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  • - [Instructor] Let's talk a little bit

  • about ion-dipole forces.

  • And before we think about how ions

  • and dipoles might interact,

  • let's just remind ourselves

  • what the difference is between ions and dipoles.

  • And I encourage you to pause this video

  • and try to refresh your own memory

  • before we refresh our memories together.

  • All right, now let's first think about ions.

  • Ions are atoms or molecules that have a net charge.

  • So for example, when chlorine gains an electron

  • and becomes the chloride ion,

  • it's an ion because it now has a net negative charge.

  • Similarly, when sodium loses an electron,

  • it now has a net positive charge.

  • So this is the sodium ion.

  • Now what's the difference between that and a dipole?

  • Well, generally speaking when we're talking about dipoles,

  • we're not talking about something

  • that has necessarily a net charge,

  • we're talking about something where the charge is separated

  • on different ends of the molecule,

  • that you have a partially positive end

  • and you have a partial negative end,

  • that there is a molecular dipole moment.

  • And a good example of a molecule that is a dipole

  • or has a dipole moment at a molecular level is water.

  • Water is a very polar molecule.

  • We've talked about this many times.

  • You have your oxygen which is quite electronegative,

  • covalently bonded to two hydrogens,

  • and those are really polar covalent bonds

  • because the oxygen's so much more electronegative

  • that it hogs the electrons, it's selfish of the electrons.

  • And since the electrons spend more time around the oxygen

  • than around the hydrogen,

  • you have a partial negative charge

  • at this end of the molecule

  • and you have partial positive charges

  • at the other end of the molecules.

  • And we describe this

  • when we talked about hydrogen bonding

  • where the partial negative end of one water molecule

  • would be attracted to the partial positive end

  • of another water molecule.

  • But, as we've talked about, hydrogen bonds,

  • which are an intermolecular force

  • are just a special case of dipole forces.

  • Things that are able to form hydrogen bonds

  • just have a very strong dipole moment,

  • because you have hydrogen bonded to an oxygen,

  • a nitrogen, and a fluorine, that is quite electronegative.

  • So now that we know the difference between ions and dipoles,

  • how might they interact?

  • Well you might guess Coulomb forces are at play.

  • The partial negative end of a dipole would be attracted

  • to a positively charged ion.

  • And I have prearranged these water molecules

  • so that you have the partial negative end

  • is facing towards this sodium positive ion.

  • And so what I'm drawing right over here,

  • these are ion-dipole forces.

  • Similarly, if you have a chloride anion, or a negative ion,

  • well then the partially positive ends of the dipoles

  • are going to be attracted,

  • and so water might arrange itself in this way

  • where the partial positive ends, the ends with the hydrogen,

  • are facing the chlorine.

  • And this is one of the reasons

  • why it's so easy to dissolve sodium chloride,

  • to dissolve table salt in water.

  • Those ions are able to separate

  • and be attracted to the water molecules which are polar,

  • which have molecular dipoles.

  • Now, if I were to ask you what's gonna dictate the strength

  • of the ion-dipole forces, think about that.

  • Pause this video, and what do you think is going to matter?

  • Well, as you can imagine, these are Coulomb forces.

  • So the strength of the charges matter.

  • So you're gonna have a stronger ion-dipole force

  • if you have stronger charges on the ions.

  • So instead of a sodium with a positive one charge,

  • if you had a calcium ion that had a positive two charge,

  • then the partially negative ends of the water molecules

  • would be even more strongly attracted.

  • You would have stronger ion-dipole forces.

  • Similarly if you have stronger dipole moments,

  • that will also make the ion-dipole forces stronger,

  • or vice versa.

  • If you had a molecule that had a weaker dipole moment,

  • you're not going to have as strong ion-dipole forces.

  • Coulomb forces are inversely proportional

  • to the distance between the charges.

  • So you're also going to have stronger ion-dipole forces

  • the closer that these things get to each other.

  • But to some degree that's true of a lot

  • of the intermolecular forces we've talked about,

  • because on some level they are all Coulomb forces.

- [Instructor] Let's talk a little bit

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B2 中上級

イオン・双極子力|分子間力と物性|AP化学|カーンアカデミー (Ion-dipole forces | Intermolecular forces and properties | AP Chemistry | Khan Academy)

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    林宜悉 に公開 2021 年 01 月 14 日
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