Permanent dipole-dipole interactions

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HelloGoodbye
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#1
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A question from last year's F321 Chemistry paper asked to name the main intermolecular force in NH3 and PH3. I wrote hydrogen bonding for NH3 which is correct and van der Waals' forces for PH3, but the answer is permanent dipole-dipole interactions.

I don't understand how you'd know that there's a permanent dipole in PH3, I thought it was van der Waals' simply because the it shows that the boiling point is lower than NH3.

Can anyone please help explain how to tell that there's a permanent dipole? I have my f321 exam tomorrow!! Thank you!
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Chlorophile
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(Original post by HelloGoodbye)
A question from last year's F321 Chemistry paper asked to name the main intermolecular force in NH3 and PH3. I wrote hydrogen bonding for NH3 which is correct and van der Waals' forces for PH3, but the answer is permanent dipole-dipole interactions.

I don't understand how you'd know that there's a permanent dipole in PH3, I thought it was van der Waals' simply because the it shows that the boiling point is lower than NH3.

Can anyone please help explain how to tell that there's a permanent dipole? I have my f321 exam tomorrow!! Thank you!
Are you sure it didn't also accept van der Waals? A similar question was in a recent AQA exam and it accepted vdW as an alternative. There is a difference in electronegativity between Phosphorus and Hydrogen so technically it is a polar molecule, but the difference is absolutely minute and I can't believe you'd be expected to know that.
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HelloGoodbye
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(Original post by Chlorophile)
Are you sure it didn't also accept van der Waals? A similar question was in a recent AQA exam and it accepted vdW as an alternative. There is a difference in electronegativity between Phosphorus and Hydrogen so technically it is a polar molecule, but the difference is absolutely minute and I can't believe you'd be expected to know that.
No, the only answer they gave is permanent dipole and it didn't say allow vdW :/
Thanks!
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the1akshay
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(Original post by HelloGoodbye)
A question from last year's F321 Chemistry paper asked to name the main intermolecular force in NH3 and PH3. I wrote hydrogen bonding for NH3 which is correct and van der Waals' forces for PH3, but the answer is permanent dipole-dipole interactions.

I don't understand how you'd know that there's a permanent dipole in PH3, I thought it was van der Waals' simply because the it shows that the boiling point is lower than NH3.

Can anyone please help explain how to tell that there's a permanent dipole? I have my f321 exam tomorrow!! Thank you!
We wouldn't normally be expected to know that. Permanent dipoles typically only happen with N, O, F Cl and Br, with F being the most electronegative. However, H is quite bad at attracting electron, whereas P is only average. And given the structure of PH3, with a lone pair, the dipoles wouldn't cancel so there would be permanent dipole-dipole interaction. I do think that's an exceptionally tricky question though, and I'm surprised they didn't accept VdW as an alternative seeing as both would be present.
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EierVonSatan
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#5
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#5
(Original post by the1akshay)
We wouldn't normally be expected to know that. Permanent dipoles typically only happen with N, O, F Cl and Br, with F being the most electronegative. However, H is quite bad at attracting electron, whereas P is only average. And given the structure of PH3, with a lone pair, the dipoles wouldn't cancel so there would be permanent dipole-dipole interaction. I do think that's an exceptionally tricky question though, and I'm surprised they didn't accept VdW as an alternative seeing as both would be present.
It's a pretty standard question, I think. Permanent dipoles exist where-ever there is a charge difference in the molecule and here it exists due to it's shape (rather than the individual bonds between atoms).

The question is asking for the main intermolecular force so allowing the alternative one would defeat that - it's a light molecule so instantaneous dipole - induced dipoles would be small.
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