2.5 Intermolecular forces
Posted by: intechemistry on: September 23, 2010
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20 Responses to "2.5 Intermolecular forces"
December 11, 2010 at 1:46 pm
another confusion here sir.
regarding the strength of the intermolecular forces. as I understood, the strength follows this way: H-bond > dipole-dipole > dispersion force.
but, there’s one ques asked about the strongest intermolecular forces between HCl molecules, the answer is dispersion forces not dipole-dipole interaction as I thought.
when it asks about strength, do we consider it as cumulative strength a particular intermolecular force that contribute to overall strength in bonding of that molecule or do we consider one intermolecular force itself?
December 11, 2010 at 3:57 pm
#1 hidayah December 11, 2010 at 12:35 pm
Re: spec not mentioning anything about van der Waals forces… Perhaps it’s an attempt to try and reduce some of the (very)casually used terminonoly, which is a long running cause of to confusion. In fact, I think I’ve changed my mind once or twice on this matter over the years, when reading alternatice authoritive/trusted sources of information. But the actual ‘name’ used is no where near as important as identifying which interactions are taking place.; What lies underneath the ‘name’ if you will/
My understanding = vdW covers a) b) and c) below
a) Interactions(attractions) between permanent dipoles, sometimes called (dipole-dipole interactions
b) Interactions(attractions) between permanent dipoles causing induced dipoles in other molecules, sometimes called permanent-induced dipoles
c) Interactions(attractions) between temporary dipoles and induced(or other temporary) dipoles.
LDF seems to be involved with b) and c) only i.e. where there are temporary and/or induced dipoles. (although LDF terminology is also used very loosley)
So this is in agreement with Chemguide. Also with these
http://www.chem.purdue.edu/gchelp/liquids/disperse.html
http://www.britannica.com/EBchecked/topic/622645/van-der-Waals-forces
Did this help?
#2 hidayah December 11, 2010 at 1:46 pm
The comparisons are usually made for similar sized molecules. After all, poly(ethene) isn’t polar but has induced-induced dipoles and as the molecules are v.v. bign the total force is greater than that of a singly permanenently polarised molecule interacting with another single molecule. It’s pretty meaningless to compare two things that differ in 4 important features so, H-bond > dipole-dipole > dispersion force is usually on a (small)molecule intercting with another (small)molecule basis.
I think my 9m2 student, “superman”, asked about this HCl thing before. The fact HCl is a gas (at RTP) means its dipole strength must be very low. I think the factors of: atoms involved, sizes, bond lengths, polarisability and polarisation, all result in HCl molecules actually being able to efficiently distrubuting any uneven charges that we might think exist when we ‘visualise’ the molecule. Note: HBr is also gaseous!
You were pobably told HCl is a gas. That was clue to realising what force was present. If HCl was a liquid, then you would be expected to say it was dipole-dipole.
Does that cover your Q’s (and does it anwer them?)
December 11, 2010 at 4:50 pm
for my first Q,so, it is safer to use terms that are mentioned specifically and understand each interaction.
ok, got it.
as for my 2nd Q, the Q doesn’t state specifically whether the HCl is gas or liquid. so, how I should answer that? both dipole-dipole and induced-dipole interactions are involved in the bonding between HCl molecules, how should I determine the strongest intermolecular forces?
December 11, 2010 at 6:59 pm
Yes. I’d say it’s safer. You can often get a handle of what’s ok and what’s not, by looking at past years papers.
Jan 2009 U2 Q20 p12
(iii) Name the strongest intermolecular force between BF3 molecules. (1)
(iv) Name the strongest intermolecular force between NH3 molecules. (1)
Answer:
(iii) Instantaneous dipole – induced dipole /
temporary dipole – induced dipole /
Induced dipole – Induced dipole /
/ London forces / van der Waals forces
No mention of rejected answers.
(iv) Hydrogen bonds / H-bonds Rejected answer: ‘Hydrogen’ alone
Sept’07 Specimen Unit 2 Q24 b)
(v) Name the strongest intermolecular force between boron trichloride molecules. (1)
Answer:
London forces / instantaneous dipole-Induced
dipole/dispersion /v der Waals
Temporary or instantaneous can be used instead of
induced (1)
Rejected answers:
“dipole” forces/ permanent dipole/ dipole-dipole vdw
They give some flexibility.
Are you sure they gave no clue? Did they give a graph of bpt for the various halogen halides? If there is no clue, then the person who wrote the Q is expecting you to remember HCl has no permanent dipole as a fact. If they give clues/graphs/data then you can deduce the answer.
December 11, 2010 at 7:41 pm
should be there is H-bonding between NH3 molecules. It is not considered as the strongest intermolecular force here. Is it because the collective strength of induced-dipole interaction largely contributes to overall strength in NH3 molecules?
so, we can totally ignore the small effect of H-bonding, though individually it is stronger than induced-dipole interaction. is that the reason sir?
December 11, 2010 at 7:55 pm
Sorry hidayah, the answer for NH3 was missing. It’s there now.
Hopefully that makes things ok now.
February 1, 2011 at 6:00 am
sir,
i just read GF AS, it stated that induced dipole is stronger than permanent dipole, but as what i understood, induced dipole is the weakest intermolecular forces.
if can, could u please check the GF page 165. maybe i interpret them wrongly
February 1, 2011 at 4:53 pm
I’ll check the GF reference later, but induced dipoles can be stronger than permanent dipoles. e.g. as witnessed by the fact poly(propene) has a higher melting point than ammonia.
But this is not a fair comparison as the size of poly(propene) and ammonia is extremely different, hence the total IMF experienced by poly(ethene) is much greater than the hydrogen bonding ammonia.
Most fair comparisons are made between two ‘things’ that only differ in one aspect, not two as in the above example (i.e. size and IMF type)
If GF says says induced are stronger than permanent then he must be talking about different sized molecules.
I will check later. I don’t have GF with me now.
February 1, 2011 at 8:28 pm
Agh! I can’t find my GF for AS level. I think I lent it out. Maybe u could take a pic of it and mms / e-mail me???
February 1, 2011 at 9:58 pm
sir unfortunately i left AS GF at college hehe i just arrived from shah alam today
sorry sir i’ll see u after hols yea 
May 22, 2012 at 8:51 pm
Sir, just to confirm the strength of forces:
1)Cov bond (intra) > Hbond (inter)> permanent dipole > London forces aka Induced/ temporary dipole CORRECT?
2)Is permanent dipole the intermolecular forces found in HCl? Only polar molecules have permanent dipole right? I know that non-polar molecules won’t have permanent dipole, but can polar molecules have London forces (sometimes called induced dipole, instantaneous dipole, temporary dipole, van der waal forces SO MANY NAMES!) by the way, which one is the most appropriate one? I always imagine that van der waals is the ‘umbrella’, branching under it are the london forces, induced dipole, yetta yetta~~
May 22, 2012 at 11:11 pm
1) Yes, if you are talking about small molecules, (i.e. molecules with only a small numbers of electrons – which you almost certainly will be talking about)
Plastics (many many many many many London forces per polymer molecule) and Iodine (many temporarily polarizible electrons) are interesting as their London forces cause these molecules to be solids!
2) The permanent dipole is disputable. HCl is a gas!!, {error corrected:} BOILING point = -85° C !!!
That’s a pretty damn lousy dipole.
Yes only polar molecules have permanent dipoles.
ALL molecules have London dispersion forces – sometimes we never bother to identify then (e.g. in water) as far more stronger, hence significant, IMF’s are present instead. If you had RM1m, you wouldn’t really care about losing 5cen.
The distinction between Van derWalls and London dispersions blurred long ago. They are interchangeable. It’s my understanding (and I’ve seen books with contradictory info on this) that VdW is the ‘umbrella’ containing H-bonding, permanent dipoles, and Ldf’s, but most people demote VdW to mean the same as the London dispersion forces.
In that case, VdF’s, VdW’s, induced dipole, temporary dipole, instantaneous dipole are all the same, pretty much bottom of the barrel kinda forces.
May 22, 2012 at 11:27 pm
Alright.
I read about people saying London force is stronger than permanent dipole. Their argument is that : ‘Boiling points from HCL to HI increases,
as going down the group the electron number increases, and therfore more van der waals, but permenant dipole decrease, due to Iodine being less electronegative therfore forming less permenant dipole forces. so the vdw forces is stronger and outweighs the weaker permenant forces, otherwise HCL would be the highst boiling point due to its having more permenant forces and better.’
The way this person reasons it make ‘some’ sense but…WHY? just afraid some questions about intermolecular forces might come out as objective question (arrange forces from weakest to strongest etc)
May 22, 2012 at 11:58 pm
To make general statements, a person has to consider the general situation. The dipole in HCl is pretty weak (it’s low bpt is direct proof of that) and it’s quite an exceptional case where a relatively large electronegativity difference between (non-uniformly) bonded atoms only gives rise to a small dipole. There are things going on in HCl which are not covered at A-level. So it’s a bad single case to make the generalisation from!
For small molecules, the permanent dipole is a more significant (stronger) than Ldf’s.
It is almost certain that you will get some Q’s based on IMF’s
H-Cl, H-Br do have permanent dipoles (it’s debatable if H-I has one)
From left to right the permanent dipole strength drops.
from left to right the Ldf’s forces grow.
The growth of the Ldf’s is much more significant than the decrease in the dipole,
so overall it’s the change in the Ldf’s which is responsible for the bpt trend.
May 23, 2012 at 5:52 am
Sir,if the Q is smthn like the June 2011 Q. bt this time describe formation of permenant dipole-dipole bond. Is this answer sufficient. (2 to 3m)
The dipole-dipole bond only arises in polar molecules that form attraction between different slightly (opposite) charged ends of molecules.
May 23, 2012 at 9:46 am
Sir why does Facer (AS) state that London dispersion force is stronger than permenant dipole-dipole force? bt Im pretty sure its wrong. dont ya think so?
May 23, 2012 at 12:39 pm
#18 | Awesome. May 23, 2012 at 5:52 am
Your answer seems pretty reasonable to me.
You’ve got the a) atraction b) polar molecules c) different polarties within the polar molecule.
#19 | On behalf of ariff
I suspect it’s a case where you either DON’T have small molecules or a case where the dipole is pretty weak. There are cases where these Ldf’s can be stronger than permanent dipoles.
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December 11, 2010 at 12:35 pm
salam sir.
sir, in the syllabus, does it totally explain dipole-dipole interaction separately from van der Waal forces? as I’ve checked in the spec, it doesn’t state anything about van der Waals forces
act, I have a bit problem in understanding it. I’ve read in chemguide (the sitelink), it states that (from what I understand), “van der Waals forces can be separated into dispersion forces also known as London forces and the other one is dipole-dipole interaction.
but for the spec, we are supposed to understand “the nature of intermolecular forces resulting from interactions between permanent dipoles, instantaneous dipoles and induced dipoles (London forces).”
this topic always make me confused everytime I’ve read it.