Intermolecular Forces - Chemistry Q Please Help

yoyojc

If anyone is able to clarify this for me? It would be a huge help because this chapter is difficult for me to understand Thank you so much.

So if it asks which compound has a higher boiling point you immediately must look at what intermolecular force is present i.e Van Der Waal, Dipole Dipole or Hydrogen Bonding (listed in order of strength)
How do you know which intermolecular bonding is present?

And when you discover which one is present, can you merely say - 'X' has a higher poiling point because it has Hydrogen Bonding (or whichever) and then explain hydrogen bonding with a diagram?

And also..
When they ask for why one has a higher boiling point and you're dealing with elements? How can you tell there?

Thank you so much

Days 298

Van der Waals are temporary dipole dipole bonds present in non polar molecules. H2

Dipole dipole is the attractive forces between the positive end of one polar molecule to the negative end of another. Example HCl

Hydrogen bonding is formed between a highly electronegative element (N,O,F) and a molecule containing Hydrogen Example H20, NH3.

Stronger the type of bond the more energy required to brake it, therefore higher boiling point.

yoho139

Important as well to note how you decide whether they are polar/have large EN differences:

0-0.4 = non-polar covalent bonding (V.d.W., as the difference isn't large enough to form permanent poles)
0.4-1.7 = polar covalent (dipole-dipole, large enough to form permanent poles)
Involving H and N, O or F only (it's defined as a bond between a small atom and H, but only those three count anyway) = Hydrogen bonds as the difference is so large that the H becomes very positively charged.

As for the boiling point question, yes. I don't usually put in diagrams and haven't seen the marking schemes expect one either.

As for the elements, do you have an example?

yoyojc

yoho139 wrote: »

Important as well to note how you decide whether they are polar/have large EN differences:

0-0.4 = non-polar covalent bonding (V.d.W., as the difference isn't large enough to form permanent poles)
0.4-1.7 = polar covalent (dipole-dipole, large enough to form permanent poles)
Involving H and N, O or F only (it's defined as a bond between a small atom and H, but only those three count anyway) = Hydrogen bonds as the difference is so large that the H becomes very positively charged.

As for the boiling point question, yes. I don't usually put in diagrams and haven't seen the marking schemes expect one either.

As for the elements, do you have an example?

So like am I right in saying ..

Okay I see Hydrogen and either (F, O or N) in a bond and I can say it's Hydrogen Bonding which is obviously the strongest and can explain it's high boiling point?

And if I get the electronegativity difference between the two atoms in the molecule and it's between 0 and 0.4 it's automatically a Van Der Waal force. Because it's so weak it's got a low boiling point.

Then if I find the electronegativity value and it's between 0.4 and 1.7 it's automatically a Dipole Dipole bond between the molecules?

If I am correct then is the electronegative charge in Hydrogen bonds molecules like H20 equalled to 0? Because they're non polar..

And then above 1.7 is Ionic?

Thank alot for your help

yoho139

If I am correct then is the electronegative charge in Hydrogen bonds molecules like H20 equalled to 0? Because they're non polar..

Eh...



Firstly, the EN difference is 1.4, so they'll be polar (there's some exceptions, like planar compounds...) and no, it's a H bonded to an O like said above - it has H-bonds. If you're saying that you were told water is non-polar, that's wrong.

1.7+ is ionic, yes.

yoyojc

Yeah apologies I'm now aware of hydrogen bonding and how to spot it I just can't get my head around how to notice when it's van Der waal or Dipole Dipole..

Can you ever say- if the electronegativity difference is between 0 and 0.4 it's van Der waal
0.4-1.7 it's dipole dipole?

Or do you always have to say oh because of its polarity (being polar or non polar)

yoho139

Polarity is what really matters here. BCl3 (for example) seems like it should be polar, but since it's trigonal planar, the center of charge is actually neutral and it's non-polar, leaving VdW as the only option.