6.5 Intermolecular Forces

Summary

This video covers intermolecular forces, explaining how molecules interact through forces like dipole-dipole interactions, hydrogen bonding, and London dispersion forces. The strength of these forces affects properties like boiling points, with stronger forces leading to higher boiling points. Dipole-dipole forces occur between polarized molecules, while hydrogen bonds form between highly electronegative elements like oxygen, fluorine, or nitrogen and hydrogen. London dispersion forces, though weak and temporary, affect all atoms due to the random motion of electrons. The video excludes molecular geometry to focus on these key interactions.

Takeaways

• 🔬 Intermolecular forces refer to interactions between molecules, not the bonds within a single molecule.
• 🔥 Boiling points help measure the strength of intermolecular forces, with higher boiling points indicating stronger forces.
• 🏗️ Metallic compounds have the highest boiling points, followed by ionic compounds, and then molecular compounds.
• ⚡ Dipole-dipole forces occur between polarized molecules (dipoles), which have equal but opposite charges on either end.
• 🔁 Dipole-dipole forces are short-range and work only between adjacent molecules.
• 💧 Water is a strong dipole with a very negative oxygen atom and positive hydrogen atoms, while carbon dioxide is nonpolar due to balanced dipoles.
• 💥 Strong dipoles can induce slight polarity in nonpolar molecules, such as oxygen becoming soluble in water.
• 🌡️ Hydrogen bonds occur when hydrogen is attached to highly electronegative elements (fluorine, oxygen, nitrogen), leading to very polar substances.
• 🌀 London dispersion forces are weak, temporary forces that occur due to random electron movement in atoms.
• 🌬️ Noble gases like helium and neon rely on weak London dispersion forces, resulting in very low boiling points.

Q & A

What are intermolecular forces?

Intermolecular forces measure interactions between molecules, not within the bonds of a molecule, but how one molecule affects another. They are responsible for holding molecules together in the liquid or solid state.

How do chemists measure the strength of intermolecular forces?

Chemists measure the strength of intermolecular forces by boiling a substance. The amount of energy needed to convert a liquid into a gas indicates how strongly the molecules are attached to one another.

What is the relationship between boiling points and intermolecular forces?

A higher boiling point indicates stronger intermolecular forces because more energy (in the form of heat) is needed to separate the molecules.

Which substances tend to have the highest boiling points?

Metallic compounds tend to have the highest boiling points, followed by ionic compounds, and then molecular compounds.

What are dipole-dipole forces?

Dipole-dipole forces occur between polarized molecules, called dipoles, which have equal but opposite charges separated by a short distance. These forces are short-range and work between adjacent molecules.

Why does iodine chloride have a higher boiling point than diatomic bromine?

Iodine chloride has a higher boiling point because it is a polar compound with dipole-dipole forces, while diatomic bromine is nonpolar and lacks these forces, resulting in a lower boiling point.

How does the structure of water contribute to its polarity?

Water is polar because the oxygen atom, being more electronegative, pulls the electrons away from the hydrogen atoms, creating a dipole with a negative charge on the oxygen side and positive charges on the hydrogen sides.

Why is carbon dioxide nonpolar despite having polar bonds?

Although the bonds between carbon and oxygen in CO2 are polar, the dipoles are oriented in opposite directions and cancel each other out, resulting in a nonpolar molecule.

What are hydrogen bonds, and why are they strong?

Hydrogen bonds occur when hydrogen is attached to highly electronegative elements like fluorine, oxygen, or nitrogen. The hydrogen essentially loses its electron, leaving a lone proton that is strongly attracted to the negative side of nearby dipoles, making the bond particularly strong.

What are London dispersion forces, and why are they weak?

London dispersion forces are weak intermolecular forces that arise from the random movement of electrons, creating temporary dipoles. These forces are short-lived and weak, which is why substances relying on them, like noble gases, have low boiling points.

Outlines

00:00

📘 Introduction to Intermolecular Forces

This paragraph introduces intermolecular forces, which describe the interactions between molecules, not the bonds within them. It explains that the strength of these forces can be measured by boiling the substance, with higher boiling points indicating stronger forces. Metallic compounds usually have the highest boiling points, followed by ionic compounds, and finally molecular compounds. The section sets up the discussion on how intermolecular forces affect molecular behavior.

05:01

🔄 Dipole-Dipole Forces Explained

Here, dipole-dipole forces are explained as interactions between polarized molecules, where opposite charges on the molecules attract each other. Dipoles, like iodine chloride (ICl), have positive and negative ends that align with opposite charges on nearby molecules, resulting in stronger intermolecular forces. The paragraph contrasts iodine chloride, a polar compound with a higher boiling point, with bromine, a non-polar compound with a much lower boiling point.

10:02

💧 Water’s Polarity and Dipole Interactions

This section discusses the additive nature of dipoles in molecules like water. Oxygen's electronegativity causes electrons to be drawn away from hydrogen atoms, resulting in a strong polarity. Carbon dioxide, by contrast, is non-polar because its dipoles cancel out. The paragraph also introduces the concept of induced dipoles, where non-polar molecules like oxygen can become weakly polar when near polar molecules like water, allowing them to dissolve.

🌬 Hydrogen Bonds: A Special Case

Hydrogen bonds, a strong form of dipole interaction, occur when hydrogen is bonded to highly electronegative elements like fluorine, oxygen, or nitrogen. This leaves the hydrogen almost proton-like, with its lone positive charge attracting the negative ends of nearby molecules. The paragraph describes how hydrogen bonding is represented and emphasizes its role in creating highly polar substances with strong intermolecular forces, such as water.

🌫 London Dispersion Forces: Weak but Pervasive

The final paragraph covers London dispersion forces, weak forces present in all atoms, caused by random electron movement that creates temporary dipoles. These forces are fleeting and result in weak attractions between atoms. As a result, substances that rely solely on London dispersion forces, like noble gases (helium, neon, argon), have very low boiling points since their intermolecular attractions are minimal.

Keywords

💡Intermolecular Forces

Intermolecular forces refer to the interactions between molecules, not the bonds within a single molecule. These forces are essential for understanding how molecules attract or repel each other, and they are often measured by examining the boiling points of substances. The video emphasizes how the strength of intermolecular forces determines how much energy is required to separate molecules from one another.

💡Boiling Point

Boiling point is the temperature at which a liquid turns into a gas, and it serves as an indicator of the strength of intermolecular forces. A higher boiling point suggests stronger intermolecular forces. The video explains that substances like metals and ionic compounds have higher boiling points due to their strong molecular attractions compared to molecular compounds.

💡Dipole-Dipole Forces

Dipole-dipole forces are the forces between polarized molecules where one end is slightly positive, and the other end is slightly negative. These forces result from the interaction of molecules with permanent dipoles. The video illustrates this with the example of iodine chloride, where the molecule's polarity leads to a higher boiling point than non-polar molecules like diatomic bromine.

💡Dipole

A dipole is a molecule with two regions of equal but opposite charge, separated by a small distance. The video uses iodine chloride as an example of a dipole, where the iodine atom is slightly positive, and the chlorine atom is slightly negative. Dipoles are important for understanding how certain molecules orient themselves and attract neighboring molecules in solutions or solids.

💡Electronegativity

Electronegativity refers to an atom's ability to attract and hold onto electrons in a chemical bond. In the video, electronegativity helps explain why molecules like iodine chloride and water have dipoles, as the more electronegative atom (chlorine in iodine chloride and oxygen in water) pulls electrons toward itself, creating polarity.

💡Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative elements such as fluorine, oxygen, or nitrogen. In the video, this concept is discussed in relation to the boiling points of compounds, explaining why substances with hydrogen bonds (like water) tend to have higher boiling points compared to those without.

💡London Dispersion Forces

London dispersion forces are weak, temporary attractions between molecules caused by random movements of electrons. These forces occur in all molecules, even non-polar ones. The video explains that these forces are responsible for the low boiling points of noble gases like helium and neon, as their weak interactions require little energy to break.

💡Polarity

Polarity refers to the uneven distribution of electron density in a molecule, creating a dipole with a positive and a negative end. The video describes how polarity affects molecular interactions and boiling points, giving examples of polar molecules like water and iodine chloride and non-polar molecules like carbon dioxide.

💡Non-Polar Molecules

Non-polar molecules do not have a dipole moment, as their electron density is distributed evenly. In the video, carbon dioxide is given as an example of a non-polar molecule because the dipoles cancel each other out. Non-polar molecules typically have lower boiling points since they lack strong intermolecular forces like dipole-dipole attractions.

💡Induced Dipole

An induced dipole occurs when a polar molecule, like water, causes a temporary shift in the electron cloud of a non-polar molecule, such as oxygen, making it slightly polar. This concept is explained in the video to show how non-polar molecules can become soluble in polar solvents, even though they do not have permanent dipoles.

Highlights

Intermolecular forces measure the interactions between molecules rather than within bonds of molecules.

Boiling points are used to measure the strength of intermolecular forces, as higher boiling points indicate stronger forces between molecules.

Metallic and ionic compounds generally have higher boiling points compared to molecular compounds due to stronger intermolecular forces.

Dipole-dipole forces occur between polarized molecules, where one end of a molecule is positive and the other is negative.

Iodine chloride (polar molecule) has a higher boiling point than diatomic bromine (non-polar molecule), illustrating the impact of dipole-dipole forces.

Molecular geometry influences polarity: water is polar due to additive dipoles, while carbon dioxide is non-polar due to cancelling dipoles.

Strong dipoles can induce polarity in non-polar molecules, enabling compounds like diatomic oxygen to dissolve in water.

Hydrogen bonds occur when hydrogen is bonded to highly electronegative elements like fluorine, oxygen, or nitrogen, leading to very strong intermolecular forces.

Hydrogen bonds are represented by dotted lines in diagrams, showing the attraction between a proton and a nearby negatively charged dipole.

London dispersion forces occur due to the random motion of electrons, creating temporary dipoles that can induce polarity in nearby atoms.

London dispersion forces are weak and temporary, contributing to low boiling points in substances like noble gases.

Noble gases such as helium, argon, and neon rely on weak London dispersion forces to remain liquids, resulting in very low boiling points.

The strength of intermolecular forces directly influences boiling points, with stronger forces requiring higher temperatures to separate molecules.

Dipole-dipole forces are short-range, affecting only adjacent molecules, and do not influence distant molecules as much.

Electronegativity differences drive the formation of dipoles, with arrows in diagrams pointing toward the more electronegative element.

Transcripts

all right in this video we're going to

be covering chapter 6 section 5 which

covers intermolecular forces just as a

little preface i'm not going to be

covering this part of this section that

covers a

molecular geometry because uh i can't

properly do justice to the

three-dimensional

shapes

of molecules

in this 2d medium

but for the intermolecular forces i feel

i can properly discuss the material

within the context of these videos so

let's get into it uh

first of all intermolecular forces

measure

interactions between

molecules

so it's not

within the bonds

of say these two molecules it's

measuring the force

of how one affects the other and the way

chemists usually measure

how

strongly two molecules will

attach to one another

is by boiling them because

if these molecules are sort of vibrating

in the liquid state

and then they heat them up further

so that they eventually float away into

the gas state once they're in this gas

state they have

properly separated

from all the molecules surrounding them

and they can measure the energy released

by this and the amount of energy that it

took

to

get these molecules to escape

in order to measure

how

much force

attach them to one another and just as a

quick rule of thumb the higher boiling

point of a substance such as a metal or

ionic compound

the more

or the stronger the intermolecular

forces between them because it takes a

higher temperature and therefore more

vibrating energy

around these molecules to get them to

separate

and as a general rule you'll find that

uh metallic metallic compounds

tend to have the highest

boiling points

followed by ionic compounds and then

lastly molecular compounds

all right so dipole dipole forces are

forces between polarized molecules and

these polarized molecules are called

dipoles which are

molecules with equal but opposite

charges on either end

that are separated by a very short

distance usually the length of the

molecule

and so

how you would

write a dipole is you would write the

formula for

a compound

let's take

iodine chloride

and you draw a sort of lewis structure

with the dash in the middle representing

the

bond

and then you would

write the positive at one end

with an arrow extending towards the

negative end

so in this instance the dipole has a

positive

iodine iodine uh

atom and a negative chlorine atom

and this is easy to remember because you

can just look at the periodic table and

the arrow will always point

towards the more electronegative element

so if you have a bunch of dipole

molecules in a solution

or in a solid

they'll orient themselves so that

their polar regions

which are charged

will uh

attract the oppositely charged regions

of adjacent molecules so these are very

short range forces only affecting the

adjacent molecules you'll notice that

the negative end of this dipole over

here isn't super attracted to the

positive end of this one over here

and these short rain forces only work

between adjacent molecules and they're

called dipole-dipole forces

as written at the top

and they're responsible for a stronger

bond than typical

intermolecular forces

which means that

dipole

compounds will tend to have a higher

boiling point for example uh

iodine chloride has a boiling point of

97 degrees celsius whereas if you were

to take uh

diatomic bromine

which of course has an electronegativity

difference of zero because it's the same

element and therefore isn't polar

and doesn't have these dipole-dipole

bonds

it has a much lower boiling point of

just 59 degrees celsius

so iodine chloride is kind of a simple

example of a dipole system

because it only has one bond that

dictates its polarity if you look at a

more complex molecule like let's say uh

water

which we know has

two hydrogens and one oxygen

when you look at the polarity difference

because oxygen is more electronegative

it'll

tend to take away from the hydrogen

electrons

and attract them more towards oxygen so

they spend more time

over here

these two

dipoles within the same molecule

are additive so

oxygen in this molecule will be

very negative over here and these two

hydrogens will tend to be

very positive

now the same thing

is true with subtraction if you were to

have two dipoles oriented oppositely

within a molecule like let's say

within carbon dioxide

which we know is carbon in the middle

with two oxygens on either end the

oxygens because they're more

electronegative

will tend to orient a dipole

towards them

causing them to

take uh carbon's electrons more of the

time than it will have them however

because these two effects cancel out

the net dipole on this is zero so carbon

dioxide is a non-polar compound

some dipoles however if they're strong

enough can also

induce

a slight polarity in otherwise nonpolar

molecules for example

in diatomic oxygen

if it gets close to a

water molecule particularly the positive

hydrogen dipole

what will happen is that the electrons

in the shared orbital

will tend to spend more time over here

making this end negative and this end

positive

now these this doesn't make the oxygen

molecule

completely

a dipole however it makes it polar

enough

where this oxygen molecule can then

become soluble

in water

so under the umbrella of

dipole interactions there are some

compounds that tend to have very high

boiling points compared to

other similar compounds and this is

because

those compounds tend to contain hydrogen

attached to very electronegative

elements such as fluorine

oxygen

or nitrogen

now what happens

is that because these elements are so

electronegative

and hydrogen only has

one electron

it almost gives up its electron entirely

to these elements within these uh bonds

and that leaves

pretty much a lone proton

and then

what is almost a negative ion over here

so it's fluorine that has an extra

electron most of the time connected to

another proton

now if you have a solution of something

very similar to this

you'll end up with a very polar

substance because this lone proton

doesn't have any electron

shielding around it

to offer you know

[Music]

a balance out for this large positive

charge and these hydrogen bonds between

various molecules

are usually represented by a dotted line

so here i'll put a bunch of oxygen and

then

add the hydrogen in their proper places

so it would be represented by

a dotted line

representing uh

the hydrogen or at least the lone proton

out here

its attraction to the negative uh

polarized side of the

water dipole

so the final intermolecular force we're

going to be covering in this video are

the

london dispersion forces

and the london dispersion forces occur

in all atoms regardless of

uh whether they have a full octet

or highly reactive and

they're due to the random motion of

electrons so let's say we have a helium

atom we'll say that's the nucleus

and it has two electrons

in the cloud around it

now because electrons are in constant

motion and there's a certain uncertainty

to their location at any point this

electron could be over here

or this one could be over here or they

could both be down here

etc and what this means is that uh

the randomness of the electrons can

cause a slight

charge on one end so let's say this

electron moved over to the right side

there would be a slight negative charge

over on this side leading to a slight

positive charge over on this side which

would affect

say an adjacent atom over here causing

it to be

slightly polarized as well

now this leads to uh

small attractions

between

adjacent molecules however

just as it can randomly form a dipole it

can just as easily disassociate into a

neutral atom

once again which means that london

dispersion forces

are very weak and very temporary and

this means that things that rely on

london dispersion forces to hold

themselves together as liquids such as

the noble gases

like helium argon

neon

etc

have very low boiling points because

there's not much attractive force

between them at all

only due to this small london dispersion

force