Chem question !!

[FutureDental88]

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I dont really understand.... When exactly is boiling point freezing point etc etc based on strength of bonds, (Hydrogen bonds etc), and when is it based on the amont of particles the compound associates into?

Help would be appreciated.

 

[FutureDental88]

Anyone??? If anyone knows. please respond . I got the test coming up in a week..

 

[Josh779]

The greater the van hoff factor (number of dissociating ions) the higher the melting point and lower freezing point. If the van hoff factor is greater than 1, your usually dealing with ionic bonds, thus the higher m.p and b.p. If the van hoff = 1 it is typically covalent/molecular which usually has lower m.p and b.p.

H-bonding and all the intermolecular forces deal with intermolecular forces (obviously), which is typically excluded from ionic bond type of questions. The order is:

H-bonding<---strongest bond i.e. highest m.p. b.p. lowest vapor pressure etc
Dipole-Dipole
London dispersion

H-bonding is the strongest because it has all three of the intermolecular forces. But remember, ionic is still far far stronger than an intermolecular force.

Hope this separates the concepts a little bit. Let me know if you need clarification. Study hard this next week! I take mine in 2 weeks!

 

[FutureDental88]

thx for the clarification Josh !!
So you'are basically saying that if its ionic bondings, then we count how many molecules it dissociates into.

And when its covalent bonding, we check for hydrogen bonding, etc etc ...

Am i correct??


Thanks again for your respons Josh. Appreciate it !

 

[Josh779]

thx for the clarification Josh !!
So you'are basically saying that if its ionic bondings, then we count how many molecules it dissociates into.

And when its covalent bonding, we check for hydrogen bonding, etc etc ...

Am i correct??


Thanks again for your respons Josh. Appreciate it !

Yup, It depends on the context of the question of course, but separating those 2 concepts (colligative vs. intermolecular forces/bonding) in your head should help you tackle any of those types of questions. Good luck!

 

[JohnnnyD]

The greater the van hoff factor (number of dissociating ions) the higher the melting point and lower freezing point....

Firstly, MP and FP are the same point. I think you meant lower freezing point/melting point and higher boiling point.

Assuming that is what you meant, it still isnt fully correct, right? Since the equation for mp is deltaT=kim, it would depend not only on the van hoff factor (i), but also the molality.

for example:
1.0M NaCl would have a higher MP and lower BP than 20.0M NaCl and they both have the same van hoff factor of 2.

Someone correct me if im wrong.

 

[Josh779]

Firstly, MP and FP are the same point. I think you meant lower freezing point/melting point and higher boiling point. [/QUOTE

Good catch, that is correct. And yeah I didn't give the whole story because assuming he knows the equation molarity will have to also be a factor. I was trying to distinguish which points I thought he was getting mixed up, which was between intermolecular and ionic bonds dissociating in a solution

If comparing solutions like your example then multiplying the van hoff by molarity will get you the answer. But if he gets a question saying adding salt to boiling water will do what? Knowing an increasing van hoff factor will increase the boiling point might help for a conceptual question.

Thanks for elaborating tho Johnny. Have you taken your DAT yet?


Edit: Failed quote lol