- [Instructor] Let's talk a little bit
about ion-dipole forces.
And before we think about how ions
and dipoles might interact,
let's just remind ourselves
what the difference is between ions and dipoles.
And I encourage you to pause this video
and try to refresh your own memory
before we refresh our memories together.
All right, now let's first think about ions.
Ions are atoms or molecules that have a net charge.
So for example, when chlorine gains an electron
and becomes the chloride ion,
it's an ion because it now has a net negative charge.
Similarly, when sodium loses an electron,
it now has a net positive charge.
So this is the sodium ion.
Now what's the difference between that and a dipole?
Well, generally speaking when we're talking about dipoles,
we're not talking about something
that has necessarily a net charge,
we're talking about something where the charge is separated
on different ends of the molecule,
that you have a partially positive end
and you have a partial negative end,
that there is a molecular dipole moment.
And a good example of a molecule that is a dipole
or has a dipole moment at a molecular level is water.
Water is a very polar molecule.
We've talked about this many times.
You have your oxygen which is quite electronegative,
covalently bonded to two hydrogens,
and those are really polar covalent bonds
because the oxygen's so much more electronegative
that it hogs the electrons, it's selfish of the electrons.
And since the electrons spend more time around the oxygen
than around the hydrogen,
you have a partial negative charge
at this end of the molecule
and you have partial positive charges
at the other end of the molecules.
And we describe this
when we talked about hydrogen bonding
where the partial negative end of one water molecule
would be attracted to the partial positive end
of another water molecule.
But, as we've talked about, hydrogen bonds,
which are an intermolecular force
are just a special case of dipole forces.
Things that are able to form hydrogen bonds
just have a very strong dipole moment,
because you have hydrogen bonded to an oxygen,
a nitrogen, and a fluorine, that is quite electronegative.
So now that we know the difference between ions and dipoles,
how might they interact?
Well you might guess Coulomb forces are at play.
The partial negative end of a dipole would be attracted
to a positively charged ion.
And I have prearranged these water molecules
so that you have the partial negative end
is facing towards this sodium positive ion.
And so what I'm drawing right over here,
these are ion-dipole forces.
Similarly, if you have a chloride anion, or a negative ion,
well then the partially positive ends of the dipoles
are going to be attracted,
and so water might arrange itself in this way
where the partial positive ends, the ends with the hydrogen,
are facing the chlorine.
And this is one of the reasons
why it's so easy to dissolve sodium chloride,
to dissolve table salt in water.
Those ions are able to separate
and be attracted to the water molecules which are polar,
which have molecular dipoles.
Now, if I were to ask you what's gonna dictate the strength
of the ion-dipole forces, think about that.
Pause this video, and what do you think is going to matter?
Well, as you can imagine, these are Coulomb forces.
So the strength of the charges matter.
So you're gonna have a stronger ion-dipole force
if you have stronger charges on the ions.
So instead of a sodium with a positive one charge,
if you had a calcium ion that had a positive two charge,
then the partially negative ends of the water molecules
would be even more strongly attracted.
You would have stronger ion-dipole forces.
Similarly if you have stronger dipole moments,
that will also make the ion-dipole forces stronger,
or vice versa.
If you had a molecule that had a weaker dipole moment,
you're not going to have as strong ion-dipole forces.
Coulomb forces are inversely proportional
to the distance between the charges.
So you're also going to have stronger ion-dipole forces
the closer that these things get to each other.
But to some degree that's true of a lot
of the intermolecular forces we've talked about,
because on some level they are all Coulomb forces.