[cuongt]

in exam today i had question which was i cant really remember but somehting like this.

explain with reference to intermolecular bonding to why there is a difference in the boiling points of NH3 and CH4. they gave that NH3 boiling was about -33 degrees celcius and CH4 was -155 degrees i think somewhere around there.
it was an extended response so yer worth 10 marks.
wat i said was that NH3 has hydrogen bonds and these are the strongest intermolecular bonds. i talked about NOF( nitrogen, oxygen, fluorine) hydrogen bonds. i said that CH4 had dispersion forces, but i didnt know if this was true though. and i said that NH3 had was trigonal pyrimidal and CH4 was tetrahedral and i didnt know wat else to write i think i dont even have enough to get 5 marks. lol.
 
this was a easy question in exam but i cant explain why water is v shaped and CO2 is linear dunno how but my mind went blank... i could only explain CO2 i said that it had no lone pair of electrons so therefore for maximum repulsion between the pairs of electrons the structure would have to be linear. VSEPR.

thanks in advance
thanks in advance  .

[Stewed_ant]

Good answer my friend, you are absolutely right, NH3 does have hydrogen bonding where as CH4 only has dispersion forces. 

You see Carbon and hydrogen bond in the CH4 is not very polar, they have similar electronegativities, what's more, the molecule CH4 is symmetrical and so it is not a polar in any direction, therefore it doesn't have polar intermolecular bonding and it certainly doesn't have hydrogen bonding, all it has is weak dispersion forces (which are due to random movements of electrons).

Coming to the NH3, the difference between the electro negativities of Hydrogen and Nitrogen are alot different and the N-H bond is polar, and since N is one of the most electronegative atoms, it is one of (you where right in including oxygen and flourine) the strongest polar covalent bonds called hydrogen bonding. Also the molecule is not symmetrical making it a polar molecule, with the lone pair of electrons on the Nitrogen making it easy for other H's on other NH3's attracted to them.

You are right that CO2 is linear because it has no lone pairs, and due to max repulsion, i don't know why you couldn't apply that to the H2O molecule though, Oxygen has two lone pairs, which have their own repulsion forces, they push the hydrogens away from them infact they have such a replusive force that the angle between the hydrogens is actually a little less than it would be than if they where tetrahedral. But you can think of the H2O of being a sort of tetrahedral with the two  lone pairs two sides of the tetrahedral, however since they aren't atoms, the molecule itself just looks like a v shape.

But you where right that NH3 is trigonal pyramidal (because of the lone pair as explained above) and that CH4 was tetrahedral.

So i think you did well in this question, if i was a marker i'd give you 9 out of 10

-hope that helped.

[Yggdrasil]

A good way to explain polarity and molecular shape is to draw lewis structures.  In science exams you shouldn't just limit youself to writing answers.  Sometimes pictures and equations can be extremely concise and effective ways of explaining answers.

[cuongt]

thanks for the answer and compliment stewed_ant  i understand now