Bonding chemistry

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tigera111
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The questions are about comparing nh3 and ph3 melting points and then going on to explain why ash3 has a higher melting point that ph3. What I want to know is if ph3 has dipole dipole and if it's that what I should be referring to in both the questions. I get confused with identifying whether a molecule is polar or not so itd be great if you could explain that too.
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UsernameF40
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(Original post by tigera111)
The questions are about comparing nh3 and ph3 melting points and then going on to explain why ash3 has a higher melting point that ph3. What I want to know is if ph3 has dipole dipole and if it's that what I should be referring to in both the questions. I get confused with identifying whether a molecule is polar or not so itd be great if you could explain that too.
Check out the Pauling Scale
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BDavies1
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(Original post by tigera111)
The questions are about comparing nh3 and ph3 melting points and then going on to explain why ash3 has a higher melting point that ph3. What I want to know is if ph3 has dipole dipole and if it's that what I should be referring to in both the questions. I get confused with identifying whether a molecule is polar or not so itd be great if you could explain that too.
NH3 has a much higher boiling point than PH3 because hydrogen bonding is possible between molecules of NH3.

H-bonding betweenPH3 molecules impossible.

If you look at the next group 5 hydride, AsH3, you will see it has a higher boiling point than PH3. This must be because in PH3 and AsH3, the main type of intermolecular force must be Van der Waals forces (rather than dipole/dipole interactions). AsH3 has more electrons than PH3 so Van der Waals forces are stronger.

Note that if dipole/dipole interactions were important in AsH3 and PH3, then you would expect PH3 to have a higher boiling point because P is more electronegative than As


You get the same trend with group 7 hydrides i.e. HF, HCl, HBr and HI. HF has a very high boiling point as H-bonding is possible. Then the order of boiling points (highest to lowest) is HI> HBr>HCl. HCl has a bigger dipole than HI sa Cl is more electronegative, but it must be that Van d W forces most important- HI bigger than HCl so its boiling point higher.
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charco
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(Original post by BDavies1)
NH3 has a much higher boiling point than PH3 because hydrogen bonding is possible between molecules of NH3.

H-bonding betweenPH3 molecules impossible.

If you look at the next group 5 hydride, AsH3, you will see it has a higher boiling point than PH3. This must be because in PH3 and AsH3, the main type of intermolecular force must be Van der Waals forces (rather than dipole/dipole interactions). AsH3 has more electrons than PH3 so Van der Waals forces are stronger.

Note that if dipole/dipole interactions were important in AsH3 and PH3, then you would expect PH3 to have a higher boiling point because P is more electronegative than As


You get the same trend with group 7 hydrides i.e. HF, HCl, HBr and HI. HF has a very high boiling point as H-bonding is possible. Then the order of boiling points (highest to lowest) is HI> HBr>HCl. HCl has a bigger dipole than HI sa Cl is more electronegative, but it must be that Van d W forces most important- HI bigger than HCl so its boiling point higher.
Be careful to check your individual examination specification when using the term "Van der Waals" forces.

These include both London dispersion forces and permanent dipole dipole interactions, but not hydrogen bonding.

Some specifications lump Van der Waals and London dispersion together as synonyms, whereas others use the (more correct according to the literature) separation of terms.

For example the IB Diploma requires that students do NOT refer to London dispersion forces as Van der Waals, but understand that all intermolecular forces that do not involve electron pair transfer are Van der Waals, whereas hydrogen bonding is not.
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