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What are Intermolecular Forces?

Jun 03, 2021
This is lesson 5.1 and today we are going to talk about

intermolecular

forces

. Our goal today is to learn

what

an

intermolecular

force is and how different intermolecular

forces

work. To start, let's define a few things, first of all, an intramolecular force is. a force within or within a molecule, so intramolecular forces are different from intermolecular forces. Intramolecular forces are covalent bonds. As you can see here in the photo, there are four water molecules, four water molecules and the red here will represent oxygen, the gray will represent hydrogen. and within that molecule you have intramolecular forces that are your covalent bonds, they are the forces that hold the molecule together.
what are intermolecular forces
Now you also have things known as intermolecular forces. An intermolecular force is a force between different molecules, so there is a force of attraction between them. different neighboring water molecules between this water molecule and this water molecule there is a force of attraction and that force of attraction we call intermolecular force so keep in mind that intermolecular forces are forces between different molecules while intramolecular forces They are forces within a single molecule and help you remember that remember that an interstate is a highway that goes between different states and therefore an intermolecular force is a force that occurs between different molecules.
what are intermolecular forces

More Interesting Facts About,

what are intermolecular forces...

Also note that these are generally attractive forces, all of these intermolecular forces are attractive. It is not repulsive in nature and the basis of all these forces is really simple, it is just electrostatic attraction which is positive and negative, they attract, opposite charges attract each other and that is the basis of all your intermolecular forces as well as all your intramolecular forces, so just dig into it and talk about these different intermolecular forces. You could say that the first one is not even an intermolecular force and those are your ionic forces. Their ionic forces are forces within ionic compounds and are simply an attraction between positive and negative ions.
what are intermolecular forces
Anions and cations attract each other, which is a good way to start when we talk about intermolecular forces because the different things that affect ionic forces are the same things that affect intermolecular forces, so keep in mind that the greater your charges, stronger the attraction is the greater the charge, the stronger the attraction between those different ions, secondly, the closer those ions are, the stronger the attraction, so a shorter distance between different ions will cause a stronger traction between those ions. Now let's try to make some sense. From this by looking at these different melting points, which is real data here, we can look at these two different things, first of all we have magnesium oxide and magnesium fluoride, let's focus on these two things, magnesium oxide has a melting point much higher and

what

does that mean?
what are intermolecular forces
It means that it's much harder to melt magnesium oxide and when you melt magnesium oxide, you effectively break down those ionic forces, they don't break all of them, but they break down significantly, so if we look at magnesium oxide and magnesium fluoride magnesium, we notice that magnesium oxide has a much stronger pull between its ions causing this higher melting point in magnesium oxide, well why does it have a stronger pull between the ions because the oxide is o2 less while Fluoride is simply f minus? Here you have a larger charge for the oxide than for the fluoride it has two negative charges instead of one negative charge, which means that there is a stronger attraction between the ions within the ionic compound, holding it together by stronger forces and , therefore, has a higher melting point.
Now also notice that magnesium fluoride has a higher charge. Melting point than magnesium chloride. Magnesium fluoride melts at 1263 degrees Celsius, while magnesium chloride is much easier to melt at 714 degrees Celsius. Why are they both made of magnesium, which is two plus and a halide that has a charge of one minus, fluorides one? less and chlorides one less, so the charges on the ions are exactly the same, the only difference is that chlorine is larger than fluorine, so its chloride ions will be larger than fluoride ions, when The ion is larger, it will take up more space, which will cause the ions to separate, which makes the distance greater, so magnesium fluoride has a stronger pull between the ions because the magnesium and fluorine ions are closer together in magnesium fluoride simply because fluoride is smaller than chloride.
Now let's go ahead and try to apply these concepts. to their proper intermolecular forces which are for covalent compounds and the first of these intermolecular forces that we really want to talk about is their dipole-dipole forces, what are dipole-dipole forces? Dipole-dipole forces are simply an attraction between two different polar molecules and So if you look here, I have hydrogen chloride and hydrogen chloride and you have an attraction between two different neighboring hydrogen chloride molecules and just to make sure we're all on the same page, what do these different symbols mean? This is a delta a. lowercase negative delta and this is a lowercase positive delta lowercase negative delta means partially negative, partly means smaller than one and this is a partially positive charge, which means it's a less than positive charge, so the real numbers for hydrogen chloride are positive 0.178 and negative 0.178 so you have much smaller than 1 positive and 1 negative this is not an ionic compound this is a polar molecule and so you have partial charges on each side of this polar molecule now you You would imagine that the positive end of one of these polar molecules is going to attract the negative end of another polar molecule, which is exactly what happens: the positive and the negative attract each other and these two polar molecules attract each other.
This happens all the time in all polar molecules. Now, before we continue, let's pause and think here. for a second this dipole dipole force do you think it is stronger than the same or weaker than the ion ion force pause here and think about it for a second it turns out that the dipole-dipole force is much weaker than the ion force ion and that is because polar molecules have charges that are much smaller than positive or negative ones. Remember that the smaller the charge, the weaker the attraction, the greater the charge, the stronger the attraction, so in general ionic forces will always be greater than dipole-dipole forces. because dipole-dipole forces involve polar molecules that have much lower charges than positive or negative ones 1. let's compare some compounds to see how it works li2o is an ionic compound while h2o, as we all know, is a covalent compound that is water lithium Oxide has a boiling point of 2600 degrees Celsius, while water has a much lower boiling point of 100 degrees Celsius and that is true in general, ionic compounds usually have much higher boiling points than covalent compounds and that shows that the attraction between the ions within lithium oxide is much stronger than the attraction between neighboring water molecules within the water, we will pause for a second and think about it, many people think that covalent bonds are not stronger than ionic bonds, most of the time, yes, but remember that covalent bonds do not hold water together, they simply hold hydrogen and oxygen together within a single water molecule rather than any type of attraction between different water molecules.
What holds water together as a material are intermolecular forces, which is the force of attraction. between the neighboring water molecules and those will be their dipole-dipole forces and you can see here that the dipole-dipole forces are much weaker than the ion forces. Now let's compare two different covalent compounds, h2o and h2s, very similar in nature, however, h2o. it has a much higher boiling point than h2s, water boils at 100 degrees celsius, while h2s boils at negative 60 degrees celsius, so you can see here that h2s has a much weaker force of attraction between the molecules neighbors, those intermolecular forces in h2s are much weaker than they are in water, why look at the different dipoles?
Water has a dipole of 1.85, while hydrogen sulfide h2s has a dipole of almost half of 0.97, which means that water is much more polar than hydrogen sulfide and that happens because oxygen It is more electronegative than sulfur and you can see that the result is that those water molecules have a stronger dipole-dipole attractive force between them than hydrogen sulfide; In fact, the dipole-dipole forces in water are so strong that people give it a special name and that is a hydrogen bond. Hydrogen bonds are a special type of intermolecular force, however, let's not be fooled by the name here, hydrogen bonds are not real bonds, okay, hydrogen bonds are okay, if they are not real bonds, what are they?
They are extra strong dipole-dipole attractions. they are actually just a form of dipole-dipole attraction and they are a special form because they are only found in molecules with nhoh or fh, that is, molecules that can form hydrogen bonds must have a bond between nitrogen and hydrogen or oxygen and hydrogen or fluorine and hydrogen, so the molecule has to have hydrogen and it also has to have some other atom that is quite electronegative. Nitrogen, oxygen, and fluorine are all pretty electronegative, so when that happens, you can have a pretty strong dipole-dipole attraction. The bonds are quite special because hydrogen is very small and when hydrogen is so small it is able to get closer to a neighboring partially negative oxygen, nitrogen or fluorine atom, allowing for a stronger dipole-dipole attraction because they are closer to each other. yes because hydrogen is very small here. is a picture of the hydrogen bond in water and you can see that oxygen is more electronegative than hydrogen, it has a partial negative charge and the hydrogens that are less electronegative have a partial positive charge, so there is an attraction between the partial negative of an oxygen. and the partial positive of the hydrogen atom of a neighboring water molecule now, because hydrogen is so small, this oxygen can get pretty close to this hydrogen, but the bond length here was not a proper bond, the length of this hydrogen bond here is significantly larger than a covalent bond between oxygen and hydrogen, so keep in mind that this here is an intramolecular force, it is a covalent bond between oxygen and hydrogen and it is much stronger and much shorter than a bond of hydrogen, the hydrogen bond itself is simply an intermolecular force, it is quite strong in terms of intermolecular forces, but it is much weaker than a covalent bond and it is much longer than a covalent bond two, but the hydrogen bond in yeah it's pretty short as far as intermolecular forces go and that's why it's a pretty strong dipole-dipole attraction, strong hydrogen.
Water bonding is one of the things that makes water have so many interesting properties compared to other small molecules and it's also very interesting that hydrogen bonds play a role in life itself. Hydrogen bonds are key to what information is stored in DNA and how it is transferred. to RNA and then why is it paired with t and g with c? The reason is hydrogen bonding. Hydrogen bonds between neighboring base pairs and DNA are what allow them to know what pairs with what and that is the entire basis of the information stored in our DNA. So far we have only been talking about polar molecules that attract each other, but what about non-polar molecules?
Can nonpolar molecules attract each other? Well, at first you might think that the answer is no, there is no kind of positive and negative relationship. in a non-polar molecule, so why could they attract everything, but it turns out they can also attract each other through something known as London dispersion forces? Well, how does that work? Firstly, one of our non-polar molecules can get what is known as an instantaneous dipole and this happens because the electrons are always in motion and therefore if the electrons are always in motion at any instant, a molecule does not polar can have a temporary dipole that we call instantaneous dipole, it is short-lived, it is not very strong, but any type of molecule can experience this instantaneous dipole, this instantaneous dipole will be able to affect all the molecules around it and what it can do is induce or cause a dipole to form in a nearby molecule, for example, this chlorine molecule up here. which is normally nonpolar has an instantaneous temporary dipole created within it simply because one or more electrons are more in aside of the molecule than on the other, causing a partial positive on one side of the molecule that is partial positive on this side. of the molecule affects its neighbor here, which makes the electrons of this molecule want to get a little bit closer to the right side of that molecule, creating a partial negative on this side of this molecule, so now we have a partial negative here he is attracted. to a partial positive in this molecule and so that the instantaneous dipole and the induced dipole attract each other and those molecules attract each other for a short period of time, we can look inside the atoms to see what is happening in a very , Very simple.
For example, helium now has very, very weak London forces, but let's see how it would work in helium, so here we have atom a and atom b, they can attract each other even though they are completely non-polar. Because? Because right here the atom b. Those electrons are simply on the left side of that atom temporarily and when they are on the left side of the atom temporarily, they will influence the electrons of the atom and cause these electrons to move towards the left sum as you can see here and this atom b would be your instantaneous dipole and this atom here would be your induced dipole and you have an attraction between these two. specifically, you have an attraction between the electrons of this atom and the nucleus of this atom.
This is probably a bit of an exaggeration in this example, but that's how it works. You always have an attraction between the electrons of an attracted atom or molecule. the nucleus of the other atom or another molecule this attraction between the instantaneous dipole and the induced dipole is called London dispersion forces this force exists between any type of molecule or any type of atom over the entire surface of the molecules and although this is a Very, very weak force, if you have very, very large molecules, it can actually be quite significant in terms of their attraction.
The larger the molecules in the material, the more london forces and therefore the greater the attraction between the molecules within that material, and so on when you get to things. like polymers, which are very, very, very large molecules, you can have a very strong London force between neighboring molecules because they are very large. Now let's go ahead and see how this will work in some examples on our next slide here. three different hydrocarbons and because these are all hydrocarbons, that is, molecules containing only carbon and hydrogen, they are all essentially non-polar, so they do not have any dipole-dipole attraction within them.
What is the difference? First of all, this one is smaller. there are only three carbons and this one and this one are both bigger this one has four carbons and this one also has four carbons and if we compare their boiling points this molecule with three carbons and this molecule with four carbons you essentially just added one carbon and two hydrogens to this molecule right here and when you did that, you noticed that the boiling point has increased significantly. Why has it increased significantly mainly because there is a greater amount of London dispersion attractive forces between neighboring molecules simply because the molecules are larger and that will cause a greater attraction between them, which will make them harder to boil, harder To separate now, what's going on here is c4h10 and this is c4h10, so they're essentially identical in terms of molar masses, but they clearly have different boiling points. one has a higher boiling point than this and the reason is that this molecule here is more elongated and therefore has a greater surface area and is able to touch its neighbors more frequently than this one here this one has a smaller amount of surface area per molecule, which makes those London dispersion forces smaller per molecule than what you have in this one now.
If you're wondering how these different intermolecular forces compare to each other, I made the table below, but keep in mind that sometimes this isn't always accurate. We are not going to follow these rules, but this is more of a general trend than a strict rule. Okay, in general, covalent bonds will be stronger than ionic bonds, and ionic bonds will be much stronger than hydrogen bonds, which are stronger than ionic bonds. They are not true bonds anyway and hydrogen bonds are especially strong dipole-dipole forces and the London dispersion forces will be the weakest of all; However, as we already noted, if your molecules are very, very large, those London dispersion forces, although small in themselves, are very significant.
Because there are so many of these London scattering attractions between neighboring molecules, those London forces can be quite significant when you have very large molecules. Lastly, let's go ahead and test what different intermolecular forces the following compounds have, so here you have ch2cl2, co2, and nh3, which is ammonia. Go ahead and pause this video and see if you can figure out what different intermolecular forces each of them have. Now the first molecule, dichloromethane or ch2cl2, is a polar molecule and therefore we know that it is a polar molecule. which must have dipole-dipole forces plus all molecules have London dispersion forces and therefore must have London dispersion forces, however, ch2cl2, although it contains hydrogen, cannot form hydrogen bonds and the The reason is that it does not have nhoh or fh, it has chlorine.
Hydrogen and carbon do not contain nitrogen, oxygen or fluorine, then carbon dioxide, as we know, is a non-polar molecule, although carbon and oxygen have a significant difference in their electronegativities due to the symmetry of the molecule, it is a non-polar molecule and therefore is a non-polar molecule, by definition, it cannot have dipole-dipole forces and it also cannot have hydrogen bonds, plus it does not have hydrogen, so it only has dispersion forces of London, finally, ammonia and h3, well, everything has dispersion forces of London, nh3. It has a trigonal pyramidal shape, so we know that it is a polar molecule and therefore it has dipole-dipole forces and it also has hydrogen bonds because of that nh bond.
In fact, if any molecule has an nhoh or fh, you automatically know that it also has dipole-dipole forces simply because hydrogen bonding is actually a type of dipole-dipole force; In fact, any molecule that has nhoh or fh must be a polar molecule due to the nature of the nhoh or fh bonds that nitrogen, oxygen, or fluorine are. There is definitely going to be a lone pair that will make it a polar molecule. Well, thanks for watching. Stay curious and see you here next time.

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