to achieve the "noble gas configuration". Hydrogen bonding is strong but not nearly as strong as ionizing. H-bonds account for waters high boiling point.


Next down the ladder is "dipole". Dipole simply means "Di"=2 "Pole"=poles of a magnet. This is used to describe any atomic bond, outside of hydrogen bonds, which exhibits polarity. Lets say, for example, you have a carbon atom, which is only moderatly electonegative. (Note: electronegativity relates to the magnitude of the positive nuclear charge, electron shielding effect, and relative atom size, as described earlier). If the carbon atom bonds with a fluorine atom, which is higly electronegative, you would describe the covalent bond as a "dipole". These are less strong then hydrogen bonds.


Finally, the weakest force is known as london dispersion forces. These are present in all molecules to some extent, but are the only force operating in things which don't have H-bonds, ions, or dipoles. London dispersion forces describe the natural tendency for electrons to create a net positive or negative charged based on their cycling repulsion. Simply stated- because electrons repulse eachother, at any given time within a molecule a "chain reaction" can occur where "A" repulses "b" to the right, which causes "B" to repulse "C" to the right, which causes "C" to repulse "D" to the right, and so on until you have a negative charge at one end and a positive charge at the other. These forces are very weak and fluctuate.


Finally, how do these forces effect phases and energy? Well, imagine I have a cup of water. All of the H20 molecules are H-bonding within themselves and with eachother. These electromagnetic forces keep them together. Also, the compacted air molecules of our Earth atmosphere are constantly pushing "down" on the water molecules, with a magnitude of pressure sufficient to keep them trapped in the liquid phase.


If I start to heat the molecules up, I am in fact supplying them with "energy", and the atoms start the vibrate and move around. If I continue to heat them up to a "boil", that means that the motion energy of the molecules pushing up EQUALS the magnitude of the atmospheric pressure pushing down. Vapor pressure = atmospheric pressure, bubbles form, and the gaseous H20 escapes into the atmosphere.


As far as energy, the basic rule is this: You have to put energy into a system in order to break strong bonds (ie. H-bonds) and replace those with weaker bonds (ie. dipoles). This is known as an "endothermic" reaction.

If you have weaker bonds creating stronger bonds, that would be an exothermic reaction, and heat is "lost" to the environment.


anyways hope that was helpful.

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ecap, thanks for your enormous post.


I would like to point out, however, that the question was already answered, and that your post didn't contain any of the information requested by the original post :0)


Exactly what i want to say. It is not as if I am not grateful to him for writing such a lengthy post but I know these stuff already. the only thing i wanted to ask was the magnitude/strength of the forces and I got the help I wanted.


But lets give the person some credit for trying to help out.

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