[xstrae]

Hi,

I have a few doubts on some of these questions:

1.Give reason: NF₃ does not undergo hydrolysis while NCl₃ undergoes hydrolysis.  
No idea about this one.

2. O-hydroxy benzaldehyde and p-hydroxy benzaldehyde. Which of these 2 is
i) a liquid at room temperature?
ii) a high melting solid?

o-hydroxy benzaldehyde has intra-molecular hydrogen bonding and p-hydroxy benzaldehyde has inter-molecular hydrogen bonding. But I am not sure how it affect the characterisitcs of the compound.

3. Which among the following has the highest boiling point and why : H₂,He,Ne,Xe,CH₄

I think its He but I am not sure..

4. Amongst BBr₃ and BF₃, which is a stronger acid and why?

No idea about this one.

5. Write down the geometry of N₂H₄

I think it has a sp3 hybridisation with a total of two lone pairs, one on each N atom. So it's shape must pyramidal.

thanks for your time.

[Dan]

Hi,

I have a few doubts on some of these questions:

1.Give reason: NF₃ does not undergo hydrolysis while NCl₃ undergoes hydrolysis.  
No idea about this one.

I would be inclined to argue bond strengths here

2. O-hydroxy benzaldehyde and p-hydroxy benzaldehyde. Which of these 2 is
i) a liquid at room temperature?
ii) a high melting solid?

o-hydroxy benzaldehyde has intra-molecular hydrogen bonding and p-hydroxy benzaldehyde has inter-molecular hydrogen bonding. But I am not sure how it affect the characterisitcs of the compound.

Ok, will intermolecular bonding cause the molecules to 'stick together' more or less?

3. Which among the following has the highest boiling point and why : H₂,He,Ne,Xe,CH₄

I think its He but I am not sure..

Why do you think it's He?

4. Amongst BBr₃ and BF₃, which is a stronger acid and why?

No idea about this one.

How does BX₃ act as an acid? What is the feature of BX₃ that causes it to be acidic? What could cause this feature to be reduced?

5. Write down the geometry of N₂H₄

I think it has a sp3 hybridisation with a total of two lone pairs, one on each N atom. So it's shape must pyramidal.

Pyramidal at each N yes, but the shape of the molecule is not pyramidal.

[xstrae]

1.Give reason: NF3 does not undergo hydrolysis while NCl3 undergoes hydrolysis.   
No idea about this one.


I would be inclined to argue bond strengths here

ok here is what I think. Cl^- is a larger anion. So, it must be more polarisable. Hence, NCl3 is more covalent than NF3. Hence, it is much easier to break it down so it undergoes hydrolysis, whereas the latter being more ionic does not break down easily. correct?

2. O-hydroxy benzaldehyde and p-hydroxy benzaldehyde. Which of these 2 is
i) a liquid at room temperature?
ii) a high melting solid?

o-hydroxy benzaldehyde has intra-molecular hydrogen bonding and p-hydroxy benzaldehyde has inter-molecular hydrogen bonding. But I am not sure how it affect the characterisitcs of the compound.


Ok, will intermolecular bonding cause the molecules to 'stick together' more or less?

I am not sure if this is correct but here goes.. o-hydroexy bezaldehyde is the liquid at room temperature because the intra-molecular h.b reduces it's boiling point. p-hydroxy benzaldehyde is much more stable as a result of stronger inter-molecular h.b. causing it behave as a high melting solid.

3. Which among the following has the highest boiling point and why : H2,He,Ne,Xe,CH4

I think its He but I am not sure..

Why do you think it's He?

Never mind the He. It was just a wild guess(what prompted me to guess it was that, it is stable,inert so I thought more energy would be required to excite its atoms)
But if that is true, it could also be Ne or Xe. so never mind..

Since boiling point is the temperature at which the atoms are able to 'break free' from the molecular forces of attraction, I suppose the one in which the forces of attraction that hold the element/compound more strongly has a higher boiling point. But I still have no idea which one..

4. Amongst BBr3 and BF3, which is a stronger acid and why?

No idea about this one.

How does BX3 act as an acid? What is the feature of BX3 that causes it to be acidic? What could cause this feature to be reduced?

well there are so many definitions for an acid, it confuses me. a lewis acid is an electron acceptor, a 'normal' one is proton donor. I think I know which one is the answer but again not sure.. It must be BBr3 because BF3 is more ionic and hence doesnt dissociate easily.

Is this correct?

[Dan]

ok here is what I think. Cl^- is a larger anion. So, it must be more polarisable. Hence, NCl3 is more covalent than NF3. Hence, it is much easier to break it down so it undergoes hydrolysis, whereas the latter being more ionic does not break down easily. correct?

Yeah I think that makes sense. I would argue that the orbital overlap is greater with N and F because the orbitals are better matched in therms of size -> stronger bond

I am not sure if this is correct but here goes.. o-hydroexy bezaldehyde is the liquid at room temperature because the intra-molecular h.b reduces it's boiling point. p-hydroxy benzaldehyde is much more stable as a result of stronger inter-molecular h.b. causing it behave as a high melting solid.

Yeah, you've got the right idea. Inter Hbonding is simply an additional intermolecular force holding the molecules together on the P isomer -> higher melting point.
In the O isomer, inter Hbonding occurs less readily, so this effect is less marked -> lower mp

Never mind the He. It was just a wild guess(what prompted me to guess it was that, it is stable,inert so I thought more energy would be required to excite its atoms)
But if that is true, it could also be Ne or Xe. so never mind..

Since boiling point is the temperature at which the atoms are able to 'break free' from the molecular forces of attraction, I suppose the one in which the forces of attraction that hold the element/compound more strongly has a higher boiling point. But I still have no idea which one..

Yes, the highlighted bit is exactly what you should be thinking about! Consider the magnitude of the van der Waals forces in each gas (since none of them are polar or ionically charged). hint: how does polarisability affect the magnitude of this force?

well there are so many definitions for an acid, it confuses me. a lewis acid is an electron acceptor, a 'normal' one is proton donor. I think I know which one is the answer but again not sure.. It must be BBr3 because BF3 is more ionic and hence doesnt dissociate easily.

Is this correct?

No, not really. BX₃ is a lewis acid. It accepts a lone pair. Think about the electronic config of B in these compounds... does BX₃ have to dissociate to accept a lone pair from a base?
Could it accept a lone pair from anything a little closer to home?

[xstrae]

Yes, the highlighted bit is exactly what you should be thinking about! Consider the magnitude of the van der Waals forces in each gas (since none of them are polar or ionically charged). hint: how does polarisability affect the magnitude of this force?

Polarisability increases the magnitude of the force. So is it Xe? since it has the largest surface area an most number of electrons.

No, not really. BX3 is a lewis acid. It accepts a lone pair. Think about the electronic config of B in these compounds... does BX3 have to dissociate to accept a lone pair from a base?
Could it accept a lone pair from anything a little closer to home?

All these are electron deficient species which readily accept an electron pair. F is the most electronegative element, so it pulls the shared electrons closer toward itself. So B has a greater tendency to accept electrons in BF3, making it more acidic?

[Dan]

Polarisability increases the magnitude of the force. So is it Xe? since it has the largest surface area an most number of electrons.

Yep, thats what I would say too. And I looked up some bps, Xe is the highest of those.

All these are electron deficient species which readily accept an electron pair. F is the most electronegative element, so it pulls the shared electrons closer toward itself. So B has a greater tendency to accept electrons in BF3, making it more acidic?

That was a good idea, but consider backdonation from the halogen too. It's actually not a very good question because the lewis acidity of these compounds varies with the nature of the base, have a look at this: http://pubs.acs.org/cgi-bin/abstract.cgi/inocaj/1999/38/i21/abs/ic990713m.html

[xstrae]

allright thanks a lot