Help with some questions on bonding !

Hi guys,

here are the questions,

1)suggest why in terms of intermolecular forces, hydrogen iodide requires more heat energy for melting than does hydrogen chloride.

2) why is the heat energy required to vapourise one mole of sodium chloride much graeter than that needed yo melt one mole of sodium chloride?

3) Describe the motion of the atoms in a macromolecular crystalline solid.

waiting for ur responses, thank you ! :colondollar:

Reply 1

pielalit

2

I only know the first one. It's because iodine has more mass than chlorine.

Reply 2

jonathan3909

11

In short-

1) HI molecule is bigger than HCl molecule thus is heavier , more shielded due to greater electron shells and hence more polarizable therfore the intermolecular forces between HI molecules are stronger.

2)Vaporisation requires all bonds to be broken where as melting doesn't(think of it in terms of ionic bonding :rolleyes:

)

3) All molecules held by strong covalent bonds[think of the word solid :wink:

......tightly packed , vibration etc..]

(edited 13 years ago)

Original post by jonathan3909

In short-

1) HI molecule is bigger than HCl molecule thus is heavier , more shielded due to greater electron shells and hence more polarizable therfore the intermolecular forces between HI molecules are stronger.

2)Vaporisation requires all bonds to be broken where as melting doesn't(think of it in terms of ionic bonding :rolleyes:

)

3) All molecules held by strong covalent bonds[think of the word solid :wink:

......tightly packed , vibration etc..]

1) incorrect - larger mass = larger van der Waals forces

Reply 4

jonathan3909

11

Original post by charco

1) incorrect - larger mass = larger van der Waals forces

Wrong-You need to be specific about the forces-

You need to realize that dispersion forces have very variable strengths depending on the size, shape and polarisability of the molecules. Describing them as weak is wrong.
For these hydrogen halides , dipole-dipole attractions decrease down the group while the dispersion forces increases down the group hence larger boiling points down the group.[remember that both are van der Waals forces so be highly specific as to which forces]

Spoiler

(edited 13 years ago)

Original post by jonathan3909

Wrong-You need to be specific about the forces-

You need to realize that dispersion forces have very variable strengths depending on the size, shape and polarisability of the molecules. Describing them as weak is wrong.
For these hydrogen halides , dipole-dipole attractions decrease down the group while the dispersion forces increases down the group hence larger boiling points down the group.[remember that both are van der Waals forces so be highly specific as to which forces]

Spoiler

At no point did I describe van der Waals forces as weak!!!! If they were weak sulphur and iodine would be gases.

My statement was:

larger mass = larger van der Waals forces

Which, strangely enough, is exactly what you say.

The only difference is that certain exam boards distinguish between permanent dipole -dipole attractions and dispersion (London) forces, while others group them both under van der Waals.

Van der Waals are generally taken to mean induced dipole-dipole attractions (i.e. dispersion forces)

Reply 6

jonathan3909

11

Original post by charco

At no point did I describe van der Waals forces as weak!!!! If they were weak sulphur and iodine would be gases.

Why did you prove me incorrect then?I stated that "the intermolecular forces between HI molecules are stronger....".Since I assumed that the OP would already know that i'm talking about van der Waals forces(I didn't wanna go in to its details)-

You said

larger mass = larger van der Waals forces

?So where was I incorrect :confused:

Original post by jonathan3909

Why did you prove me incorrect then?I stated that "the intermolecular forces between HI molecules are stronger....".Since I assumed that the OP would already know that i'm talking about van der Waals forces(I didn't wanna go in to its details)-

You said ?So where was I incorrect :confused:

Tecnically you weren't, but I suggest that

more shielded due to greater electron shells and hence more polarizable

is not what the exam boards want in order to achieve the marks. They are looking for reference to either van der Waals forces, or temporary induced dipole attractions, related to particle mass.

Reply 8

jonathan3909

11

Original post by charco

Tecnically you weren't, but I suggest that

is not what the exam boards want in order to achieve the marks. They are looking for reference to either van der Waals forces, or temporary induced dipole attractions, related to particle mass.

If you read my first post it says:

"In short"

I could have gone on forever~Since we are not allowed to tell the whole answers , I thought the OP would get the idea through "intermolecular forces".

Original post by jonathan3909

If you read my first post it says:

"In short"

I could have gone on forever~Since we are not allowed to tell the whole answers , I thought the OP would get the idea through "intermolecular forces".

OK