Edexcel chemistry - unit 2 19th january 2012

how do u work out if a compound has an overall dipole moment or not?

Ok initially, when you have a compound you have to look for whether the compound as either of the following compounds - F,O,N, Cl. If it has these elements then it causes bond polarity. The presence of this elements will only be able to tell you whether the bonds are polar or not, but will not tell you whether the molecule is polar. IF Tthe shape is symmetric (linear,trigonal planar or tetrahedral) & the dipoles cancel out then the overall molecule is non polar thus it only has london forces. If it is not then it can either be permanent dipole dipole attractions or hydrogen bonding...

hope this helps

can anyone tell why the answer to both the question are B for part1 and A for part 2?

Not sure but this is what I think

For a), hydrogen bonding in water

For b) the longer chain has the higher boiling point, so its a choice between the two butanes and it would be chlorobutane beacuse the C-Cl bond is stronger than a C-I bond. I don't know why the answer should be A.

can anyone tell why the answer to both the question are B for part1 and A for part 2?

Can anyone help me out over here...pleasee???!?!?

In the june 2009 paper unit 2...Question 21
Could anyone tell me why HBr doesnot have hydrogen bonding where as HF has ??

for part b) it says why is propanone able to dissolve a wide range of substances?? I dont understand,,how does it have hydrogen bonding ??? It only has permanent dipole dipole attractions & london forces isnt it ???

Any help provided is much appreciated

thanks

a. HF can also hydrogen bond, but water can hydrogen bond twice per molecule.

b. Same length carbon chain but iodine has more electrons so stronger london forces.

but isnt a C-CL bond stronger then a C-I bond, meaning it will have a higher boiling point?

Yes it is. But the molecule itself is more stronger due to the bigger electron cloud and more electrons, so more energy required.

I'm about to get off, so this should be quick.

Hydrogen bonding will occur when a hydrogen atom is bonded to a very electronegative atom such as F, O and N.
Br is not as electronegative as the elements I've listed above, so does not have hydrogen bonds.

You are correct in saying that propanone doesn't have hydrogen bonding, however, when it dissolves in water, it can FORM hydrogen bonds between the water molecules due to the carbon double bond O.

Remember, when it talks about dissolving, you have to concentrate on the INTER-MOLECULAR forces.
So propanone has london and permanent dipole dipole forces WITHIN its molecule but hydrogen bonding between water.
Also, propanone can dissolve in non polar solvents because it has similar bonds/can make similar bonds with the solvent.

Sorry, for the capital letters, though it would emphasise the point.

Hope it helps

Yes it is. But the molecule itself is more stronger due to the bigger electron cloud and more electrons, so more energy required.

Thank you so much
No worries about the capital letters..its helped me

its because it has stronger van der waals forces isnt it ??

Next quiz question:

State the reagents and conditions for the following organic reactions.

(a) 1-bromobutane to butan-1-ol

(b) 1-chloropropane to prop-1-ene

(c) 1-iodopropane to propylamine

2 marks for each!

a >> heat under reflux with AQUEOUS sodium hydroxide

b >>> head under reflux with Alcoholic sodium hydroxide

C >>> heat it with alcoholic NH3


did it from the top of ma head, hope i am right