forces- chemistry a level
why is permanent dipole dipole forces stronger than induced dipole dipole? i searched it up and no one seems to give a reason for it?
Original post by vix.xvi
why is permanent dipole dipole forces stronger than induced dipole dipole? i searched it up and no one seems to give a reason for it?
They are not necessarily stronger.
The dipole in a molecule with a permanent dipole is stronger than an instantaneous dipole, or a dipole causes by the instantaneous dipole (induced dipole). Just accept it as at a level you don’t need to go into much detail about it. an instantaneous dipole is just caused by electron density randomly fluctuating, and it doesn’t really fluctuate enough to create a dipole as large as those in a molecule with a permanent dipole. And you only really have permanent dipole forces if there is a significant difference in electronegativity, eg between C and H there’s a difference of 0.4 but you don’t say it has permanent dipole forces. And in large molecules eg hexanal, the London forces are stronger than the pd-d forces as there is a large number of electrons, so the size of the molecule makes a difference.
(edited 3 years ago)
Original post by hibam
Because London dispersion forces are temporary, they're weaker than the permanent dipole-dipole attraction
Original post by Pigster
They are not necessarily stronger.
Original post by i_love_dogs
The dipole in a molecule with a permanent dipole is stronger than an instantaneous dipole, or a dipole causes by the instantaneous dipole (induced dipole). Just accept it as at a level you don’t need to go into much detail about it. an instantaneous dipole is just caused by electron density randomly fluctuating, and it doesn’t really fluctuate enough to create a dipole as large as those in a molecule with a permanent dipole. And you only really have permanent dipole forces if there is a significant difference in electronegativity, eg between C and H there’s a difference of 0.4 but you don’t say it has permanent dipole forces. And in large molecules eg hexanal, the London forces are stronger than the pd-d forces as there is a large number of electrons, so the size of the molecule makes a difference.
tysm!!
Original post by vix.xvi
why is permanent dipole dipole forces stronger than induced dipole dipole? i searched it up and no one seems to give a reason for it?
HCl(g) has pdd with a BP of -85 oC
I2(s) has idd only with a BP of 185 oC
Are pdd stronger than idd?
Original post by Pigster
HCl(g) has pdd with a BP of -85 oC
I2(s) has idd only with a BP of 185 oC
Are pdd stronger than idd?
I2(s) has idd only with a BP of 185 oC
Are pdd stronger than idd?
ohh
hmmmm
then why did my teacher say they are ? :/ im rly confused now
Original post by Pigster
HCl(g) has pdd with a BP of -85 oC
I2(s) has idd only with a BP of 185 oC
Are pdd stronger than idd?
I2(s) has idd only with a BP of 185 oC
Are pdd stronger than idd?
The main intermolecular force in HCl is London forces as Cl isn't very electronegative it doesn't really form hydrogen bonds. And as I2 is a relatively large molecule, it has lots of electrons so the London forces are quite strong.
Original post by i_love_dogs
The main intermolecular force in HCl is London forces as Cl isn't very electronegative it doesn't really form hydrogen bonds. And as I2 is a relatively large molecule, it has lots of electrons so the London forces are quite strong.
Hang on... chlorine isn't very electronegative. I don't think you'll find a whole lot of agreement on that one. There are only three elements with a higher electronegativity!
HCl is most definitely polar, with a difference in electronegativity values of around 0.4. Without the dipole is wouldn't be anywhere near as soluble in water as it is.
Original post by vix.xvi
ohh
hmmmm
then why did my teacher say they are ? :/ im rly confused now
hmmmm
then why did my teacher say they are ? :/ im rly confused now
I didn't make a fair comparison.
As you should know, the strength of idd mostly depends on the number of e-. More e- = stronger idd.
A better one is to look at Br2, which has 70 e- and a BP of 59 oC. This has idd only.
And compare it to ICl (iodine monochloride), which also has 70 e- but has a BP of 97 oC. This has idd (all molecules have e-, so have idd) but it also has pdd. The extra force due to the pdd makes the intermolecular attraction stronger, hence...
Comparing HCl to I2 isn't a fair comparison since HCl has 18 e- so has very weak idd as well as the pdd, whereas I2 has 106 e- and hence has much stronger idd. The idd in I2 are clearly stronger than the combined idd and pdd of HCl.
If I had have been fair and picked a non-polar (i.e. idd only) molecule with 18 e-... well methane is the closest I can think of right now with 16 e-, that has a BP of -162 oC. This shows that the idd in methane is much lower than the combined idd and pdd in HCl.
Original post by Pigster
Hang on... chlorine isn't very electronegative. I don't think you'll find a whole lot of agreement on that one. There are only three elements with a higher electronegativity!
HCl is most definitely polar, with a difference in electronegativity values of around 0.4. Without the dipole is wouldn't be anywhere near as soluble in water as it is.
HCl is most definitely polar, with a difference in electronegativity values of around 0.4. Without the dipole is wouldn't be anywhere near as soluble in water as it is.
OOPS I think hcl is a weird one as it doesn't form H bonding, I think its something to do with the size of cl.
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