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intermolecular forces vs bonds ?? (GCSE CHEMISTRY)

if an element has a low melting point, is it due to the intermolecular forces being weak or the bonds being weak (when it says 'bonds' i'm assuming that means covalent ??)

i keep getting confused between the two as the covalent bonds are really strong as well?
Original post by harlz_chalamet
if an element has a low melting point, is it due to the intermolecular forces being weak or the bonds being weak (when it says 'bonds' i'm assuming that means covalent ??)

i keep getting confused between the two as the covalent bonds are really strong as well?

So, covalent bonds are very strong. Covalent bonds are between the atoms in a molecule, so for example between diatomic elements such as oxygen or hydrogen.
However, between the molecules are the intermolecular forces. This is only for molecules, so things like metals do not have intermolecular forces (because they don't exist in a molecular form). When you boil water for example, you are breaking the weaker intermolecular bonds and you get a gas, but that gas is made up of H2O molecules. The bonds between the H and the O are still there.
What element are you thinking of?
If you are looking at different melting points of metals, then any difference would be from the size of the metal cation, the charge etc.
Original post by booklover1313
So, covalent bonds are very strong. Covalent bonds are between the atoms in a molecule, so for example between diatomic elements such as oxygen or hydrogen.
However, between the molecules are the intermolecular forces. This is only for molecules, so things like metals do not have intermolecular forces (because they don't exist in a molecular form). When you boil water for example, you are breaking the weaker intermolecular bonds and you get a gas, but that gas is made up of H2O molecules. The bonds between the H and the O are still there.


Original post by booklover1313
What element are you thinking of?
If you are looking at different melting points of metals, then any difference would be from the size of the metal cation, the charge etc.

this is excellent thank you!

i was doing a past paper and this was the question:

Why does iodine have a higher boiling point than chlorine?
[1 mark]
Tick one box.
Iodine is ionic and chlorine is covalent
Iodine is less reactive than chlorine
The covalent bonds between iodine atoms are stronger
The forces between iodine molecules are stronger


the one in bold is the correct answer but i thought it was the one above, about the covalent bonds.

Screenshot 2022-04-23 at 13.12.03.png
and this was another question in the same paper, the answer being "bonds are strong" for the high boiling point.



so in conclusion for ionic compounds and covalent compounds, do their boiling points depend on different things?
so covalent = intermolecular forces
and ionic = ??



thank you ever so much :smile:
(edited 1 year ago)
Reply 4
Original post by harlz_chalamet
and ionic = ??

Ionic is the electrostatic force of attraction between a positively charged ion and a negatively charged ion in an ionic compound. It is a strong force which gives the ionic compound a high melting point
@harlz_chalamet so for the iodine vs chlorine one, it is talking about the boiling points. If we write equations for what happens when you boil chlorine or iodine, it would be
Cl2(l) -> Cl2(g)
and I2(l) -> I2(g)
So you see that we are just changing the state from a liquid to a gas. We then have either chlorine molecules or iodine molecules as a gas. They still exist as the diatonically, covalent bonded molecule, but the difference is that they are a gas rather than a liquid. When you boil it, you are supplying energy that is enough to overcome the forces of attraction between the molecules. We are breaking the weaker intermolecular forces. The covalent bond remains when it is boiled. We have Cl2(g) not Cl(g).
So any difference in boiling point can't be to do with the strength of the covalent bonds, because they aren't changed during the boiling, and they are still present afterwards.
What we are getting rid of are the intermolecular forces, so inter=between molecules. So the strength of these will influence how much energy you need to put in to boil it, and therefore the boiling point. If the intermolecular forces are weak, not much energy will be needed to overcome them, so the boiling point will be low. If the intermolecular forces are strong, more energy will be needed to overcome them, and so it will boil at a higher temperature.
So, we are told iodine has a higher boiling point than chlorine. Therefore, the intermolecular forces must be stronger.
This is because the iodine atoms are bigger, so the iodine molecules are bigger. You only need to know why for A level, but for your interest, larger molecules = more electrons = more intermolecular forces.

Does that make sense?

And for the other question, yes you are right, the bonds are different for covalent vs ionic compounds.
Covalent ones, such as water, or chlorine, exist as molecules. There are clearly defined little units of 'one water molecule', made up of H2O. However, for ionic ones, they exist in a giant lattice structure. Taking salt, NaCl as an example, it doesn't exist in little parcels of one Na and one Cl. Thousands are all bonded together in the lattice structure. If you take a crystal of salt, all the Na and all the Cl will be bonded in one giant structure. There is no such thing as a salt molecule. Therefore, since there are no molecules, there are no intermolecular forces.
Instead, when you melt or boil salt, you actually have to break the bonds between the atoms, not between the molecules (as when you boil water). It is the very strong ionic bond that you are breaking. That needs a lot of energy to break, so salts will have high boiling points.

I hope that helps!
y

Original post by booklover1313
@harlz_chalamet so for the iodine vs chlorine one, it is talking about the boiling points. If we write equations for what happens when you boil chlorine or iodine, it would be
Cl2(l) -> Cl2(g)
and I2(l) -> I2(g)
So you see that we are just changing the state from a liquid to a gas. We then have either chlorine molecules or iodine molecules as a gas. They still exist as the diatonically, covalent bonded molecule, but the difference is that they are a gas rather than a liquid. When you boil it, you are supplying energy that is enough to overcome the forces of attraction between the molecules. We are breaking the weaker intermolecular forces. The covalent bond remains when it is boiled. We have Cl2(g) not Cl(g).
So any difference in boiling point can't be to do with the strength of the covalent bonds, because they aren't changed during the boiling, and they are still present afterwards.
What we are getting rid of are the intermolecular forces, so inter=between molecules. So the strength of these will influence how much energy you need to put in to boil it, and therefore the boiling point. If the intermolecular forces are weak, not much energy will be needed to overcome them, so the boiling point will be low. If the intermolecular forces are strong, more energy will be needed to overcome them, and so it will boil at a higher temperature.
So, we are told iodine has a higher boiling point than chlorine. Therefore, the intermolecular forces must be stronger.
This is because the iodine atoms are bigger, so the iodine molecules are bigger. You only need to know why for A level, but for your interest, larger molecules = more electrons = more intermolecular forces.

Does that make sense?

And for the other question, yes you are right, the bonds are different for covalent vs ionic compounds.
Covalent ones, such as water, or chlorine, exist as molecules. There are clearly defined little units of 'one water molecule', made up of H2O. However, for ionic ones, they exist in a giant lattice structure. Taking salt, NaCl as an example, it doesn't exist in little parcels of one Na and one Cl. Thousands are all bonded together in the lattice structure. If you take a crystal of salt, all the Na and all the Cl will be bonded in one giant structure. There is no such thing as a salt molecule. Therefore, since there are no molecules, there are no intermolecular forces.
Instead, when you melt or boil salt, you actually have to break the bonds between the atoms, not between the molecules (as when you boil water). It is the very strong ionic bond that you are breaking. That needs a lot of energy to break, so salts will have high boiling points.

I hope that helps!

you're genuinely a lifesaver thank you ever so much.
just to see what i comprehended:

covalent structures = you need to break the INTERMOLECUALR bonds as for both solid, liquid and gases, for example Br2, it will always be Br2 so you are never breaking the covalent bond itself

for ionic compounds = just breaking the bonds between the atoms: you are not breaking the electrostatic forces because those actually bond the metal and non-metals together


ahhh thank you so so so much :smile:
Original post by harlz_chalamet
y


you're genuinely a lifesaver thank you ever so much.
just to see what i comprehended:

covalent structures = you need to break the INTERMOLECUALR bonds as for both solid, liquid and gases, for example Br2, it will always be Br2 so you are never breaking the covalent bond itself

for ionic compounds = just breaking the bonds between the atoms: you are not breaking the electrostatic forces because those actually bond the metal and non-metals together


ahhh thank you so so so much :smile:

You are 90% of the way there!!!
You are right about the covalent stuff, but with the ionic compounds, it is the ionic bonds that are being broken, that is what I meant when I said the bonds between the atoms (sorry I wasn't very clear)! The ionic bond is the electrostatic forces of attraction between the cation and anion. When you melt NaCl or boil it, you get Na+ and Cl-.
By doing that you are breaking apart the ionic bond, which needs lots of energy.
Original post by booklover1313
You are 90% of the way there!!!
You are right about the covalent stuff, but with the ionic compounds, it is the ionic bonds that are being broken, that is what I meant when I said the bonds between the atoms (sorry I wasn't very clear)! The ionic bond is the electrostatic forces of attraction between the cation and anion. When you melt NaCl or boil it, you get Na+ and Cl-.
By doing that you are breaking apart the ionic bond, which needs lots of energy.




so when melting an ionic compound, you are breaking the ELECTROSTATIC forces (which are also known as just the bonds between the atoms?)

you don't understand how much this has helped me thank you :smile:
Original post by harlz_chalamet
so when melting an ionic compound, you are breaking the ELECTROSTATIC forces (which are also known as just the bonds between the atoms?)

you don't understand how much this has helped me thank you :smile:

You're so welcome!
Yeah, when melting the ionic compound, you are breaking the ionic bond, which is the electrostatic forces. In an exam I would say specifically the ionic bonding, so that you are showing the examiners you know that it is ionic (as opposed to being a metallic bond, or covalent).
Original post by booklover1313
You're so welcome!
Yeah, when melting the ionic compound, you are breaking the ionic bond, which is the electrostatic forces. In an exam I would say specifically the ionic bonding, so that you are showing the examiners you know that it is ionic (as opposed to being a metallic bond, or covalent).

THANK YOUUU :smile:
do you happen to do chemistry A-Level as you are so knowledgeable! (i'm taking it next year along with biology and psychology haha)
Reply 12
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Original post by harlz_chalamet
THANK YOUUU :smile:
do you happen to do chemistry A-Level as you are so knowledgeable! (i'm taking it next year along with biology and psychology haha)

I did chemistry in the IB (currently on a gap year) and I'm starting a chemistry degree in October, so if you have any other chemistry questions I'm always happy to help!

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