Silicon dioxide Watch

iqra-s
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I was looking at the 2017 chemistry paper from aqa paper 1. One of the questions was asking about why silicon dioxide has a higher melting point than sulfur trioxide. I know that sio2 is a giant covalent structure and so3 is simple molecular but i would have said because sio2 is giant there are many vdw forces to overcome compared to so3. However the mark scheme doesnt mention this and says that covalent bonds are stronger than the vdw in so3 but arent covalent bonds inTRAmolecular????? I thought they don't break when melting??? I understand that sio2 is more of a covalent network and each atom is bonded to another but the mark scheme rejects answers that refer to covalent bonds between molecules of sio2..... am i missing something??
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Katiemick8
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Is it do with the fact that to melt a giant covalent structure, you have to break the intramolecular bonds i.e between atoms, as giant covalent structures are a kind of matrix where everythings covalently bonded to each other
And to melt a simple molecular structure you have to break intermolecular bonds as its more of a sea of molecules, the molecules are only connected by vdw forces.


Idk its been a while since I did AS chem, but I remember something about having to only break intermolecular bonds when a substance is simple, but having to actually break the bonds between atoms when its giant covalent as each atom is covalently bonded to each other

So to "release" the molecules you have to break the covalent bonds between atoms. Whilst giant structures do have vdw, they also have covalent bonds that almost act like intermolecular forces.


So then obviously as you know covalent bonds are stronger than vdw forces, so thats what the mark scheme is referencing
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iqra-s
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(Original post by Katiemick8)
Is it do with the fact that to melt a giant covalent structure, you have to break the intramolecular bonds i.e between atoms, as giant covalent structures are a kind of matrix where everythings covalently bonded to each other
And to melt a simple molecular structure you have to break intermolecular bonds as its more of a sea of molecules, the molecules are only connected by vdw forces.


Idk its been a while since I did AS chem, but I remember something about having to only break intermolecular bonds when a substance is simple, but having to actually break the bonds between atoms when its giant covalent as each atom is covalently bonded to each other

So to "release" the molecules you have to break the covalent bonds between atoms. Whilst giant structures do have vdw, they also have covalent bonds that almost act like intermolecular forces.


So then obviously as you know covalent bonds are stronger than vdw forces, so thats what the mark scheme is referencing
Thanks so much!! so is it right to say that intramolecular forces are broken when melting sio2?? i know i dont need to know this but im annoying like that!
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clareeey
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(Original post by iqra-s)
Thanks so much!! so is it right to say that intramolecular forces are broken when melting sio2?? i know i dont need to know this but im annoying like that!
No, you're breaking the bonds between atoms in Si02. There are no discrete molecules because of the giant structure. And intramolecular means between molecules.

Saying you have to overcome intermolecular van der waals forces for Si03 is correct
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Katiemick8
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(Original post by iqra-s)
Thanks so much!! so is it right to say that intramolecular forces are broken when melting sio2?? i know i dont need to know this but im annoying like that!
Yeah thats the general gist of it
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iqra-s
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(Original post by clareeey)
No, you're breaking the bonds between atoms in Si02. There are no discrete molecules because of the giant structure. And intramolecular means between molecules.

Saying you have to overcome intermolecular van der waals forces for Si03 is correct
ohhh i did read that somewhere else but different places say different things.... in sio2 there isnt a defnitive molecule right? sio2 kind of acts as an empirical formula??
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clareeey
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(Original post by iqra-s)
ohhh i did read that somewhere else but different places say different things.... in sio2 there isnt a defnitive molecule right? sio2 kind of acts as an empirical formula??
Yeah, that's a good way of putting it. Each piece of Silicon dioxide, like a grain of sand is probably million of each, but in that ratio
https://www.123rf.com/photo_17236694...structure.html
something like this and it'll pretty much go on forever at this scale
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iqra-s
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(Original post by clareeey)
Yeah, that's a good way of putting it. Each piece of Silicon dioxide, like a grain of sand is probably million of each, but in that ratio
https://www.123rf.com/photo_17236694...structure.html
something like this and it'll pretty much go on forever at this scale
thanks for clearing that up!! my annoying self is confused about whether there are intermolecular forces in sio2??? just a quick google search tells me there are but is it not important in the actual melting of sio2?
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clareeey
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(Original post by iqra-s)
thanks for clearing that up!! my annoying self is confused about whether there are intermolecular forces in sio2??? just a quick google search tells me there are but is it not important in the actual melting of sio2?
Covalent bond between atoms in SiO2 are far stronger than any intramolecular/ van der waals forces. So for melting, when you have to break the bonds, the bond strength is all that matters.

As I said before there aren't any discrete molecules in Si02https://chemistry.stackexchange.com/...vercome-broken

there are however weak dipole interactions that are present in anything that has charge- https://www.chem.fsu.edu/chemlab/chm...terforces.html

https://www.chemguide.co.uk/atoms/st.../giantcov.html
scroll down to the bit about it
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iqra-s
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(Original post by clareeey)
Covalent bond between atoms in SiO2 are far stronger than any intramolecular/ van der waals forces. So for melting, when you have to break the bonds, the bond strength is all that matters.

As I said before there aren't any discrete molecules in Si02https://chemistry.stackexchange.com/...vercome-broken

there are however weak dipole interactions that are present in anything that has charge- https://www.chem.fsu.edu/chemlab/chm...terforces.html

https://www.chemguide.co.uk/atoms/st.../giantcov.html
scroll down to the bit about it
thanks for clearing it up!! i can now move on in m revision lol
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username3911134
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covalent bonds>intermolecular forces.
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