AS chem questions

sweetiepieh

1) Why is diamond more dense than graphite?

Diamond forms 4 covalent bonds with other carbon atoms resulting in a very compact shape. Graphite on the other hand forms "sheets of carbon atoms joined together by 3 covlaent bonds. The extra electrons are delocalised. In between each sheet there is a space of about 2.5x the length of the C-C bond. This means that alot of space in graphite has litlle mass lowering the density.

sweetiepieh

2) Explain why HF has a higher boiling temperature than the other hydrogen halides and why there is an increase from HCl to HI

Hydrogen fluoride contains hydrogen bonds whereas the others do not. hydrogen bonding is only present when hydrogen is bonded to an extremly electronegative atom, such as fluorine. This hydrogen bonding means the intermolecular forces are greater and therefore the boiling point is higher.
Hydrgoen chloride contains no hydrogen bonds and so it has a lower boiling point than HF.
HCl has a lower boiling point than HI because HI has a greater total number of electrons and so the van de waals forces between HI molecules are greater than HCl molecules. There is also a difference between electronegativities, but this is not the deciding factor as this would cause HCl to have a higher boiling point than HI.

3) Identify the strongest intermolecular force present in the liquid form of each Hydrogen halide
HF --- Hydrogen bonding
HCl --- dipole-dipole
HBr --- dipole-dipole
HI --- dipol-dipole Unsure about HCl,HBr and HI as it says in their liquid form.

sweetiepieh

4) The mass spectrum of a sample of Cl shows the 3 molecular peaks. Calculate the RMM of Cl in this sample...

Molecule % abundance
35Cl- 35Cl 56.25
35Cl- 37Cl 37.50
37Cl- 37Cl 6.25

First work out the average RMM of each molecule so:
35 = 56.25%
36 = 37.50%
37 =6.25 %

Then times the average RMM of the molecule by the percentage:
35 x 56.25% = 19.69
36 x 37.50% = 13.50
37 x 6.25 % = 2.31

Add them together to get the RMM of the chlorine in the sample:
19.69 + 13.50 +2. 31 = 35.50

Hope that helps :P

Reply 2

Demolion

1)

Diamond is more dense than graphite because the atoms are all bonded together with

\sigma

bonds. These are very short so the atoms are close together and the structure is dense.

Graphite has sheets of hexagons of C bonded using

\sigma

bonds, but it also has a

\pi

system above and below the planes. These are relatively long compared to the

\sigma

bonds, and also do not actually bind the sheets together. This is done using van-de-vaal's dispersion forces, which are much weaker than either covalent bond, so are far longer. As such graphite is less dense than diamond.

(For more details on the

\pi

system look up the bonding in benzene.)

Reply 3

Excalibur

15

Demolion

1)

Diamond is more dense than graphite because the atoms are all bonded together with

\sigma

bonds. These are very short so the atoms are close together and the structure is dense.

Graphite has sheets of hexagons of C bonded using

\sigma

bonds, but it also has a

\pi

system above and below the planes. These are relatively long compared to the

\sigma

bonds, and also do not actually bind the sheets together. This is done using van-de-vaal's dispersion forces, which are much weaker than either covalent bond, so are far longer. As such graphite is less dense than diamond.

(For more details on the

\pi

system look up the bonding in benzene.)

Ahhhh! This was the stuff I was asking before - the sheets are held together by Van der Waal forces, so what does the π thing got to do with it??

Reply 4

BlakDog

3) Identify the strongest intermolecular force present in the liquid form of each Hydrogen halide
HF --- Hydrogen bonding
HCl --- dipole-dipole
HBr --- dipole-dipole
HI --- dipol-dipole Unsure about HCl,HBr and HI as it says in their liquid form.

Which dipole is it?

permenant-permernent dipole or instantaneous-induced dipole?

Reply 5

Edit: (poorly phrased) -- > Exaclibur: The bonds in diamond would hybridise into sp3 bonds and graphite bonds would hybridise to sp2 bonds, but dont quote me on it. I believe then the left over electrons create planes of delocalised electrons. Also vdw are the stronger than permant dipole-dipole.
This is only an educated guess, but i know diamond bonds hybridse into sp3 and i believe graphite hybridise into sp2 as they are flat.

samsonlcy: permant dipole-dipole.

There is NO pi-pi overlap between sheets (according to accepted views of the bonding involved). The bonding between sheets in graphite is only VDW.

As each of the p orbitals from the sp2 hybridised carbons of graphite overlaps laterally this leaves no possibility of any other overlap.

Reply 7

music788

3

BlakDog

samsonlcy: permant dipole-dipole.

isnt it just permenant dipole? because it isn't joined to another dipole.. its just HCl! or am i confusing myself :confused:

Reply 8

music788

isnt it just permenant dipole? because it isn't joined to another dipole.. its just HCl! or am i confusing myself :confused:

Yes but there are more is more than one molecule of HCl!. Permant dipole-dipole says its between a permanent dipole and another permenant dipole, which is what this is, i think.

Reply 9

Excalibur

15

Aren't Van der Waals stronger than permanent dipole-dipole though?

Reply 10

Fire_X1

they are van der waals forces or you can call them Induced dipole forces...just checked the mark scheme

Reply 11

permanent - permanent dipoles (stronger of the two here)

forms when two molecules (e.g. hydrogenbromide) that has a permanent dipole is attracted to each other. Thus, forming a permanent -permanent diople.

instantaneous induced diople (weaker of the two)

forms with all types of molecules. electrons are continually moving around a nucleus so instantaneous dipole form within a molecule. As other molecules approach this molecule, they will get induced thus forming an instantaneous induced dipole.
As electron clouds are always moving, these bonds will also be forming and breaking. The more electrons a molecule has, the higher the boiling point.

Van der waals
not in my course. Sorry! :smile:

Reply 12

Yeah van de waals is stronger (london forces are the same).
Sorry got a bit confused.

Reply 13

BlakDog

Yeah van de waals is stronger (london forces are all the same).
Sorry got a bit confused.

What board are you doing. just making sure I havnt missed anything

edxcel

oh. OCR so im okok

OP

Thanks to everyone for their replies but can i just clarify...

Identify the strongest intermolecular force present in the liquid form of each Hydrogen halide
HF --- Hydrogen bonding :smile:

HCl --- dipole-dipole :confused:

HBr --- premenant- permenant dipole :confused:

HI --- dipole-dipole :confused:

Reply 17