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AP Chemistry 3.1 Structure and Arrangement of Atoms. Which substance exhibits the weakest intermolecular forces and possibly the lowest boiling point?

Language:
English Language

Transcript

00:04

And here's your Shmoop du jour, brought to you by the force. [Girl using the force at a birthday party]

00:07

Nifty party trick, but not a good explanation tactic on exams…

00:13

Here's our question….

00:14

The boiling point for a covalent compound can be predicted by understanding intermolecular

00:19

forces. Which substance below exhibits the weakest intermolecular forces and possibly

00:25

the lowest boiling point?

00:28

Alright. Well, the question writer threw us a bone by telling us that the substance with [Men fighting over a bone]

00:36

the weakest intermolecular forces will have the lowest boiling point.

00:41

So all we have to do is figure out which substance has the weakest forces between its molecules.

00:47

How do we do this?

00:48

Use the forceeee…

00:54

All intermolecular forces are ultimately just an attraction between a positive charge and [Positive and negative charge connected with love heart]

01:00

a negative charge, but the type of molecule will dictate how much charge is involved.

01:06

The more charge, the more attraction.

01:08

There are essentially four strengths of charge interactions that we can rank.

01:12

The strongest is ionic bonding, because it involves attractions between ions that have

01:18

at least one full positive or negative charge. [Ions attract each other from sodium and chlorine]

01:21

Another type of fairly strong interaction is a dipole interaction. A dipole interaction

01:27

occurs when a molecule has a bond dipole between a very electronegative atom, and a less electronegative

01:33

atom.

01:34

This strong dipole creates a partial positive charge on the less electronegative atom and

01:38

a partial negative charge on the more electronegative atom.

01:41

The partial negative charges are attracted to the partial positive charges, making the [Example of normal dipole interaction]

01:46

molecules stick together, but not as tightly as if they were full charges.

01:51

So…maybe not the molecules you want on your side in a rumble… [Ionic bonds fighting a dipole]

01:56

Any dipole in a molecule will do this, but hydrogen bonding is a special case in which

02:01

these interactions are particularly strong. [Hydrogen bonds in a cup of water]

02:03

A hydrogen bond is a dipole interaction that occurs in a molecule that has hydrogen atoms

02:08

bonded to a very electronegative atom, typically oxygen or nitrogen like the example shown

02:14

here.

02:15

Here’s what’s on our list so far: There’s one last force to consider. Electrons

02:24

in a molecule can move around and shift their charge density. This can cause more of the [Electrons moving around in a molecule]

02:28

negative charges of electrons in a molecule to temporarily end up on one side of the molecule,

02:34

leaving that side slightly negatively charged and the other side slightly positively charged

02:40

for a brief instant in time. The negatively charged side of one molecule can attract the

02:45

positively charged side of another, and the bigger the molecule, the greater the area

02:49

over which this attraction occurs and the stronger it is. [Lots of molecules with negatively charged electrons]

02:53

These attractive forces are very weak for one important reason:

02:56

The unbalanced charge lasts less time than it takes to jump to hyperspace. [Spacecraft jumps through hyperspace]

03:02

So here's our final lineup of forces:

03:11

Now we need to figure out what forces are holding together the molecules in the question..

03:15

We can get through this at light speed. [Spaceship appears]

03:16

H2O and BF3

03:19

both have dipoles, but H2O can make hydrogen bond, while BF3 sadly cannot.

03:25

That means H2O has the strongest intermolecular forces, followed closely by BF3. [H2O rocket wins race ahead of BF3]

03:31

Both CO2 and SeO2 only have London Dispersion Forces, but CO2 is smaller than SeO2, so that's that... Carbon

03:41

is higher on the periodic table, therefore smaller than selenium, so the forces holding [Carbon and selenium circled on periodic table]

03:45

it together are weaker.

03:46

So to make a very long story short, the answer to the question is C. Small, but proud. [Darth Vader points to answer C]

03:52

Much like a wise little green dude we know…

03:54

The right answer, C is…used the force, we did… [Yoda makes Darth Vader disappear]

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