Name: Hydrogen bonding | Intermolecular forces and properties | AP Chemistry | Khan Academy
Uploaded: 2020-03-27T17:32:40.000Z
Duration: 6 min 39 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

- [Instructor] Let's talk about hydrogen bonds.

Depicted here, I have three different types of molecules.

Each ammonia molecule has one nitrogen bonded

I have something you're probably very familiar with,

in fact, you're made up of it, which is water.

And then here on the right, I have hydrogen fluoride.

Now, why are these types of molecules interesting?

And what does that have to do with hydrogen bonds?

And the simple answer is, in each of these cases,

they're going to be polar covalent bonds.

You are going to have a bond dipole moment that goes

from the hydrogen to the more electronegative atom,

from the hydrogen to the more electronegative atom.

The more electronegative atom is going to hog the electrons.

The electrons are gonna spend more time around that.

those are gonna have partial positive charges.

you would have a net dipole for the entire molecule

And the polarity comes from both the asymmetry,

and you have a very electronegative atom bonded to hydrogen,

oxygen, very electronegative atom, bonded to hydrogen.

So this end of the molecule is partially negative.

or these ends of the molecule are partially positive.

For hydrogen fluoride, this end is partially positive.

when these molecules interact with each other?

The nitrogen end right over here, of this ammonia,

could be attracted to one of these hydrogens

that has a partially positive charge right over there.

Or this hydrogen, the partial positive charge,

between the partial positive hydrogen end

and the partially negative end of another molecule,

And they are an intermolecular force that will be additive

from, say, things like London dispersion forces,

which makes you have a higher boiling point

And to make that clear, you can look at this chart.

You can see all of these molecules are formed

between period two elements and hydrogen.

In fact, all of these molecules have similar molar masses,

methane, ammonia, hydrogen fluoride, and water.

If we were just thinking about London dispersion forces,

London dispersion forces are proportional

which is proportional to the electron cloud size,

And generally speaking, as you go from molecules formed

with period two elements to period three elements

to period four elements to period five elements,

there is that general upward trend of the boiling point,

and that's due to the London dispersion forces.

But for any given period, you do see the separation.

you see a lot of separation for the molecules formed

These molecules, despite having similar molar masses,

So there must be some other type of intermolecular forces

at play above and beyond London dispersion forces.

What you have at play are the hydrogen bonds.

they also don't have the same boiling point,

And the separation that you see here in boiling points,

In particular, dipole-dipole forces would be at play.

But what you can see is the spread is much higher

for these molecules formed with nitrogen and hydrogen,

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Hydrogen bonding | Intermolecular forces and properties | AP Chemistry | Khan Academy

specific

entire

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