L26 IMFs P2. Induced Dipole Attractions

Summary

The video by Fogline Academy delves into the intriguing phenomenon of induced dipole attractions, or London dispersion forces. While nonpolar molecules like nitrogen and halogens appear to lack attractions due to their symmetrical electron distribution, they can still condense into liquids or solids thanks to temporary induced dipoles. These fleeting attractions occur because electrons in atoms can shift momentarily, causing temporary positive and negative regions. Larger atoms with more electrons are more easily polarizable, exhibiting stronger induced dipole interactions. This principle explains why molecules with larger electron clouds have higher boiling and melting points. The video also highlights a fun analogy of molecular velcro, illustrating how larger surface areas between molecules enhance attraction, influencing substances' physical properties like boiling points.

Highlights

• Temporary induced dipoles allow nonpolar molecules to temporarily attract. 🌀
• London forces explain liquid and solid forming in nonpolar molecules. ❄️
• Larger electron clouds in big atoms lead to stronger attractions. 💪
• Boiling points increase with molecular weight due to dipole strength. 🔥
• Shape and molecular structure influence the strength of dispersion forces. 💼

Key Takeaways

• Nonpolar molecules can have attractive forces due to induced dipoles. 🌟
• Induced dipoles explain the liquid and solid states of nonpolar substances. 🌊
• Larger atoms with bigger electron clouds exhibit stronger induced attractions. 🌌
• Molecular weight correlates with polarizability and boiling points. 📈
• Molecular 'velcro' illustrates how shape affects intermolecular forces. 🤹‍♂️

Overview

Ever wondered how nonpolar molecules like nitrogen or bromine can exist in liquid and solid states? Enter the world of induced dipole attractions—a fascinating realm where temporary electron shifts momentarily create positive and negative regions, allowing these molecules to, quite literally, stick together! 🌟

Fogline Academy shines a light on how larger molecules are more easily 'polarizable,' meaning their electron clouds shift more dramatically, hence intensifying these temporary attractions. It's a dance of electrons that makes big molecules like iodine solid at room temperature and gives bigger atoms a knack for forming liquids. 🌊

Imagine molecular velcro, where the more area you have, the stronger the bond. This analogy perfectly captures how larger molecular surfaces with more electron clouds exhibit stronger dispersion forces, ultimately affecting things like boiling points. It's why pentane boils at room temperature while dimethyl propane doesn't—it's all about those attractive forces! 🤹‍♀️

Chapters

• 00:00 - 01:00: Introduction to Induced Dipole Attractions The chapter introduces the concept of induced dipole attractions, highlighting their similarity to ionic bonds in terms of involving attractions between positive and negative partial charges on molecules. It poses a question about the nature of nonpolar molecules like nitrogen in this context.
• 01:00 - 03:00: London Forces and Dispersion Forces This chapter discusses the existence of halogen molecules like bromine and iodine in liquid or solid states despite being nonpolar. It highlights the question of how nonpolar diatomic molecules with identical atoms sharing electrons equally can exist in condensed states such as liquids and solids at room temperature (notably iodine, which is a solid).
• 04:00 - 06:00: Strength of Induced Dipole Attractions The chapter discusses the concept of induced dipole attractions, also known as London forces or dispersion forces. It emphasizes that although certain molecules lack distinct positive or negative sides, there are still attractive forces at play. These forces allow the molecules to adhere together, facilitating the formation of condensed liquids or solids.
• 06:00 - 09:00: Boiling Points of Halogens and Hydrocarbons This chapter explains the boiling points of halogens and hydrocarbons, emphasizing the role of intermolecular attractions. It discusses how even in neutral and nonpolar atoms or molecules, like a helium atom—a noble gas that doesn't bond—the electron distribution can vary. This is attributed to the electron's nature of being like a cloud, leading to instantaneous polarities and attractions despite an overall neutral charge.
• 09:00 - 12:00: Molecular Velcro and Effect of Shape This chapter discusses the concept of electron distribution within atoms and molecules, specifically focusing on moments when the electron density is asymmetric. Such asymmetry can lead to a concentration of electrons on one side of the atom. This distribution results in a temporary negative charge that can influence adjacent electron clouds by repelling them. The chapter explores the implications of this fluctuating electrical nature.
• 12:00 - 15:00: Ranking Boiling Points of Different Molecules The chapter discusses how a temporary dipole can be induced in molecules. When a cloud shifts a neighboring electron cloud away, it creates an asymmetrical distribution of electron density. This causes one side of the atom to be slightly positive and the other to be slightly negative, leading to an attraction between these induced dipoles.

L26 IMFs P2. Induced Dipole Attractions Transcription

• 00:00 - 00:30 In this video, we're gonna talk briefly about  what are known as induced dipole attractions.   So now that we've established that intermolecular  forces are in some ways analogous to ionic bonds,   that is they involve attractions between positive  and negative partial charges on molecules,   brings up an interesting question, which is how  is it that nonpolar molecules, such as nitrogen or
• 00:30 - 01:00 the halogens, bromine, iodine, and so forth, how  is it that these molecules can exist as liquids,   or in some cases even solids, iodine at room  temperature is a solid, how is it that these   things can exist in these condensed states if  they're completely nonpolar because, of course,   by definition these diatomic molecules have two  identical atoms that share electrons equally,
• 01:00 - 01:30 so there is no positive or negative side of  such molecules, and yet there must be some sort   of attractive force between these molecules that  allows them to stick together and form a condensed   liquid or solid. So, these types of attractions  go by various names, often called induced dipole   attractions, London forces, dispersion forces,  essentially the concept of these induced dipole
• 01:30 - 02:00 attractions is that in atoms or molecules that  are completely neutral and nonpolar, for example,   let's take a helium atomm remember helium is a  noble gas so it doesn't bond to anything, and in   theory at any given instant the electron should  be uniformly distributed around the atom. But   since electrons we think of as being this sort of  cloud or amorphous moving material, it's possible
• 02:00 - 02:30 that the electron distribution, or the electron  density can for at least a moment be asymmetric   call in an atom or molecule. And if, for example,  we have electrons concentrated on one side of the   atom in the cloud at some instant, that will then  create a slight excess of negative charge that   may repel the electrons in the neighboring cloud.  And if that happens, that fluctuating electrical
• 02:30 - 03:00 cloud can temporarily induce a dipole because  as it shifts the neighboring electron cloud away   from itself it creates an asymmetry in the two  clouds. And as soon as that asymmetry occurs,   it means that one side of one of the atoms  is slightly positive, while the other side is   slightly negative, and has more electron density.  And as a result there's an attraction between the
• 03:00 - 03:30 slight positive of one atom and the slight  negative of the neighboring atom. And so we   would say that we have these mutually reinforcing  induced dipoles. Now of course. in a gas or even   in a liquid because these are temporary dipoles  that are just caused by fluctuations in electrical   clouds, these induced dipoles and these temporary  polarities will disappear almost as soon as
• 03:30 - 04:00 they're formed. So, in one moment we'll have this  induced dipole in this attraction, in the next   instant these electron clouds will fluctuate back  in the other direction. However, if you have a   large number of atoms or molecules, if you sort of  time averaged over all of the atoms and molecules   in the sample there is enough of these temporary  fluctuating attractions to allow these materials,
• 04:00 - 04:30 if it's cool enough, to condense into a liquid  or solid state. And so once again these temporary   fluctuating dipoles are called dispersion forces,  or induced dipole, induced dipole attractions. Now, the strength of these induced dipole  attractions turns out to be very important in   understanding the trends in all sorts of physical  properties of lots of different substances. And so
• 04:30 - 05:00 we want to talk a little bit about the idea of  what causes these induced dipole or dispersion   forces to be stronger in some substances than  others. So basically, the concept here is if we   were to compare two different atoms, one that's  relatively small and one that's relatively large,   and think about the size of that electron cloud  and what happens as we shift the electron cloud
• 05:00 - 05:30 and distort it. We recognize that in a larger  atom with a large electron cloud, that's not   held as tightly, that electron cloud can shift  more dramatically than it can in a small atom.   And as a result, it creates a larger asymmetry,  where there's more positive and negative, in that   large atom, than there would be in a smaller atom  that shifts. And so we would say that these larger
• 05:30 - 06:00 atoms are more polarizable, meaning it's easier to  induce this temporary dipole and polarity in these   large atoms. Some evidence to show that that's  the right way to think about things is to look,   for example, some substances that are very similar  but have different sized electron clouds. And so,
• 06:00 - 06:30 for example, we could look at me halogens:  fluorine, chlorine, bromine, iodine, that   are all in the same column, they're all diatomic  nonpolar molecules. And if we look at the boiling   points and melting points of these substances,  we recognize that, of course, for fluorine,   the melting and boiling point is lower than it is  for chlorine. As we move from chlorine to bromine,
• 06:30 - 07:00 it goes up even more. In fact, fluorine and  chlorine are both gases at room temperature,   while bromine is a liquid, so it's boiling  point is above room temperature, and then as   we move from bromine to iodine it goes up even  more, that is the iodine molecules are even   more strongly attracted and are in fact solid at  room temperature. Now, since the size of electron   clouds goes up with the number of electrons, and  since the number of electrons goes up with the
• 07:00 - 07:30 number of protons and also neutrons, there is  a strong correlation between molecular weight   and polarizability. So, in essence molecules  that have a larger molecular weight will tend   to have larger atoms with larger electron clouds  that are more easily distorted and so they will   tend to have higher melting and boiling points.  Now, a similar but slightly different version of
• 07:30 - 08:00 that argument can be seen here, where we have a  graph of boiling point versus molecular weight,   for a series of hydrocarbons: pentane,  hexane, heptane, octane, nonane. Now,   at first one thing that might be confusing about  this is that in every one of these molecules
• 08:00 - 08:30 we're talking about the same sized atoms. We  have carbon atoms and hydrogen atoms, and so   regardless of which molecule we're talking about  the polarizability of a carbon atom is roughly the   same in all of them same for the hydrogen atom,  and yet as the chain of the molecule gets longer   and longer. We can see that the boiling point goes  up, and so obviously there must be stronger and   stronger attraction between molecules. And one  common way to think about this is in terms of
• 08:30 - 09:00 what they often call molecular velcro, as being  a nice analogy to describe dispersion forces,   and simply the concept here is that like having  pieces of velcro that are, say, attached to wood,   if you have two relatively small pieces of wood  that have velcro on them and are stuck together,   they will, those two boards or pieces of wood will  be easier to pull apart, then if say you had two
• 09:00 - 09:30 longer boards that were covered with velcro  and stuck together. It would take more effort   to separate those two boards. And so essentially  even though per square inch there's no difference   in the attraction of the velcro, the fact that you  have more square inches or more area of velcro on   the long molecules means that will be harder  to separate those molecules and they'll have   higher boiling points. And that way of thinking  is reinforced even further in thinking about the
• 09:30 - 10:00 effect of shape on these dispersion forces. So,  for example, let's take two different isomers of   c5h12 on the left we have pentane. five carbons in  a row, and on the right we have dimethyl propane,   an isomer of that right but arranged in the  slightly different way, a structural isomer.
• 10:00 - 10:30 And if we think about the molecular velcro on  these molecules we consider that in the case   of pentane, where the carbons are all in what  we would call a straight chain arrangement,   there's a larger area for interaction between a  larger number of those carbon and hydrogen atoms,   then there is in the dimethyl propane, which is  sort of a more globular kind of shaped molecule.   And not as many atoms can interact with each other  on adjacent molecules smaller area for interaction
• 10:30 - 11:00 less attractive force. And, of course, this means  that the boiling points for the two substances are   different, in fact, quite dramatically different  so much, so that pentane on the left has a boiling   point that's above room temperature, that is  pentane is a liquid, whereas dimethyl propane,   on the right, has a boiling point that's below  room temperature. And so it's a gas at room
• 11:00 - 11:30 temperature, even though the two substances have  exactly the same molecular weight. So finally,   we can take those concepts and apply them you can  challenge yourself to rank the boiling points of   the four substances listed here. So, I would  encourage you to pause the video and to think
• 11:30 - 12:00 about what we just talked about and try to rank  the boiling points of these four substances. Of course, one of the first things we need to  think about is what are the molecular weights   and structures of these different substances. Of  course, we know that oxygen, o2, is a diatomic   molecule with two oxygen atoms, molar mass of  32. Nitrogen is another diatomic molecule, n2,
• 12:00 - 12:30 with a molar mass of 28. We remember butane from  our study of organic chemistry, is CH3CH2CH2CH3,   whereas methyl propane is CH3CHCH3, with a methyl  group sticking off the middle carbon. Of course,
• 12:30 - 13:00 they're both c4h10, so they both have a  molar mass of 58, but they're two different   structural isomers. And of course, we know  that based on what we just talked about,   that the smallest boiling point or the lowest  boiling point of these should be for nitrogen,
• 13:00 - 13:30 since it has the lowest molecular weight. Then,  second should be oxygen, 32. And then, the   highest boiling point should be the two isomers  of butane. And of course, the straighter version   butane should have a higher boiling point than  the methyl propane, which because of this shape   does not stick together as well. So, if we look  at the actual boiling points of these substances,   which are listed here, we recognize that is, in  fact, the case nitrogen has the lowest of these.
• 13:30 - 14:00 And you'll note that I put these in both Kelvin  and Celsius, since the celsius temperatures are   mostly negative numbers makes a little harder  to compare. If we look instead at the absolute   boiling point, and Kelvin we can see that, in  fact, that nitrogen is the lowest number, 77, and   it goes up next oxygen, then methyl propane, and  finally the highest of these butane has expected.