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1 Question..

15

The electronegativities of hydrogen, carbon and chlorine are 2.1, 2.5 and 3.0 respectively. Use values to explain why the boiling temperature of hydrogen chloride is greater than that of methane.

any ideas?

Reply 1

charco

Study Forum Helper

18

permanent dipole-dipole interactions between the polar H-Cl molecules.
CH4 is non-polar.

Reply 2

free radical

galadriel100

The electronegativities of hydrogen, carbon and chlorine are 2.1, 2.5 and 3.0 respectively. Use values to explain why the boiling temperature of hydrogen chloride is greater than that of methane.

any ideas?

the answer is: HCl has a higher boiling point than CH4, because chlorine is 0.5 times more electronegative than carbon.

let me explain...

try to imagine the bonding in each molecule:

consider a HCl molecule: one hydrogen bound with one chlorine: H---:-Cl
the ':' represents a pair of shared electrons. because chlorine has a much higher electronegativity compared to hydrogen, it has more of an 'influence' on the sigma(?) bond, thus the pair of electrons orbit closer to the chlorine.

now consider a CH4 molecule: four hydrogens bound to one carbon,


          H
          |
          ..
          |
    H--:--C--:--H
          |
          ..
          |
          H

again, the '..' and ':' represent a pair of shared electrons. now, because carbon and hydrogen have similar electronegativities, the shared pair of electrons will be more or less in the center of the bond (slightly more towards the carbon, but it really doesn't matter)...

anyway, because the chlorine is more electronegative than the carbon, the chlorine is going to have a greater pull on the electrons, meaning that more energy (in the form of heat) will be required to break the bond. i don't think it matters that there are 3 times as many bonds in the CH4 molecule...

hope that helps,

fr

Reply 3

iceman_jondoe

18

sounds right

Reply 4

charco

Study Forum Helper

18

Unfortunately it's not!

When a substance boils it's bonds are NOT broken - if this were the case then hydrogen and oxygen would be released everytime a kettle boiled for tea....

The explanation of distorted sigma bonds is correct, but this only causes the bonds to be polarised. It is the attraction between dipoles on different molecules (intermolecular forces) that increases the bp.

Reply 5

free radical

It's got to be said, charco knows his stuff!

Thanks for correcting me. :smile:

Reply 6

charco

Study Forum Helper

18

no hay de que..

Reply 7

galadriel100

OP

15

charco

Unfortunately it's not!

When a substance boils it's bonds are NOT broken - if this were the case then hydrogen and oxygen would be released everytime a kettle boiled for tea....

The explanation of distorted sigma bonds is correct, but this only causes the bonds to be polarised. It is the attraction between dipoles on different molecules (intermolecular forces) that increases the bp.

That's what I thought.

So would you say HCl has dipole dipole forces between the molecules and CH4 has dispersion forces (non polar) between the molecules. Seeing that dispersion forces are weaker than dipole dipole (I think), HCl will naturally have a higher boiling point as more energy is needed to break the dipole dipole forces??

Reply 8

charco

Study Forum Helper

18

yes

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