Exploring Intermolecular Forces and Their Effects

In this video, we're going to focus on intermolecular forces. We're going to go over ion-ion interactions, ion-dipole, dipole-dipole interactions, including hydrogen bonds. And we're going to talk about the difference between inter-and intramolecular forces, as well as going over London dispersion forces and Van der Waal forces. And towards the end of the video, we're going to go over some examples. I'm going to give you a list of compounds, and then you can determine what type of interaction is found. in each compound. So let's go ahead and begin. So let's start with the ion-ion interaction. So let's say if we have two ions, the sodium plus ion and the chloride ion. Opposite charges attract. A positive charge and a negative charge are attracted to each other. And so they're pulled together by a force called an electrostatic force. That electrostatic force is proportional to the charge and inversely related to the distance between them. So for example, let's say if we have the calcium plus 2 ion and the oxide ion. Notice that the magnitude of the charges is greater. So therefore, the interaction between... Calcium and oxide is greater than the interaction between sodium and chloride because the charge is greater. So the higher the charge, the greater the ionic interaction. So as charge increases, the electrostatic force will increase. And as the size of the ions increase, the electrostatic force will decrease. And there's another equation that's associated to this interaction. It's called lattice energy. Lattice energy is proportional to the magnitude of the charges and inversely related to the distance between them. But for electrostatic force, the distance is squared, but for lattice energy, it's not squared. So, here's a question for you. Consider these two ionic compounds, aluminum nitride versus magnesium oxide. Now, In both compounds, we have a metal and a nonmetal, so we know it's ionic in nature. But which of these two ionic compounds will have a higher melting point? The one that's going to have a higher melting point is the one that has more lattice energy. It's the one where the electrostatic forces between the two ions are greater. Now, aluminum has a positive 3 charge. Nitride has a negative 3 charge. Magnesium has a plus 2 charge. And so oxygen has a minus 2 charge. If you multiply Q1 and Q2, if you find the products of their charges, for aluminum nitride it's 3 times 3, so it's 9. And for magnesium oxide it's going to be 2 times 2, so it's going to be 4. So therefore we should expect that aluminum nitride should have a higher melting point because the ion-ion interactions are very strong in aluminum nitride. So... Let's compare sodium fluoride and potassium chloride. Between these two, which one has more lattice energy? Which one's going to have a higher melting point? So, sodium is smaller than potassium. Potassium is larger, because in a periodic table, potassium is below sodium. And they both have a plus one charge. Now, fluoride... smaller than chloride. So the magnitude of the charge is the same, but because sodium fluoride is smaller than sodium chloride, we should expect that sodium fluoride should have a higher melting point. It has more lattice energy because remember that we said that as the size of the ions increase, the lattice energy will decrease. So the lattice energy for potassium chloride is smaller because Potassium chloride is larger. But in the case of sodium chloride, as you decrease the size of the ions, the lattice energy goes up, and the strength of the electrostatic force, or the ion-ion interaction that holds the sodium and fluoride ions together, is going to be stronger because those ions are smaller. So now, let's go over the ion-dipole interaction. You know what an ion is? An ion is basically a particle with an unequal number of protons and electrons. So what is a dipole? But real quick, let's go over definition of ions. So, looking at aluminum, this is the aluminum atom. It has a charge of zero. The 13 is the atomic number, the mass number is 27. Aluminum has 13 protons because the atomic number is 13. Protons are always equal to the atomic number. It has 14 neutrons which is the difference between the mass number and the atomic number and has 13 electrons because it's an atom. So as you can see an atom has an equal number of protons and electrons but the aluminum ion has a positive 3 charge. The protons is still equal to 13. The number of protons is always equal to the atomic number. The neutrons is 14. Now the number of electrons is equal to the atomic number. I'm going to write an for atomic number minus the charge. So in the case of aluminum it's going to be 13 minus plus 3. So aluminum has 10 electrons. So this is an ion. Ions have charges and they have unequal number of protons and electrons. So the aluminum plus three ion has 13 protons, each proton has a positive charge, and it has 10 electrons. If you add plus 13... and negative 10 you're going to get a net charge of positive 3 so that's what an ion is a dipole is a substance that has two charges where one side is positive and the other side is negative so this is a dipole Di means two, so there's two poles of charge. So molecules that is overall neutral, where one side is positive and the other side is negative, that is a dipole. A good example is carbon monoxide. Because oxygen is more electronegative than carbon, oxygen is going to pull the electrons toward itself. And therefore, oxygen is going to carry a partial negative charge, and carbon is going to have a partial positive charge. So this molecule is said to be a dipole. It's polar. Whenever one side is positive and the other side is negative, it's a polar molecule. And so that's what it's meant by the term dipole. It has two poles of charge. So the interaction between the sodium cation, a cation is an ion with a positive charge, and water, this interaction is an ion dipole. Water is a polar molecule, and the reason why water is polar is because one side is negative and the other side is positive. So the oxygen atom of water has a partial negative charge. The hydrogen has a partial positive charge because oxygen is... is more electronegative than hydrogen so oxygen pulls the electrons toward itself so water is always polar because water always have a net dipole moment it is said to be called a permanent dipole A permanent dipole is a molecule that is always polar all the time. It never takes a break. Now the oxygen atom, which has a partial negative charge, is attracted to the sodium cation because it has a positive charge. So this electrostatic interaction, that is called an ion-dipole interaction. Because it's the interaction between an ion and a polar molecule, and that's why it's called an ion-dipole interaction. So let's go over another example of an ion-dipole. So another example is between the chloride ion, which is an anion. Anions are ions that have a negative charge. And between water. Now the hydrogen part of water has a partial positive charge and chlorine has a negative charge. So opposites attract, so these two atoms attract to each other. And so this is another example of an ion-dipole interaction. So if you were to dissolve sodium chloride in water, The sodium cation is going to be surrounded by water molecules. And all of the oxygen atoms which are attracted to the positive charge of the sodium cation, they're going to surround the sodium cation. So it's going to look like this. And so that's how water can dissolve salt, because the oxygen part of water is going to surround the sodium cation, and the chloride part is going to be surrounded by the hydrogen atoms that are found in water. And because hydrogen has a partial positive charge, it's going to be attracted to... the negative charge of the chloride ion. And so, as you can see, the chloride ion solvates, or the water solvates the chloride ion. So, these are all ion-dipole interactions, and that's how water can dissolve certain chloride, because they form strong ion-dipole interactions. So, the next... Intermolecular force that we're going to go over is the dipole-dipole interaction. So usually this is between two polar molecules. So an example will be between two carbon monoxide molecules. Now as we mentioned before, we said that the oxygen part of carbon monoxide has a partial negative charge. And the carbon... has a partial positive charge. So these two atoms are attracted to each other. And so therefore that is the dipole-dipole interaction. So as you can see opposite charges or opposite atoms attract. Like charges repel, opposite charges attract. And that's always going to be the case. So between two polar molecules you're going to have a dipole-dipole interaction. So another example could be like between one molecule of HBr and another molecule of HBr. So bromine is more electronegative than hydrogen. And so the hydrogen of one molecule is attracted to the bromine atom of another molecule. And that interaction is known as a dipole-dipole interaction. Now, Another specialized type of dipole-dipole interaction is the hydrogen bond. So let's look at the case associated with water. So anytime you have an interaction between hydrogen and nitrogen, oxygen, or fluorine, and if it's an intermolecular interaction, you have a hydrogen bond. So keep in mind, this OH bond is not the hydrogen bond. That is a covalent bond. In a covalent bond, the electrons are shared. Now, there's two types. You have polar covalent bonds and nonpolar covalent bonds. In the nonpolar covalent bonds, the electrons are shared equally. But in a polar covalent bond, the electrons are shared unequally. So, in a bond between hydrogen and oxygen, Well, oxygen is more electronegative, so it pulls the electrons toward itself. And so it doesn't share the electrons equally, and that's why oxygen develops this partial negative charge. And hydrogen, which is electron deficient, develops a partial positive charge. However, the partially positive hydrogen atom is attracted to the partially negative oxygen atom of another water molecule. And the interaction between two different... water molecules or two separate water molecules that is the hydrogen bond so because the hydrogen bond is between two water molecules and not within a water molecule the hydrogen bond is said to be an inter molecular force the word enter means between so an intermolecular force or interaction in the case of water is between two water molecules and so the hydrogen bond is a type of intermolecular interaction now the covalent bond it's inside or within a single water molecule so the covalent bond is an example of an intra molecular interaction or molecular bond so the word intra means inside or within so that's the difference between an intermolecular bond or intermolecular force with versus an intermolecular force. So an intermolecular bond or force is inside a molecule within a single molecule and an intermolecular bond or force is between two separate molecules. So hydrogen bonds, ion-ion interactions, ion-dipole, dipole-dipole interactions, these are considered intermolecular interactions or forces. The next type of Intermolecular force that we're going to talk about is the London dispersion force also known as the van der Waal force. Now these type of forces, they're found in everything. Any type of molecule or ion have London dispersion forces. However, these forces are most significant in nonpolar molecules. Nonpolar molecules only contain London dispersion forces. So now let's understand what exactly is the London dispersion force. London dispersion forces they come from very very weak dipole interactions So let's consider the neon atom. Let's say that's the neon nucleus And here's the entire atom and there's electrons around it now I'm just going to draw six electrons, but neon has much more electrons than six but For the sake of illustrative purposes. I don't want to draw like 18 electrons, so I'm just going to draw six Now, notice that these electrons are distributed evenly around this atom. When the electrons are distributed evenly, it's not polar. It's going to be nonpolar. We don't have a distortion of charge. One side is not positive and the other side is not negative. It's neutral. Now, sometimes the electrons or the electron cloud can be distorted. So sometimes you might have more electrons on one side of the atom as opposed to the other side. And when that happens, what we have is a temporary dipole or momentary dipole. A dipole that lasts for a very short time. So at this instant, one side is negative and the other is positive. So right now this is a temporary dipole. Now think about what effect This atom is going to have on another neon atom. So let's say if another neon atom was very close to it. It sees that there's a deficiency of electrons on this side. So this side has a negative charge, this side has a slight positive charge. So the electrons in this atom, they're going to drift towards the left side. And so as they drift towards the left side, they're going to develop a negative charge on the left and a positive charge on the right. and the interaction between these temporary dipoles that is the London dispersion force and these samples are very very weak so London dispersion forces are weaker than that normal type of action so notice that this dipole was created by this type of so whenever a dipole is created by another molecule or atom this is called an induced dipole Because it was created by another atom or molecule so whenever you have a created dipole It's called an induced dipole, but specifically a temporary induced dipole because it doesn't last for very long And so that's why London dispersion forces and van der Waal forces. They're associated with the term Temporary induced dipoles so keep in mind Hydrogen bonds are very very strong dipole interactions Lunan dispersion forces are very very like weak dipole-dipole interactions. So now let's review the strength of the intermolecular forces. So the strongest is the ion-ion interaction and then after that we have the ion-dipole interaction. That's pretty strong. And then after that the next strongest interaction is the hydrogen bond, which is a very very powerful dipole-dipole interaction. It's a specialized type. And then you have your normal dipole-dipole interactions, which is typically between two polar molecules. By the way, if it has hydrogen bonds, it's a polar molecule as well. And then the weakest is the LDF, the London Dispersion Forces, or the Van der Waal forces, the Temporary Induced Dipole Interactions. So you need to know the order of the strength of those intermolecular forces. So now let's go over a list of compounds. And what I want you to do is indicate the strongest. intermolecular force that is found in that compound. So let's start with magnesium oxide. So magnesium is a metal, oxide is a nonmetal, and so there are ions that make up this crystal or this ionic compound. And because there's ions we see that it has an ion-ion interaction. By the way for each of these examples feel free to pause the video and see if you can figure it out yourself. So what about KCl, potassium chloride, and water. So what is the strongest intermolecular interaction that is found between these two compounds? So potassium has a positive charge, and it's going to interact with water, particularly the oxygen atom of water, since oxygen has a partial negative charge. And so that interaction is known as an ion-dipole interaction. And keep in mind the chloride ion, which has a negative charge, is going to interact with the hydrogen part of water, which has a partial positive charge. And so this is also considered an ion-dipole interaction. So what about methane? What type of interaction is found in this compound? Now, whenever you have a hydrocarbon, that is a compound that is made up of only carbon and hydrogen, know that this compound is nonpolar. So other examples would be like benzene C6H6, ethane C2H6. If it is composed only of hydrogen and oxygen, it's nonpolar. And if you have a nonpolar molecule, the only type of force that's found in it is the LDF forces, the London dispersion forces. Now let's focus on methane. The carbon-hydrogen bond has a very, very weak dipole moment. Carbon has an electronegativity value of 2.5 and for hydrogen it's 2.1. Because the electronegativity difference is less than 0.5, the bond is considered to be nonpolar. And even though there's a small dipole moment, it points towards the carbon atom because carbon is more electronegative. Now because all of the hydrogen atoms are the same, all of these little dipole moments, they cancel. Notice that they all point in opposite directions. And so therefore, the net dipole moment is zero. So this molecule is completely nonpolar because all the dipole moments cancel. There's no net dipole moment. And so, because it's nonpolar, the only type of force that it has is London dispersion forces. Now even though there's no net dipole moment, sometimes an induced dipole can occur. Sometimes, as you mentioned, temporary induced dipoles may occur within... non-polar molecules, but they're very very weak. So those temporary induced dipoles are considered LDF forces. So what about the interaction between carbon dioxide and well, just carbon dioxide for now. What type of intermolecular forces are found in this molecule? If you draw the Lewis structure, carbon dioxide looks like this. It's a linear molecule, it has a bond angle of 180, and the carbon is sp-hybridized. Now, if you focus on a carbon-oxygen bond, oxygen has an electronegativity value of 3.5, and for carbon it's 2.5. So the electronegativity difference is greater than 0.5, it's about 1. And so the carbon-oxygen bond is considered to be a polar bond. So it has a very strong dipole. And whenever you want to draw the dipole moment, it points towards the more electronegative atom. But notice that it has two dipoles. So even though the bond is polar, notice that these arrows, they pretty much cancel because they point in opposite directions. So because it cancels, there is no net dipole. And so carbon dioxide is considered to be a nonpolar molecule. So because it's nonpolar, the predominant force is going to be LDF. London dispersion forces and not dipole-dipole interactions. So make sure you are aware of that. So what about Sulfur dioxide what type of intermolecular forces are found in this molecule? So to draw the Lewis structure let's add up the valence electrons sulfur has six valence electrons and Oxygen has six and there's two of them. So the total is 18 Now, if you subtract 18 by the highest multiple of 8, just under 18, multiples of 8 are 8, 16, 24, but 24 is too high. So, if you subtract it by 16, you're going to get the number of electrons, or dots, that is on the central sulfur atom. So, sulfur has two dots, or one lone pair. And oxygen likes to form two bonds, typically. So, this is the Lewis structure of sulfur dioxide. So... Is this molecule polar or nonpolar? Now, the sulfur-oxygen bond is fairly polar. Sulfur has an electron negativity value of 2.5, and oxygen is 3.5, so the bond is polar. But now, these two arrows, do they cancel? It turns out that they do not cancel because of the bent shape of this sulfur dioxide molecule, and also the presence of this lone pair. Now, let's go into physics. So, we have a vector that goes in this direction, and another vector that goes in this way. Do these two vectors cancel? Now, each vector has a y component, and they have an x component. or horizontal component. Notice that the horizontal components cancel, the blue lines, because one goes to the right and the other goes to the left, so they cancel out. However, the vertical components do not. The red lines, they both point in the same direction, so there's a net dipole moment that goes in a downward direction. And so therefore, sulfur dioxide is considered to be a polar molecule. So, if it's polar, that means that it has dipole interactions. so let's draw the dipole interactions for sulfur dioxide let's draw between two sulfur dioxide molecules so oxygen there's a partial negative charge and sulfur has a partial positive charge So the sulfur in one molecule and the oxygen in another molecule, they will be attracted to each other because opposites attract. And so this is an intermolecular force because that interaction is between two separate molecules. But specifically, it's called a dipole-dipole interaction. Now keep in mind, a dipole is basically a polar molecule. So the reason why we have a dipole-dipole is because there's two polar molecules. It's an interaction between two polar molecules. And so this is the dipole-dipole interaction between the sulfur dioxide molecules. Hydrofluoric acid. What type of intermolecular forces are found in this molecule? Now, this is hydrogen bonds because whenever H is bonded to N, O, or F, hydrogen bonds can occur. But keep in mind, the hydrogen bond is not the bond between H and F. That's an intramolecular bond or covalent bond. specifically a polar covalent bond, since the electrons are shared unequally. But the hydrogen bond exists between these two molecules, or between the hydrogen atom of one molecule and the fluorine atom of another. So opposites attract. The partially positive hydrogen atom is attracted to the partially negative fluorine atom, and that's the hydrogen bond. Notice that the electronegativity difference between hydrogen and fluorine It's very large. Hydrogen has an EN value of 2.1, and for fluorine, it's the most electronegative element. It's 4.0. So, it's a combination of two things. The very high electronegativity of fluorine, and also the very small size of hydrogen and fluorine. Those combined effects make hydrogen bonds very powerful. Now, keep in mind, we said that the electrostatic force between either two atoms... of opposite charge is kq1q2 over r-squared. So as the magnitude of the charge, q, increases, the electrostatic force increases, and also the lattice energy goes up. And as the radius, or the size, decreases, the electrostatic force increases. So look at the difference in electronegativity. Because it's so high, that hydrogen bond is pretty strong. And also, The size of hydrogen fluorine, because it's so small, also increases the electrostatic force, which means that hydrogen bonds are very powerful forms of dipole-dipole interactions. Consider the interaction between methanol, CH3OH, and lithium chloride. Now, lithium chloride is ionic, and methanol, it's a covalent compound, so you know this is going to be an ion-dipole. Lithium has a positive charge and like water oxygen has a partial negative charge, but I'm going to draw it like this so the interaction between the partially negative oxygen atom and the positive The positively charged lithium cation that is an ion dipole interaction, and then we have the chloride ion which is attracted to the hydrogen part of methanol. Now this hydrogen has a partial positive charge, and so it's attracted to the negative charge of the chloride ion, and so that is also considered to be an ion-dipole interaction. So what about CH2O and CO? CH2O is formaldehyde. It looks like this. And CO is carbon monoxide. The carbon part has a partial positive and the oxygen atom is partially negative. So, there's an interaction between the oxygen of one molecule and the carbon of another. And, there's also another interaction. between these two atoms because this is partially negative that's partially positive and so all of these are dipole interactions because both these molecules are polar so this interaction and this one is considered a dipole dipole interaction so now we're going to talk about which molecule has a higher boiling point so we're going to go through some examples so let's compare I2 versus Br2 now the first thing you want to do is identify the strongest type of intermolecular force that's found in these molecules now whenever you have a molecule composed of one element it's nonpolar now because both of these are nonpolar the only type of intermolecular force that is found in it are LDF London dispersion forces Now, when you're comparing two molecules that only have London dispersion forces, you need to look at the size. Iodine has more electrons than bromine because it has a higher atomic number. As the number of electrons go up, the... polarizability of iodine goes up as well, which means that iodine has a greater chance or greater probability of having its electron cloud distorted, which means that it can form temporary induced dipoles easier, which means that it has more London dispersion forces. If it has more London dispersion forces, it has more overall intermolecular forces, and therefore iodine is going to have a higher boiling point than bromine. So let's say if you get a question like Cl2, I2, Br2, and F2. How would you rank the following molecules in order of increase in boiling point? Now, on the periodic table, fluorine is the lightest, and you have chlorine, bromine, and then iodine. So, as the size increases, the boiling point is going to increase because there's more lead in dispersion forces. All of these molecules are nonpolar, so they only have... London dispersion forces or van der Waal forces as their predominant intermolecular force so because iodine is the biggest Iodine which has like a violet purple color is going to have the highest boiling point But we want to rank it in order of increase in boiling point, so let's start with the smallest So the first one is fluorine. It's the weakest because it has it's the lightest gas molecule And so it has the least amount of intermolecular forces so next we have chlorine And then after chlorine we have bromine. Bromine have like a reddish color and then followed by that we have iodine. So boiling point increases as you go towards iodine because it has more intermolecular forces, it's larger. It has more electrons and so it's more polarizable and it has more LDF, London dispersion forces. It turns out that fluorine is a gas at room temperature and the same is true for chlorine, it's a gas. Bromine is a liquid. But iodine is a solid at room temperature because iodine has the highest boiling point. So what about CH3OH versus CH4? Which one has the higher boiling point? Now, CH4 is nonpolar, but methanol is polar because of the OH bond. Whenever you see an OH bond, it's always polar. Now, because it has... that hydroxyl bond or that OH group, it also has hydrogen bonds. Methane, which is nonpolar, only has LDF forces. If it has hydrogen bonds, particularly methanol, it also has like a dipole interactions and everything has LDF forces. So as we can see, methanol has more intermolecular forces than methane, which is CH4. So therefore, we should expect that methanol it's going to have a higher boiling point because it has hydrogen bonds and methane doesn't have it So it turns out that methane is a gas at room temperature and methanol is a liquid at room temperature. Gases have low boiling points. Solids have very high boiling points, by the way. So let's compare propanol, CH3CH2CH2OH. with methanol, CH3OH. So which one has a higher boiling point? Now both of these molecules have hydrogen bonds, so they both are very polar. However, one of them has a higher boiling point than the other. so they both have H bonds and they both have dipole interactions and they both have LDL forces but so whenever they both have hydrogen bonds next thing is look at is the size propanol is significantly bigger than methanol and because it has a larger size it has more intermolecular forces due to the presence of more London dispersion forces so the more atoms you have the more the electrons there are and so the greater the polarizability increases. So therefore, you're going to have more London dispersion forces because temporary induced dipole moments can be easily created with molecules that have a lot of electrons. So therefore, as the size increases, the LDF forces increases and so the boiling point is going to go up. So propanol has a higher boiling point than methanol because it has more intermolecular forces, particularly LDF forces. Whenever the boiling point goes up, the vapor pressure goes down. So if you get a question that asks you which one is more volatile, you would say methanol. Methanol is more volatile because it has a higher vapor pressure but a low boiling point. So propanol has a higher boiling point, so I'm just going to say high BP, but it has a relatively low vapor pressure compared to methanol. Methanol, which has less intermolecular forces, is going to have... correspondingly lower boiling point but it's gonna have a higher vapor pressure so methanol can easily escape to the gas phase excuse me the gas phase compared to propanol propanol it takes more energy for it to vaporize and so propanol is gonna be more on the liquid side where methanol it's a liquid at room temperature but it has a lower boiling point so it takes less energy for it to get to the gas phase now Let's go back to propanol and methanol. So we know that propanol has a higher boiling point. But now, let's talk about the solubility, particularly in water. Which compound is more soluble in water? Now, solubility really doesn't depend on the size. It depends more on the polarity. So the OH part of the molecule is the polar region of the molecule. So, life dissolves like polar substances can dissolve in water. Nonpolar substances, they don't dissolve well in water. Now, we said that any time you have a compound that contains carbon and hydrogen, that part of the molecule is nonpolar. Carbon-hydrogen bonds are relatively nonpolar. So, the CH3 part of this molecule is nonpolar, and this region is nonpolar as well. So, which compound do you expect? is going to have a higher solubility in water. Methanol is going to have a higher solubility in water because it's very polar. The non-polar region is very small. Propanol can still dissolve in water, but it's going to have a correspondingly lower solubility in water compared to methanol. The reason being is it has a larger non-polar region, and that part of the molecule doesn't like water. It wants to stay away from water. So the longer... the hydrocarbon chain is, the less soluble it is in water. So propanol, as a line structure, looks like this. It has three carbons, one, two, three. Octanol, which has eight carbons, octanol doesn't dissolve well in water. Its solubility is extremely very low. For the most part, it doesn't dissolve in water. So this is considered relatively nonpolar, even though... It has a polar head, but because of this huge non-polar tail, which is 8 carbons, overall this molecule is considered to be non-polar. But propanol and methanol are still relatively polar. They dissolve well in water, but octanol doesn't dissolve very well in water. So consider these two compounds. This is called neopentane. Pentane is a compound that has five carbon atoms. And on the right we have normal pentane. So which of these two compounds has a higher boiling point. Pantane or neopantane. Now the molecular weight is the same. Both of these compounds have 5 carbons and 12 hydrogens. Because they have the same chemical formula, but because they have different structures, they're called isomers. Specifically, constitutional isomers. Same chemical formula, but they're connected differently. So if they have the same molar mass, how do we know which one is going to have a higher boiling point? It turns out that straight-chain alkanes have a higher boiling point than branched alkanes. So this structure is branched, so it's going to have a lower boiling point. So the reason why it has a low boiling point is because the surface area of neopentane is smaller than that of pentane. Because pentane is a straight-chain alkane, it has a larger surface area. And so that's why it's going to have a higher boiling point. Because as the surface area increases, there's more contact space so that there's more room for... temporary induced dipole interactions to occur. If you decrease the surface area, then there's going to be less interactions between molecules and so there's going to be less ion dipole, I mean less temporary induced dipole interactions and thus less London dispersion forces. So the greater the surface area, the more the, you're going to have more intermolecular forces, more interactions between molecules and so you can have a higher boiling point and that's why the straight chain alkane has a higher boiling point than the branch alkane. So, since pentane has a higher boiling point, neopentane has a higher vapor pressure. So neopentane is more volatile. It can evaporate better than pentane. Let's compare these three compounds, H2O, H2S, and H2Se. Now, on a periodic table, You have oxygen, sulfur, and selenium. Go ahead and rank these compounds in order of increase in boiling point. Now, H2O has hydrogen bonds. And H2S and H2SC are polar. Because if you draw H2S, they all have a bent shape. So all of these molecules are polar. So because they're polar, they're going to have dipole interactions. Now... If you want to rank them in order of increasing boiling point, the one that has hydrogen bonds is going to have the highest boiling point. So that's going to be water. Now, between the remaining two, the one that's going to have a higher boiling point is the one with the bigger size. So selenium is bigger than sulfur. So H2Se is going to have a higher boiling point than H2S. So the first thing you look at is hydrogen bonds. The one that has hydrogen bonds is going to have the higher boiling point. Then after that, look at size. Even though oxygen is the smallest, because it has hydrogen bonds, it's going to have the highest boiling point. After that, comparing sulfur and selenium, selenium is bigger. It has more electrons, so it's going to have more LDF, more London dispersion forces. And so that's why H2S, E, has a higher boiling point than H2S. So water has the highest boiling point, but hydrosulfuric acid, H2S, is going to have the highest vapor pressure. So water... is a liquid at room temperature but H2S is a gas at room temperature because it has a lower boiling point. Consider these four compounds HF, HBr, HI, and HDL. Rank them in order of decrease in boiling point. So fluorine and then comes chlorine and then bromine and then iodine. I wanted to list them the way they're written in the periodic table. So iodine is the heaviest. However, HF has hydrogen bonds, so therefore HF is going to have the highest boiling point. And we're going to write this in decreasing order of boiling point. So the second highest is the one that's going to be the heaviest between chlorine, bromine, and iodine. Because iodine is bigger, it's going to be the next in line. So between HI, HDL, and HBR, none of them has hydrogen bonds. So now you have to look at size. So BR is bigger than CL, so the next one is going to be HBR. And then after that, HDL is going to have the lowest. So HF has the highest boiling point. Boiling point increases towards HF. And HDL is going to have the lowest boiling point. So it's going to have the highest vapor pressure.

I'm going to give you a list of compounds, and then you can determine what type of interaction is found. in each compound. So let's go ahead and begin. So let's start with the ion-ion interaction. So let's say if we have two ions, the sodium plus ion and the chloride ion.

Opposite charges attract. A positive charge and a negative charge are attracted to each other. And so they're pulled together by a force called an electrostatic force. That electrostatic force is proportional to the charge and inversely related to the distance between them.

So for example, let's say if we have the calcium plus 2 ion and the oxide ion. Notice that the magnitude of the charges is greater. So therefore, the interaction between...

Calcium and oxide is greater than the interaction between sodium and chloride because the charge is greater. So the higher the charge, the greater the ionic interaction. So as charge increases, the electrostatic force will increase.

And as the size of the ions increase, the electrostatic force will decrease. And there's another equation that's associated to this interaction. It's called lattice energy. Lattice energy is proportional to the magnitude of the charges and inversely related to the distance between them. But for electrostatic force, the distance is squared, but for lattice energy, it's not squared.

So, here's a question for you. Consider these two ionic compounds, aluminum nitride versus magnesium oxide. Now, In both compounds, we have a metal and a nonmetal, so we know it's ionic in nature. But which of these two ionic compounds will have a higher melting point?

The one that's going to have a higher melting point is the one that has more lattice energy. It's the one where the electrostatic forces between the two ions are greater. Now, aluminum has a positive 3 charge.

Nitride has a negative 3 charge. Magnesium has a plus 2 charge. And so oxygen has a minus 2 charge.

If you multiply Q1 and Q2, if you find the products of their charges, for aluminum nitride it's 3 times 3, so it's 9. And for magnesium oxide it's going to be 2 times 2, so it's going to be 4. So therefore we should expect that aluminum nitride should have a higher melting point because the ion-ion interactions are very strong in aluminum nitride. So... Let's compare sodium fluoride and potassium chloride.

Between these two, which one has more lattice energy? Which one's going to have a higher melting point? So, sodium is smaller than potassium. Potassium is larger, because in a periodic table, potassium is below sodium. And they both have a plus one charge.

Now, fluoride... smaller than chloride. So the magnitude of the charge is the same, but because sodium fluoride is smaller than sodium chloride, we should expect that sodium fluoride should have a higher melting point.

It has more lattice energy because remember that we said that as the size of the ions increase, the lattice energy will decrease. So the lattice energy for potassium chloride is smaller because Potassium chloride is larger. But in the case of sodium chloride, as you decrease the size of the ions, the lattice energy goes up, and the strength of the electrostatic force, or the ion-ion interaction that holds the sodium and fluoride ions together, is going to be stronger because those ions are smaller. So now, let's go over the ion-dipole interaction.

You know what an ion is? An ion is basically a particle with an unequal number of protons and electrons. So what is a dipole?

But real quick, let's go over definition of ions. So, looking at aluminum, this is the aluminum atom. It has a charge of zero.

The 13 is the atomic number, the mass number is 27. Aluminum has 13 protons because the atomic number is 13. Protons are always equal to the atomic number. It has 14 neutrons which is the difference between the mass number and the atomic number and has 13 electrons because it's an atom. So as you can see an atom has an equal number of protons and electrons but the aluminum ion has a positive 3 charge. The protons is still equal to 13. The number of protons is always equal to the atomic number.

The neutrons is 14. Now the number of electrons is equal to the atomic number. I'm going to write an for atomic number minus the charge. So in the case of aluminum it's going to be 13 minus plus 3. So aluminum has 10 electrons. So this is an ion.

Ions have charges and they have unequal number of protons and electrons. So the aluminum plus three ion has 13 protons, each proton has a positive charge, and it has 10 electrons. If you add plus 13...

and negative 10 you're going to get a net charge of positive 3 so that's what an ion is a dipole is a substance that has two charges where one side is positive and the other side is negative so this is a dipole Di means two, so there's two poles of charge. So molecules that is overall neutral, where one side is positive and the other side is negative, that is a dipole. A good example is carbon monoxide.

Because oxygen is more electronegative than carbon, oxygen is going to pull the electrons toward itself. And therefore, oxygen is going to carry a partial negative charge, and carbon is going to have a partial positive charge. So this molecule is said to be a dipole.

It's polar. Whenever one side is positive and the other side is negative, it's a polar molecule. And so that's what it's meant by the term dipole.

It has two poles of charge. So the interaction between the sodium cation, a cation is an ion with a positive charge, and water, this interaction is an ion dipole. Water is a polar molecule, and the reason why water is polar is because one side is negative and the other side is positive. So the oxygen atom of water has a partial negative charge.

The hydrogen has a partial positive charge because oxygen is... is more electronegative than hydrogen so oxygen pulls the electrons toward itself so water is always polar because water always have a net dipole moment it is said to be called a permanent dipole A permanent dipole is a molecule that is always polar all the time. It never takes a break. Now the oxygen atom, which has a partial negative charge, is attracted to the sodium cation because it has a positive charge.

So this electrostatic interaction, that is called an ion-dipole interaction. Because it's the interaction between an ion and a polar molecule, and that's why it's called an ion-dipole interaction. So let's go over another example of an ion-dipole. So another example is between the chloride ion, which is an anion.

Anions are ions that have a negative charge. And between water. Now the hydrogen part of water has a partial positive charge and chlorine has a negative charge. So opposites attract, so these two atoms attract to each other. And so this is another example of an ion-dipole interaction.

So if you were to dissolve sodium chloride in water, The sodium cation is going to be surrounded by water molecules. And all of the oxygen atoms which are attracted to the positive charge of the sodium cation, they're going to surround the sodium cation. So it's going to look like this.

And so that's how water can dissolve salt, because the oxygen part of water is going to surround the sodium cation, and the chloride part is going to be surrounded by the hydrogen atoms that are found in water. And because hydrogen has a partial positive charge, it's going to be attracted to... the negative charge of the chloride ion.

And so, as you can see, the chloride ion solvates, or the water solvates the chloride ion. So, these are all ion-dipole interactions, and that's how water can dissolve certain chloride, because they form strong ion-dipole interactions. So, the next... Intermolecular force that we're going to go over is the dipole-dipole interaction.

So usually this is between two polar molecules. So an example will be between two carbon monoxide molecules. Now as we mentioned before, we said that the oxygen part of carbon monoxide has a partial negative charge.

And the carbon... has a partial positive charge. So these two atoms are attracted to each other.

And so therefore that is the dipole-dipole interaction. So as you can see opposite charges or opposite atoms attract. Like charges repel, opposite charges attract.

And that's always going to be the case. So between two polar molecules you're going to have a dipole-dipole interaction. So another example could be like between one molecule of HBr and another molecule of HBr. So bromine is more electronegative than hydrogen.

And so the hydrogen of one molecule is attracted to the bromine atom of another molecule. And that interaction is known as a dipole-dipole interaction. Now, Another specialized type of dipole-dipole interaction is the hydrogen bond. So let's look at the case associated with water. So anytime you have an interaction between hydrogen and nitrogen, oxygen, or fluorine, and if it's an intermolecular interaction, you have a hydrogen bond.

So keep in mind, this OH bond is not the hydrogen bond. That is a covalent bond. In a covalent bond, the electrons are shared. Now, there's two types.

You have polar covalent bonds and nonpolar covalent bonds. In the nonpolar covalent bonds, the electrons are shared equally. But in a polar covalent bond, the electrons are shared unequally.

So, in a bond between hydrogen and oxygen, Well, oxygen is more electronegative, so it pulls the electrons toward itself. And so it doesn't share the electrons equally, and that's why oxygen develops this partial negative charge. And hydrogen, which is electron deficient, develops a partial positive charge. However, the partially positive hydrogen atom is attracted to the partially negative oxygen atom of another water molecule. And the interaction between two different...

water molecules or two separate water molecules that is the hydrogen bond so because the hydrogen bond is between two water molecules and not within a water molecule the hydrogen bond is said to be an inter molecular force the word enter means between so an intermolecular force or interaction in the case of water is between two water molecules and so the hydrogen bond is a type of intermolecular interaction now the covalent bond it's inside or within a single water molecule so the covalent bond is an example of an intra molecular interaction or molecular bond so the word intra means inside or within so that's the difference between an intermolecular bond or intermolecular force with versus an intermolecular force. So an intermolecular bond or force is inside a molecule within a single molecule and an intermolecular bond or force is between two separate molecules. So hydrogen bonds, ion-ion interactions, ion-dipole, dipole-dipole interactions, these are considered intermolecular interactions or forces. The next type of Intermolecular force that we're going to talk about is the London dispersion force also known as the van der Waal force. Now these type of forces, they're found in everything.

Any type of molecule or ion have London dispersion forces. However, these forces are most significant in nonpolar molecules. Nonpolar molecules only contain London dispersion forces.

So now let's understand what exactly is the London dispersion force. London dispersion forces they come from very very weak dipole interactions So let's consider the neon atom. Let's say that's the neon nucleus And here's the entire atom and there's electrons around it now I'm just going to draw six electrons, but neon has much more electrons than six but For the sake of illustrative purposes.

I don't want to draw like 18 electrons, so I'm just going to draw six Now, notice that these electrons are distributed evenly around this atom. When the electrons are distributed evenly, it's not polar. It's going to be nonpolar.

We don't have a distortion of charge. One side is not positive and the other side is not negative. It's neutral.

Now, sometimes the electrons or the electron cloud can be distorted. So sometimes you might have more electrons on one side of the atom as opposed to the other side. And when that happens, what we have is a temporary dipole or momentary dipole. A dipole that lasts for a very short time. So at this instant, one side is negative and the other is positive.

So right now this is a temporary dipole. Now think about what effect This atom is going to have on another neon atom. So let's say if another neon atom was very close to it.

It sees that there's a deficiency of electrons on this side. So this side has a negative charge, this side has a slight positive charge. So the electrons in this atom, they're going to drift towards the left side.

And so as they drift towards the left side, they're going to develop a negative charge on the left and a positive charge on the right. and the interaction between these temporary dipoles that is the London dispersion force and these samples are very very weak so London dispersion forces are weaker than that normal type of action so notice that this dipole was created by this type of so whenever a dipole is created by another molecule or atom this is called an induced dipole Because it was created by another atom or molecule so whenever you have a created dipole It's called an induced dipole, but specifically a temporary induced dipole because it doesn't last for very long And so that's why London dispersion forces and van der Waal forces. They're associated with the term Temporary induced dipoles so keep in mind Hydrogen bonds are very very strong dipole interactions Lunan dispersion forces are very very like weak dipole-dipole interactions. So now let's review the strength of the intermolecular forces. So the strongest is the ion-ion interaction and then after that we have the ion-dipole interaction.

That's pretty strong. And then after that the next strongest interaction is the hydrogen bond, which is a very very powerful dipole-dipole interaction. It's a specialized type. And then you have your normal dipole-dipole interactions, which is typically between two polar molecules. By the way, if it has hydrogen bonds, it's a polar molecule as well.

And then the weakest is the LDF, the London Dispersion Forces, or the Van der Waal forces, the Temporary Induced Dipole Interactions. So you need to know the order of the strength of those intermolecular forces. So now let's go over a list of compounds.

And what I want you to do is indicate the strongest. intermolecular force that is found in that compound. So let's start with magnesium oxide.

So magnesium is a metal, oxide is a nonmetal, and so there are ions that make up this crystal or this ionic compound. And because there's ions we see that it has an ion-ion interaction. By the way for each of these examples feel free to pause the video and see if you can figure it out yourself. So what about KCl, potassium chloride, and water.

So what is the strongest intermolecular interaction that is found between these two compounds? So potassium has a positive charge, and it's going to interact with water, particularly the oxygen atom of water, since oxygen has a partial negative charge. And so that interaction is known as an ion-dipole interaction. And keep in mind the chloride ion, which has a negative charge, is going to interact with the hydrogen part of water, which has a partial positive charge.

And so this is also considered an ion-dipole interaction. So what about methane? What type of interaction is found in this compound?

Now, whenever you have a hydrocarbon, that is a compound that is made up of only carbon and hydrogen, know that this compound is nonpolar. So other examples would be like benzene C6H6, ethane C2H6. If it is composed only of hydrogen and oxygen, it's nonpolar.

And if you have a nonpolar molecule, the only type of force that's found in it is the LDF forces, the London dispersion forces. Now let's focus on methane. The carbon-hydrogen bond has a very, very weak dipole moment.

Carbon has an electronegativity value of 2.5 and for hydrogen it's 2.1. Because the electronegativity difference is less than 0.5, the bond is considered to be nonpolar. And even though there's a small dipole moment, it points towards the carbon atom because carbon is more electronegative. Now because all of the hydrogen atoms are the same, all of these little dipole moments, they cancel.

Notice that they all point in opposite directions. And so therefore, the net dipole moment is zero. So this molecule is completely nonpolar because all the dipole moments cancel.

There's no net dipole moment. And so, because it's nonpolar, the only type of force that it has is London dispersion forces. Now even though there's no net dipole moment, sometimes an induced dipole can occur.

Sometimes, as you mentioned, temporary induced dipoles may occur within... non-polar molecules, but they're very very weak. So those temporary induced dipoles are considered LDF forces. So what about the interaction between carbon dioxide and well, just carbon dioxide for now. What type of intermolecular forces are found in this molecule?

If you draw the Lewis structure, carbon dioxide looks like this. It's a linear molecule, it has a bond angle of 180, and the carbon is sp-hybridized. Now, if you focus on a carbon-oxygen bond, oxygen has an electronegativity value of 3.5, and for carbon it's 2.5. So the electronegativity difference is greater than 0.5, it's about 1. And so the carbon-oxygen bond is considered to be a polar bond.

So it has a very strong dipole. And whenever you want to draw the dipole moment, it points towards the more electronegative atom. But notice that it has two dipoles.

So even though the bond is polar, notice that these arrows, they pretty much cancel because they point in opposite directions. So because it cancels, there is no net dipole. And so carbon dioxide is considered to be a nonpolar molecule.

So because it's nonpolar, the predominant force is going to be LDF. London dispersion forces and not dipole-dipole interactions. So make sure you are aware of that. So what about Sulfur dioxide what type of intermolecular forces are found in this molecule?

So to draw the Lewis structure let's add up the valence electrons sulfur has six valence electrons and Oxygen has six and there's two of them. So the total is 18 Now, if you subtract 18 by the highest multiple of 8, just under 18, multiples of 8 are 8, 16, 24, but 24 is too high. So, if you subtract it by 16, you're going to get the number of electrons, or dots, that is on the central sulfur atom. So, sulfur has two dots, or one lone pair. And oxygen likes to form two bonds, typically.

So, this is the Lewis structure of sulfur dioxide. So... Is this molecule polar or nonpolar?

Now, the sulfur-oxygen bond is fairly polar. Sulfur has an electron negativity value of 2.5, and oxygen is 3.5, so the bond is polar. But now, these two arrows, do they cancel?

It turns out that they do not cancel because of the bent shape of this sulfur dioxide molecule, and also the presence of this lone pair. Now, let's go into physics. So, we have a vector that goes in this direction, and another vector that goes in this way. Do these two vectors cancel?

Now, each vector has a y component, and they have an x component. or horizontal component. Notice that the horizontal components cancel, the blue lines, because one goes to the right and the other goes to the left, so they cancel out. However, the vertical components do not. The red lines, they both point in the same direction, so there's a net dipole moment that goes in a downward direction.

And so therefore, sulfur dioxide is considered to be a polar molecule. So, if it's polar, that means that it has dipole interactions. so let's draw the dipole interactions for sulfur dioxide let's draw between two sulfur dioxide molecules so oxygen there's a partial negative charge and sulfur has a partial positive charge So the sulfur in one molecule and the oxygen in another molecule, they will be attracted to each other because opposites attract. And so this is an intermolecular force because that interaction is between two separate molecules. But specifically, it's called a dipole-dipole interaction.

Now keep in mind, a dipole is basically a polar molecule. So the reason why we have a dipole-dipole is because there's two polar molecules. It's an interaction between two polar molecules.

And so this is the dipole-dipole interaction between the sulfur dioxide molecules. Hydrofluoric acid. What type of intermolecular forces are found in this molecule?

Now, this is hydrogen bonds because whenever H is bonded to N, O, or F, hydrogen bonds can occur. But keep in mind, the hydrogen bond is not the bond between H and F. That's an intramolecular bond or covalent bond. specifically a polar covalent bond, since the electrons are shared unequally.

But the hydrogen bond exists between these two molecules, or between the hydrogen atom of one molecule and the fluorine atom of another. So opposites attract. The partially positive hydrogen atom is attracted to the partially negative fluorine atom, and that's the hydrogen bond.

Notice that the electronegativity difference between hydrogen and fluorine It's very large. Hydrogen has an EN value of 2.1, and for fluorine, it's the most electronegative element. It's 4.0.

So, it's a combination of two things. The very high electronegativity of fluorine, and also the very small size of hydrogen and fluorine. Those combined effects make hydrogen bonds very powerful.

Now, keep in mind, we said that the electrostatic force between either two atoms... of opposite charge is kq1q2 over r-squared. So as the magnitude of the charge, q, increases, the electrostatic force increases, and also the lattice energy goes up.

And as the radius, or the size, decreases, the electrostatic force increases. So look at the difference in electronegativity. Because it's so high, that hydrogen bond is pretty strong.

And also, The size of hydrogen fluorine, because it's so small, also increases the electrostatic force, which means that hydrogen bonds are very powerful forms of dipole-dipole interactions. Consider the interaction between methanol, CH3OH, and lithium chloride. Now, lithium chloride is ionic, and methanol, it's a covalent compound, so you know this is going to be an ion-dipole. Lithium has a positive charge and like water oxygen has a partial negative charge, but I'm going to draw it like this so the interaction between the partially negative oxygen atom and the positive The positively charged lithium cation that is an ion dipole interaction, and then we have the chloride ion which is attracted to the hydrogen part of methanol.

Now this hydrogen has a partial positive charge, and so it's attracted to the negative charge of the chloride ion, and so that is also considered to be an ion-dipole interaction. So what about CH2O and CO? CH2O is formaldehyde. It looks like this. And CO is carbon monoxide.

The carbon part has a partial positive and the oxygen atom is partially negative. So, there's an interaction between the oxygen of one molecule and the carbon of another. And, there's also another interaction.

between these two atoms because this is partially negative that's partially positive and so all of these are dipole interactions because both these molecules are polar so this interaction and this one is considered a dipole dipole interaction so now we're going to talk about which molecule has a higher boiling point so we're going to go through some examples so let's compare I2 versus Br2 now the first thing you want to do is identify the strongest type of intermolecular force that's found in these molecules now whenever you have a molecule composed of one element it's nonpolar now because both of these are nonpolar the only type of intermolecular force that is found in it are LDF London dispersion forces Now, when you're comparing two molecules that only have London dispersion forces, you need to look at the size. Iodine has more electrons than bromine because it has a higher atomic number. As the number of electrons go up, the... polarizability of iodine goes up as well, which means that iodine has a greater chance or greater probability of having its electron cloud distorted, which means that it can form temporary induced dipoles easier, which means that it has more London dispersion forces. If it has more London dispersion forces, it has more overall intermolecular forces, and therefore iodine is going to have a higher boiling point than bromine.

So let's say if you get a question like Cl2, I2, Br2, and F2. How would you rank the following molecules in order of increase in boiling point? Now, on the periodic table, fluorine is the lightest, and you have chlorine, bromine, and then iodine.

So, as the size increases, the boiling point is going to increase because there's more lead in dispersion forces. All of these molecules are nonpolar, so they only have... London dispersion forces or van der Waal forces as their predominant intermolecular force so because iodine is the biggest Iodine which has like a violet purple color is going to have the highest boiling point But we want to rank it in order of increase in boiling point, so let's start with the smallest So the first one is fluorine. It's the weakest because it has it's the lightest gas molecule And so it has the least amount of intermolecular forces so next we have chlorine And then after chlorine we have bromine. Bromine have like a reddish color and then followed by that we have iodine.

So boiling point increases as you go towards iodine because it has more intermolecular forces, it's larger. It has more electrons and so it's more polarizable and it has more LDF, London dispersion forces. It turns out that fluorine is a gas at room temperature and the same is true for chlorine, it's a gas. Bromine is a liquid.

But iodine is a solid at room temperature because iodine has the highest boiling point. So what about CH3OH versus CH4? Which one has the higher boiling point? Now, CH4 is nonpolar, but methanol is polar because of the OH bond.

Whenever you see an OH bond, it's always polar. Now, because it has... that hydroxyl bond or that OH group, it also has hydrogen bonds.

Methane, which is nonpolar, only has LDF forces. If it has hydrogen bonds, particularly methanol, it also has like a dipole interactions and everything has LDF forces. So as we can see, methanol has more intermolecular forces than methane, which is CH4.

So therefore, we should expect that methanol it's going to have a higher boiling point because it has hydrogen bonds and methane doesn't have it So it turns out that methane is a gas at room temperature and methanol is a liquid at room temperature. Gases have low boiling points. Solids have very high boiling points, by the way.

So let's compare propanol, CH3CH2CH2OH. with methanol, CH3OH. So which one has a higher boiling point?

Now both of these molecules have hydrogen bonds, so they both are very polar. However, one of them has a higher boiling point than the other. so they both have H bonds and they both have dipole interactions and they both have LDL forces but so whenever they both have hydrogen bonds next thing is look at is the size propanol is significantly bigger than methanol and because it has a larger size it has more intermolecular forces due to the presence of more London dispersion forces so the more atoms you have the more the electrons there are and so the greater the polarizability increases. So therefore, you're going to have more London dispersion forces because temporary induced dipole moments can be easily created with molecules that have a lot of electrons.

So therefore, as the size increases, the LDF forces increases and so the boiling point is going to go up. So propanol has a higher boiling point than methanol because it has more intermolecular forces, particularly LDF forces. Whenever the boiling point goes up, the vapor pressure goes down. So if you get a question that asks you which one is more volatile, you would say methanol.

Methanol is more volatile because it has a higher vapor pressure but a low boiling point. So propanol has a higher boiling point, so I'm just going to say high BP, but it has a relatively low vapor pressure compared to methanol. Methanol, which has less intermolecular forces, is going to have...

correspondingly lower boiling point but it's gonna have a higher vapor pressure so methanol can easily escape to the gas phase excuse me the gas phase compared to propanol propanol it takes more energy for it to vaporize and so propanol is gonna be more on the liquid side where methanol it's a liquid at room temperature but it has a lower boiling point so it takes less energy for it to get to the gas phase now Let's go back to propanol and methanol. So we know that propanol has a higher boiling point. But now, let's talk about the solubility, particularly in water.

Which compound is more soluble in water? Now, solubility really doesn't depend on the size. It depends more on the polarity.

So the OH part of the molecule is the polar region of the molecule. So, life dissolves like polar substances can dissolve in water. Nonpolar substances, they don't dissolve well in water.

Now, we said that any time you have a compound that contains carbon and hydrogen, that part of the molecule is nonpolar. Carbon-hydrogen bonds are relatively nonpolar. So, the CH3 part of this molecule is nonpolar, and this region is nonpolar as well. So, which compound do you expect? is going to have a higher solubility in water.

Methanol is going to have a higher solubility in water because it's very polar. The non-polar region is very small. Propanol can still dissolve in water, but it's going to have a correspondingly lower solubility in water compared to methanol. The reason being is it has a larger non-polar region, and that part of the molecule doesn't like water.

It wants to stay away from water. So the longer... the hydrocarbon chain is, the less soluble it is in water. So propanol, as a line structure, looks like this.

It has three carbons, one, two, three. Octanol, which has eight carbons, octanol doesn't dissolve well in water. Its solubility is extremely very low. For the most part, it doesn't dissolve in water.

So this is considered relatively nonpolar, even though... It has a polar head, but because of this huge non-polar tail, which is 8 carbons, overall this molecule is considered to be non-polar. But propanol and methanol are still relatively polar.

They dissolve well in water, but octanol doesn't dissolve very well in water. So consider these two compounds. This is called neopentane.

Pentane is a compound that has five carbon atoms. And on the right we have normal pentane. So which of these two compounds has a higher boiling point. Pantane or neopantane. Now the molecular weight is the same.

Both of these compounds have 5 carbons and 12 hydrogens. Because they have the same chemical formula, but because they have different structures, they're called isomers. Specifically, constitutional isomers.

Same chemical formula, but they're connected differently. So if they have the same molar mass, how do we know which one is going to have a higher boiling point? It turns out that straight-chain alkanes have a higher boiling point than branched alkanes.

So this structure is branched, so it's going to have a lower boiling point. So the reason why it has a low boiling point is because the surface area of neopentane is smaller than that of pentane. Because pentane is a straight-chain alkane, it has a larger surface area.

And so that's why it's going to have a higher boiling point. Because as the surface area increases, there's more contact space so that there's more room for... temporary induced dipole interactions to occur. If you decrease the surface area, then there's going to be less interactions between molecules and so there's going to be less ion dipole, I mean less temporary induced dipole interactions and thus less London dispersion forces.

So the greater the surface area, the more the, you're going to have more intermolecular forces, more interactions between molecules and so you can have a higher boiling point and that's why the straight chain alkane has a higher boiling point than the branch alkane. So, since pentane has a higher boiling point, neopentane has a higher vapor pressure. So neopentane is more volatile. It can evaporate better than pentane.

Let's compare these three compounds, H2O, H2S, and H2Se. Now, on a periodic table, You have oxygen, sulfur, and selenium. Go ahead and rank these compounds in order of increase in boiling point. Now, H2O has hydrogen bonds. And H2S and H2SC are polar.

Because if you draw H2S, they all have a bent shape. So all of these molecules are polar. So because they're polar, they're going to have dipole interactions. Now... If you want to rank them in order of increasing boiling point, the one that has hydrogen bonds is going to have the highest boiling point.

So that's going to be water. Now, between the remaining two, the one that's going to have a higher boiling point is the one with the bigger size. So selenium is bigger than sulfur.

So H2Se is going to have a higher boiling point than H2S. So the first thing you look at is hydrogen bonds. The one that has hydrogen bonds is going to have the higher boiling point.

Then after that, look at size. Even though oxygen is the smallest, because it has hydrogen bonds, it's going to have the highest boiling point. After that, comparing sulfur and selenium, selenium is bigger.

It has more electrons, so it's going to have more LDF, more London dispersion forces. And so that's why H2S, E, has a higher boiling point than H2S. So water has the highest boiling point, but hydrosulfuric acid, H2S, is going to have the highest vapor pressure. So water...

is a liquid at room temperature but H2S is a gas at room temperature because it has a lower boiling point. Consider these four compounds HF, HBr, HI, and HDL. Rank them in order of decrease in boiling point.

So fluorine and then comes chlorine and then bromine and then iodine. I wanted to list them the way they're written in the periodic table. So iodine is the heaviest. However, HF has hydrogen bonds, so therefore HF is going to have the highest boiling point.

And we're going to write this in decreasing order of boiling point. So the second highest is the one that's going to be the heaviest between chlorine, bromine, and iodine. Because iodine is bigger, it's going to be the next in line.

So between HI, HDL, and HBR, none of them has hydrogen bonds. So now you have to look at size. So BR is bigger than CL, so the next one is going to be HBR.

And then after that, HDL is going to have the lowest. So HF has the highest boiling point. Boiling point increases towards HF. And HDL is going to have the lowest boiling point. So it's going to have the highest vapor pressure.

Transcript for:Exploring Intermolecular Forces and Their Effects

Transcript for:
Exploring Intermolecular Forces and Their Effects