6.5 Intermolecular Forces
Summary
TLDRThis 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
📘 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.
🔄 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.
💧 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.
Mindmap
Keywords
💡Intermolecular Forces
💡Boiling Point
💡Dipole-Dipole Forces
💡Dipole
💡Electronegativity
💡Hydrogen Bonding
💡London Dispersion Forces
💡Polarity
💡Non-Polar Molecules
💡Induced Dipole
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
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