General Chemistry Intermolecular Forces & Liquids

Hi, welcome back to Educator.com.0000

Today's session in general chemistry is going to be concerning intermolecular forces and liquids.0002

As always we are going to start off with a brief introduction.0012

Then we are going to talk about the intermolecular forces first of polar compounds and finally of nonpolar compounds.0015

After we talk about the intermolecular forces in these classes of compounds, we will then0024

get into properties of liquids and how intermolecular forces affect these properties.0030

As always we will go ahead and do a brief summary of the presentation followed by some sample problems.0038

For the introduction, what we are looking at is basically the attractive forces that hold molecules together.0048

If you have a molecule and an identical molecule comes along right beside it,0056

what is responsible for those two molecules interacting with each other or attracting each other?0063

We call these forces of attraction intermolecular forces.0067

For my presentation, I am going to be abbreviating intermolecular forces as IMF.0073

What we are going to see is a good application of IMF.0080

We will see that the strength of a liquid's IMF can easily influence how it behaves physically.0085

Let's now get into the intermolecular forces for polar molecules.0092

The first type of IMF for polar molecules is what we call an ion-dipole force.0098

An ion-dipole force is going to occur between a polar molecule and an ionic compound.0104

For the example that I want to look at on the particulate level, this is going to be table salt dissolved in water.0111

Here I have water and table salt.0118

As soon as I dissolve table salt in water, the sodium chloride is going to break up into ions.0126

We have Na⁺ and Cl^1-.0131

We have already discussed polarity previously.0136

We know that water is partial positive here, partial positive here, and partial negative there.0139

Something interesting is going to happen.0146

The partial negative end of oxygen is going to get attracted to the full positive ion that is sodium.0148

In addition, the partial positive end of water is going to get attracted to the full ion that is chloride.0160

This is what we call an ion-dipole force.0171

It is basically the attraction between an actual ion that has a full charge and a compound that has a dipole moment.0175

Coulomb's law describes the force of attraction between ions and/or dipoles.0188

Basically Coulomb's law tells us that this attraction is going to0194

be proportional to the following: q₁q₂ over r²0200

where q₁ and q₂ are the relative charges of the cation and anion; charges of cation and anion.0210

r is just the distance between the two nuclei; ionic distance.0227

We see a very important proportionality.0237

That the force of attraction is going to be directly proportional to the magnitude of the charges.0242

It is going to be inversely proportional to 1 over r² or inversely proportional to the atomic size.0254

Once again this is what we call Coulomb's law.0264

It tells us, it really quantifies the degree of attraction between charges and/or dipoles.0267

There is one important physical parameter that describes what we just illustrated.0280

That is an ion interacting with a solvent.0285

This is what we call the enthalpy of solvation which is also known as the enthalpy of hydration.0290

Basically what it is is the following.0299

It is another type of ΔH like we learned before.0302

It is also a ΔH of solvation.0307

It describes the energy change when an ion becomes solvated.0311

In other words, when an ion becomes surrounded by solvent molecules.0323

What we want to know, remember mother nature favors low energy.0336

The more negative this energy, the more likely an ion will be dissolved or solvated by water.0339

An ion is more likely to be solvated by water the larger it is and the more positive the charge.0346

Let's go ahead and move on then.0358

Ion-dipole was the first type of IMF for a polar compound.0364

Let's now move on to the second type.0369

The second type of IMF that can occur in liquids is what we call dipole-dipole forces.0372

Dipole-dipole forces occur between molecules that have a permanent dipole moment.0379

Then the strength of the dipole-dipole force tends to increase with overall polarity.0386

For example, if we take a carbon-hydrogen bond versus an OH bond,0391

the OH bond is the more polar of the two and therefore will have a stronger IMF.0397

How does this all relate to boiling point?0411

Basically the stronger the IMF, the more energy required to overcome the attraction.0415

When we think of energy, we can think of boiling point.0421

Boiling point is the temperature at which the boiling process is going to occur.0425

Remember temperature is the measure of average kinetic energy.0428

Really when we say the more energy required to overcome the attraction, we really mean a higher boiling point.0432

This was again dipole-dipole forces.0442

The next type of intermolecular force is what we call hydrogen bonding.0445

Hydrogen bonding you can think of as a very strong type of dipole-dipole.0450

This is going to occur in a molecule that has hydrogen that is directly bonded to nitrogen, oxygen, or fluorine.0454

You will recognize these three elements being the most electronegative.0462

Therefore their bond with hydrogen will be the most polar.0467

Hydrogen bonding is important of course biologically because we are going to find out that liquids that0474

have hydrogen bonding tend to be number one, nonvolatile, and number two, have relatively higher boiling points.0480

You can think of water as our typical example.0487

Hydrogen here, partial positive, partial positive, partial negative.0493

We can have another water molecule come into play just right there.0497

We can have an interaction between the partial negative oxygen with the partial positive oxygen.0502

You can imagine another water molecule just like that; we can have even more attraction.0508

What we see is basically that we get a network of hydrogen bonding; network of strong hydrogen bonding.0520

Why is this biologically important?0537

We know water because of its strong hydrogen bonding is going to be nonvolatile.0539

Imagine if all the world's oceans all of a sudden evaporated.0546

Life would be unbearable; our planet would be too hot.0551

Because of hydrogen bonding, water does not evaporate.0554

That allows us to have a good cooling effect by the ocean.0557

Again that is also going to lead to a high boiling point.0563

Once again it is in our best interest to have water to be a very stable compound.0568

Again it is due to hydrogen bonding.0575

What we are going to go into now, now that we are done with polar molecules,0579

let's get into the intermolecular forces for nonpolar molecules.0583

We are first going to look at the solubility of a nonpolar gas in a polar solvent.0588

Wait a second, I thought polar and nonpolar are not supposed to mix.0596

But it turns out that for gases, there can be a slight degree of mixing.0599

The typical example is going to be carbon dioxide in water which is basically your average carbonated beverage like soda.0605

We all know that CO₂ is nonpolar.0614

Let's go ahead and look at its Lewis structure.0619

Each oxygen is partial negative; the carbon is partial positive.0623

What can happen is the following.0629

A water molecule can come into play right here just like that.0632

Partial positive, partial positive, and partial negative.0641

Even though carbon dioxide is nonpolar, at any one point in time, at any point in time,0646

an electron cloud can get slightly distorted; electron density can distort.0660

As soon as we get a distortion, that instant we form a temporary dipole moment.0672

It is that temporary dipole moment which can then interact with the polar water molecule.0685

Let's go ahead and take a look at the interaction between two nonpolar molecules.0698

The interaction between two nonpolar molecules is what we call a London dispersion force or a van der Waals force.0703

Another name for it is induced dipole attraction.0712

Let's go ahead and take a look at molecular fluorine as an example.0716

Here is our typical Lewis structure for a molecular fluorine.0722

Because it is homonuclear diatomic, it is nonpolar overall.0726

However, at any point in time, we can have distortion of the electron cloud once again; distortion of electron density.0730

You can imagine then that at any one point in time, if what I am drawing represents the F₂ molecule,0746

we can get one side to be partial positive and we can get one side to be partial negative.0754

This is not going to last forever.0760

It is a temporary dipole moment just like it was for carbon dioxide.0761

Basically if I have this temporary dipole moment, another F₂ molecule can come into play, can come right alongside it.0770

It too can also have a temporary dipole moment.0781

We can get an interaction between the partial positive end of one F₂0784

molecule with the partial negative end of the second F₂ molecule.0789

Because of the temporary nature though of this interaction, we can also0794

infer that London dispersion forces are going to be incredibly weak.0799

In fact, dispersion forces are the weakest type of intermolecular force due to temporary nature of attraction.0806

This brings into play a very important concept.0829

You are going to see this concept in this chapter.0832

You are going to see this concept in organic chemistry quite a bit actually when you get to that level.0835

To what extent does an electron cloud or electron density get distorted?0841

The extents to which this happen is known as polarizability.0848

Basically the more polarizable the molecule, the easier it is to0854

have an induced dipole moment and therefore the stronger this attractive force.0860

Let's go ahead and look at the molecular halogens as our example.0866

F₂, CL₂, Br₂, and I₂.0873

Under standard conditions, fluorine is a gas; chlorine is a gas.0880

Bromine is a liquid; I₂ is a solid.0885

Why is that though?--why are we going from gas to liquid to solid?0890

In other words, as we go down this column, why does the intermolecular forces increase?0894

Because that is what explains for going to a more condensed state from gas to liquid to solid.0906

Why does the IMF increase?--the answer is because of polarizability.0911

Iodine is the largest out of these molecules.0921

Because it is the largest, the electrons are not held as tightly.0927

Because the electrons are not held as tightly, we get easier distortion.0939

More likely for electron cloud distortion to occur.0945

Because it is more likely for the electron cloud distortion to occur, we can form stronger dispersion forces.0955

In other words, polarizability increases with molar mass.0976

Once again polarizability tends to increase with molar mass; that is it.0991

That pretty much explains why we change physical state as you go down the column of0997

the halogens from fluorine to iodine, from a gas all the way to a solid.1003

We finished our discussion on the attractive forces that can occur for liquids.1012

Let's go ahead and apply them now.1019

We are going to apply these to the physical properties of liquids.1021

There are several of them.1026

The first physical property that we are going to be talking about is what we call standard molar enthalpy of vaporization.1027

It is yet another energy; it is another type of enthalpy.1034

The definition of ΔH of vaporization is the energy required to vaporize one mole of a liquid under standard conditions.1038

But in order to vaporize a liquid, you have to overcome the intermolecular forces,1046

the IMF whether it be dipole-dipole, hydrogen bonding, or dispersion.1051

In other words, the stronger a liquid's IMF, the more energy required and the larger the ΔH of vaporization.1056

Let's go ahead and look at the trends and boiling points of representative liquids.1064

Here we will examine two inorganic liquids, water and hydrogen sulfide.1069

For water, let's look at the IMF; we have hydrogen bonding here.1075

For hydrogen sulfide, there is no hydrogen bonding; only dipole-dipole and van der Waals.1080

Because hydrogen bonding is going to be the strongest type of intermolecular force here,1089

we conclude that water has the stronger IMF and therefore the higher ΔH of vaporization.1094

That is how we apply intermolecular forces to solving these types of problems.1110

Let's go ahead and take a look at some representative organic liquids.1118

The first type of organic liquids I want to talk about are what we call aliphatic hydrocarbons.1122

Aliphatic hydrocarbons are basically straight-chain alkanes.1126

Here we have CH₃CH₂CH₃ and CH₃CH₂CH₂CH₃.1131

First of all, if you recall the electronegativities of carbon and hydrogen are very similar.1137

In fact, they are nonpolar bonds; both of these molecules are nonpolar.1142

Because both of these are nonpolar, the only type of intermolecular force are van der Waals in each of them.1149

What is the only difference then?--the only difference is chain length or molar mass.1156

We see here that this has the longer chain which means higher molar mass.1162

Just like we said previously, as the molar mass increases, the polarizability1171

also tends to increase, therefore the strength of the intermolecular force.1176

Therefore this molecule here, CH₃CH₂CH₂CH₃, has the stronger IMF1180

because of intermolecular forces... excuse me... of the higher molar mass and stronger IMF.1186

The next two, CH₃CH₂OH versus CH₃OCH₃.1197

In this left molecule, we have hydrogen bonding and dipole-dipole.1202

In the right molecule, there is no hydrogen bonding; only dipole-dipole and van der Waals.1210

We are going to obviously pick the one with the hydrogen bonding.1221

That is going to give us the stronger IMF and therefore the higher ΔH of vaporization.1224

These are again just examples of what we call aliphatic hydrocarbons, straight-chain alkanes.1247

We now move on to branched out hydrocarbons.1252

For branched hydrocarbons, it is important to go ahead and draw the Lewis structures out.1257

This one we did already, CH₃CH₂CH₂CH₃.1263

Nothing is going on here; this is aliphatic.1268

But when we move to its counterpart on the right side, we are1271

going to get a new type of structure that looks like this.1275

This is what we call a branched alkane; this is no longer aliphatic.1283

What we have to take away from this is the following.1290

That any type of branching is going to lower the surface area of your compound.1292

If the surface area is lower, that means there is less area of the molecule to be exposed to an adjacent molecule.1304

In other words, the IMF is going to be lower also.1314

This molecule here that is branched has the weaker IMF and therefore the lower ΔH of vaporization.1320

Next another physical property is what we call vapor pressure.1331

Vapor pressure is going to be commonly measured in our traditional units1337

for a gas pressure which is atm, torr, and even kilopascal.1341

What vapor pressure is is the following.1347

It is going to be the pressure of a vapor that is directly above the surface of its liquid in dynamic equilibrium.1349

If you take a small beaker and you cover the beaker and you close it, let's say you have a liquid in here.1358

That liquid is going to go into a gas phase naturally.1368

But not all of it because as soon as the gas forms, it is going to go back and condense back down to a liquid.1375

This is in constant motion and not a static condition.1381

This is what we call dynamic equilibrium; it is a dynamic process.1387

Basically whenever we have this type of closed system, if we measure the pressure of1393

the vapor right above the liquid, that is what we call vapor pressure.1399

Now that we have a little drawing to help support the idea of vapor pressure, we can now put it into play.1408

Vapor pressure is actually going to parallel what you and I know as smell.1415

Basically a liquid with a strong odor, we are detecting its gas phase with our nose.1421

A liquid with strong odor means it vaporizes easily.1427

Its vapor pressure is going to be relatively high.1430

If you compare gasoline versus water, gasoline we know smells very strongly.1433

Water has no odor; we can conclude that gasoline has a higher vapor pressure.1445

The technical term for this with a higher vapor pressure is what we call volatile; volatile.1456

We say water is therefore is nonvolatile.1463

We do have an equation that allows us to quantify the relationship between vapor pressure and temperature.1473

This is what we call the Clausius-Clapeyron equation.1484

The Clausius-Clapeyron equation is basically the natural log of P₂ over P₁ which is equal to...1488

ΔH of vaporization over R times 1 over T₁ minus 1 over T₂.1507

All this is saying is the following.1515

If T₂ is greater than T₁, that automatically implies that P₂ is greater than P₁.1519

In other words, vapor pressure increases with temperature; P increases with temperature.1527

Just think about that.1535

A gas station is going to smell a lot worse on a warmer day.1537

It is because the vapor pressure of any gasoline that is dropped is going to be much higher than on a cooler day.1541

That is again the Clausius-Clapeyron equation.1550

The next physical property of liquids is what we call normal boiling point.1553

Normal boiling, we all know what boiling point is.1557

It is the temperature at which a liquid is going to boil.1560

But normal boiling point is specifically the process that occurs at atmospheric pressure.1562

Since boiling requires energy, a high boiling point implies that lots of energy is needed to boil a liquid.1569

A high boiling point means very strong IMF which also means1577

higher ΔH of vaporization which also means lower vapor pressure.1588

Let me go ahead and summarize that.1601

Strong IMF, high boiling point, high ΔH of vaporization, lower vapor pressure.1603

Simply knowing the intermolecular force, we can take an educated guess at a lot of the physical properties of a liquid.1618

Now onto the last physical properties we will cover.1630

This one is what we call surface tension.1634

Surface tension is something measurable.1636

It basically quantifies the tendency of a liquid to minimize its surface area.1639

To do this, it is going to form spherical drops.1644

A liquid with a relatively high surface tension easily forms spherical droplets1649

such as water which has very strong IMF due to hydrogen bonding.1655

Liquids with low surface tension, they tend not to form spherical droplets; nonspherical droplets.1663

Finally we reached the last physical parameter of liquids.1686

It is what we call viscosity; viscosity is also measurable.1693

It basically quantifies a liquid's resistance to flow.1698

What that really translates to is how thick a liquid is.1702

If you think of something like honey or like syrup, we say those are very viscous liquids.1706

They are very resistant to flow.1712

Basically if an IMF is very large, the viscosity tends to also be large.1714

That makes sense that the intermolecular force of attraction is large.1720

Liquid is not going to want to separate from each other.1724

It is going to want to stay intact and stay together.1726

In other words, it is going to be somewhat thick if you will or not as flow-like as water or something like that.1728

Again this is what we call viscosity.1741

We now reach our summary slide.1746

Basically in this chapter, we introduced the types of intermolecular forces that are relevant to liquids.1750

Number one is what we call ion-induced dipole; number two is what we call dispersion.1757

Number three is what we call dipole-dipole; number four is what we call hydrogen bonding.1762

Remember ion-induced dipole is going to be between a free ion and a polar molecule.1768

Dispersion is going to occur between nonpolar compounds.1780

Dipole-dipole and hydrogen bonding are going to be for polar compounds.1784

After we went over all of the different types of intermolecular forces, we then applied them1791

and used them to make educated guesses on how a molecule is going to behave physically.1796

We saw that the IMF can exert strong influence on a liquid's physical properties such as boiling point and vapor pressure.1803

In other words, by using IMFs, we can make a strong educated guess on the relative volatility of a liquid.1813

For sample problem one, let's go ahead and determine which of the following liquids will have the lower vapor pressure.1823

As soon as I see this molecule here which is OH, I see hydrogen bonding immediately.1830

That tells me this molecule is going to have relatively strong intermolecular forces.1836

Here CH₃OCH₃, that is going to look a lot like this.1841

I don't have any hydrogen bonding at all.1847

But I do have dipole-dipole and of course van der Waals or dispersion.1849

For here, CH₃CH₂CH₃, there is none of that.1858

There is no dipole-dipole; there is no hydrogen bonding.1864

This molecule is completely nonpolar.1867

I am only left with van der Waals or dispersion.1871

What low vapor pressure means is that we are going to have strong intermolecular forces.1878

Therefore the molecule that is expected to have the lower vapor pressure is going to have the strongest intermolecular forces.1886

That is the CH₃CH₂OH right there.1893

Finally sample problem two, we are going to take the same compounds.1899

But now we are going to determine which of the following will have the largest ΔH of vaporization.1903

As soon as I see largest ΔH of vaporization, that means a lot of1912

energy is required to vaporize one mole of a liquid under standard conditions.1916

That tells me I am dealing with a liquid with relatively strong IMF.1920

Using the same rationale, it is going to be the CH₃CH₂OH again expected1927

to have the largest ΔH of vaporization because this has hydrogen bonding.1931

This one in the middle has only really dipole-dipole and dispersion.1938

This one here has only dispersion.1946

What I encourage you to do is when you tackle these types of problems,1951

you should always try to from the formula get the structure, the Lewis structure that is,1955

because that will really help you see any of types of bonds.1963

Step two is then go ahead and identify the types of IMF present.1966

Hopefully that is enough to distinguish the liquids from each other.1979

But if IMF identical, then what you need to go by is really1984

the size of the molecule or its molar mass; go by molar mass.1993

Again what was the word that describes the effect of molar mass and size on distortion of electron cloud?1999

This is all related to polarizability; good.2006

That was our general chemistry lecture on intermolecular forces and liquids.2015

I want to thank you guys for your attention.2021

I will see you next time on Educator.com.2024