Sciencemadness Discussion Board - General Chemistry II

General Chemistry II

chstudent24 - 8-7-2009 at 08:55

Hey everyone. I took chem 1 my freshman year and did alright. I attempted to take chem 2 a few times and had to drop. Now its 4 years later, I have a job and I've completed everything in my degree but this last annoying course. Since I'm working, I'm taking this class via correspondence, so I can take my time and really understand things. I'll post my questions/confusions here since this subject really seems to confuse me.

The first question

What kind of intermolecular attractive force is shown in each of the following cases? (They show little circle diagram shapes with 5 dots connection each)

a. HF....HF
b. FF.....FF
c. Na+.....OH2
d. SO2.....SO2

I'm really stumped on how to even begin.

entropy51 - 8-7-2009 at 09:22

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JohnWW - 8-7-2009 at 09:27

I would say:
(a) hydrogen bonding;
(b) Van-Der-Waals intermolecular forces (which are the weakest of the intermolecular forces other than gravitation);
(c) solvation of a cation due to electrostatic force arising from the polarity of the O-H covalent bonds with a slight negative charge on the O and the slight degree of ionization of water (and involving also weak dative bonding of electron pairs on the oxygens to the empty 3s and 3p orbitals on Na+, but only a very few salts with hydrated Na+ cations can be isolated as solids by crystallization at low temperatures, such as natron or washing soda, Na2CO3.10H2O, and Glauber's salt or thenardite, Na2SO4.10H2O); and
(d) partly the same as in (b) plus substantial contribution from intermolecular dipole electrostatic attraction as in (c) due to the polarity of the S=O bonds, with the Os being slightly negative and the S being slightly positive.
None of the intermolecular bonds are actual chemical bonds.

[Edited on 9-7-09 by JohnWW]

1281371269 - 8-7-2009 at 09:31

Entropy that's not strictly true - help will be given for free but you have to pose the questions in more subtle ways. Try calling the thread 'Intermolecular Forces' and mention a failed practical somewhere in your post. You'll get loads of replies.

The answers 'ionic' and 'covalent' presumably are simpler than required here?

entropy51 - 8-7-2009 at 09:59

I think so. In solution and solids you have attractive forces that are not chemical bonds, which is what you have guessed. More likely the three kinds of van der waals forces: (1) dipole-dipole, (2) dipole-induced dipole, and (3) induced dipole-induced dipole.

But charging for tutoring might buy Polverone a better web hosting service.

A better first post might have started off "my textbook describes the types of intermolecular forces as blah blah blah" just to show that you had at least read the book before asking someone else to do your homework.

[Edited on 8-7-2009 by entropy51]

1281371269 - 8-7-2009 at 10:10

But it might reduce my lab budget even further!

'Van der Waals forces include attractions between atoms, molecules, and surfaces. They differ from covalent and ionic bonding in that they are caused by correlations in the fluctuating polarizations of nearby particles (a consequence of quantum dynamics).'

I'm trying to understand this, however...
It says though that they mainly refer to organics, whereas the bonds given in the first post seem to be inorganic?

[Edited on 8-7-2009 by Mossydie]

DJF90 - 8-7-2009 at 10:23

Van der waals forces act on every molecule, whether they be organic or inorganic. The force arrises due to momentary unequal elecron density in the bond (as the electons are orbiting the nuclei - you can't expect them to sit in equally between the two nuclei (only for a small proportion of time

) causing a momentary dipole. This can then interact with another momentary dipole on an adjacent molecule, induce a dipole on an adjacent molecule, or interact with a permanent dipole on an adjacent molecule.

chstudent24 - 12-7-2009 at 14:37

Thanks for the replies everyone. I apologize for not doing a great job on my first post. I'm not here to steal answers for homework. I don't care about chemistry, but I accept the fact that i have to take this course, so I will put forth the work. I don't need any answer responses, i find a question or something leading me to the answer more helpful.

vulture - 12-7-2009 at 14:57

Quote:

I don't care about chemistry, but I accept the fact that i have to take this course, so I will put forth the work.

I appreciate your honesty. However, let's perform a little thought experiment. Assume someone comes to a forum which is about your passion, asks a question which can be deduced with some elbow grease and then says they don't really care about the subject.

Would you be inclined to offer this person your assistance whilst withholding your scorn?

chstudent24 - 12-7-2009 at 20:04

Quote: Originally posted by vulture

Quote:

I don't care about chemistry, but I accept the fact that i have to take this course, so I will put forth the work.

I'm just being purely honest with the situation this subject frustrates me a lot, i'm putting this out there since i'm always open to change. I'm hoping the passion of you guys will rub off on me.

But whatever the case i'll put more elbow grease into it before posting again. I'll post up my work/attempt and thought process which will be more helpful to me and also the people willing to help.

Cheers

[Edited on 13-7-2009 by chstudent24]

chstudent24 - 12-7-2009 at 20:41

Ok so here is my updated thought process please let me know if I'm wrong in my thinking.

a. HF....HF

Ok so this is a Hydrogen bond since they exist when H is covalently bonded to N, O, or F which we have here. And since F has a very high EN this makes it very polar which hydrogen bonds are

b. FF.....FF

This is a London Dispersion since the F2 and F2 cancel out the EN of each other creating a non polar molecule which defines a London Dispersion force

c. Na+.....OH2

This is an ionic bond because it first invovles a bond between a metal and non metal and is formed by 2 oppositley charged ions.

d. SO2.....SO2

Dipole-Dipole because this force requires 2 polar molecules. The polarity can be easily determined from the bent shape of these molecules

Viscosity (double checking understanding)

chstudent24 - 12-7-2009 at 20:51

Is the viscosity of glycerol larger or smaller than that of 1-propanol?

Viscosity= The resistance of a liquid to flow. From reading around I found that Glycerol is described as a viscous substance. So I'd assume the viscosity of glycerol would larger than that of 1-propanol. The book says viscosity is found via timing a flow through a tube or measuring the rate at which a steal sphere falls through a liquid. These test make perfect sense to me, but how can I determine this on my own (like during a test) with just having the name of a substance like i did in this question?

12AX7 - 12-7-2009 at 21:02

You use a viscometer. There are lots of methods, but keep in mind that, fluid flow being what it is, there are lots of ways to measure -- it's not as simple a standard as, say, measuring the density of a solid block of matter. It also depends on the shear rate, to varying amounts, with varying materials (see: rheology). Wikipedia has a list of common viscometer types.

In your last sentence, are you asking for a table of viscosities of common materials (such as glycerol)? I imagine such a thing exists, have you tried searching?

Tim

kclo4 - 12-7-2009 at 21:25

Perhaps they are expecting one to expect the viscosity by the structure of the molecule? Gylcerol while having three carbon atoms like propanol, it has three hydroxyl groups, while propanol has only one. I'd imagine hydrogen bonds might play a roll in viscosity?
It doesn't seem like the questions really wants a number sort of answer, but it may.

Intermoleculer Forces (again)

chstudent24 - 12-7-2009 at 21:32

What types of intermolecular force is (are) common to a) Xe and methanol (CH3OH), b) CH3OH and acetonitrile (CH3CN), c) NH3 and HF?

XE-CH3OH

London-molecule isn't non polar so can't be
Dipole-Dipole-moleclue is polar so dipole-dipole
Hydrogen-no N, O, F or C on the left so can't be
Ionic- Can't be because this isnt between a metal and nonmetal

CH3OH-CH3CN

London-the EN is very close i want to say this is a london
Dipole-Dipole-EN come close , the math would say its a close polar molecule so i'd say Dipole-Dipole
Hydrogen-I see H bonded to O and C but not sure if i'm making a mistake here to label this a hydrogen bond
Ionic- Can't be because this isn't between a metal and nonmetal

NH3-HF

London-isn't polar so can't be
Dipole-Dipole-is polar so does have this but hydrogen since of very high polarity
Hydrogen- H is bonded to an N and F so i'd assume this is a hydrogen bond
Ionic- Can't be because this isn't between a metal and nonmetal

not exactly sure if the question was asking which do they all have in common jointly, if it was I would say Dipole-Dipole from what i wrote above. Please correct any flaws with my reasoning!

chstudent24 - 12-7-2009 at 21:36

thank you for the responses. After looking back at the question I figure it was trying to have me focus on the shape of the molecule to identify the viscosity.

vulture - 13-7-2009 at 11:22

Quote:

NH3-HF

This seems to be a trick question. Ammonia and hydrogen fluoride will form ammonium fluoride, which is a salt. That should give you a clue tot the interaction.