Permanent dipole-dipole interactions

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.

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.