First intermolecular force is the force that exists between an ion and a polar compound,
so this is called ion-dipole.
Now, an example of ion-dipole is we could have NaCl, remember this is sodium chloride,
it’s ionic, and we throw it into water.
Now, ionic compounds are polar compounds.
And they’re highly polar because, remember, we’ve talked about this before, we say if
your difference in electronegativity is 0.4 or greater, you are going to be polar.
If it’s greater than 1.7 and higher, you’re going to be ionic.
So the difference in their electronegative numbers is very high, so it’s going to be
a polar type of compound.
All ionic compounds are polar.
And we’re going to learn that water, which is called the universal solvent, it’s called
universal because it’s highly polar, so it dissolves a lot of polar things.
So polar ionic compound, polar solvent, which is water, polar and polar will interact.
So the polar water is going to split this up into its ions.
Na is group 1A so it’s going to be Na+ aqueous plus Cl- aqueous . Remember Cl is in group
7A, so it’s -1.
Now, what does aqueous mean?
All aqueous means is that the water is actually wrapping itself around the ions.
This is the ion-dipole interaction.
Now if we wanted to see what it looks like, let’s say we took the sodium ion, now aqueous
again means that the water is surrounding it, but which parts of the water are surrounding
it?
We should realize that oxygen is very electronegative.
This compound here is polar if we follow the rules.
Because it’s polar, we are going to say that one end is partially positive; the other
end is partially negative.
Oxygen has a larger electron negative number so this end will be partially negative.
The hydrogens are less electronegative, so they are going to be partially positive.
Sodium is positively charged, and remember which side of the water is going to be attracted
with it?
Opposites attract.
So the negative portion of the water will be attracted to it, the partially negative
portions.
It’s not going to be one molecule surrounding the ion, it’s going to be multiple water
molecules surrounding it because we are in solution, it’s in a bucket of water.
There’s hundreds and hundreds of water molecules all around, so they all try to surround the
ion.
So it’s the interaction between the ion, the positive ion, and the partially negative
end of the water molecule.
This little dash green line means that it’s not a full bond, it’s just an interaction.
They are communicating with each other; they’re partially attracted to each other.
Intermolecular forces are not bonding forces.
Remember, we are going to say that intramolecular forces are bonding forces; intermolecular
forces are not bonding forces.
They are not bonding; they are just attracted to one another.
So these little green lines are just an attraction for the positive ion to the partially negative
oxygen.
And if we drew chlorine, we’d show that the partially positive ends, the hydrogens,
will be attracted to the negative chlorine.
That will be our ion-dipole interaction.
So here is an example of ion-dipole and we are going to say basically, fundamentally,
we have ion-dipole anytime we have an ionic compound dissolving in a polar liquid, like
water.
Water is polar based on the rules that we learned.