Is melting/boiling point determined by intermolecular forces or covalent/ionic bonds?

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LoveAmore
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I'm kind of confused about what influences melting/boiling point, whether it's the bonds or intermolecular forces. My GCSE chem revision guide says the covalent bonds but I was always taught that it was due to the intermolecular forces
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BDavies1
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giant covalent substances- (essentially only diamond graphite and SiO2)- to melt them you need to break covalent bonds so melting point is affected by the strength of the covalent bonds

all other covalent substances- H2O, CO2, C6H12O6, polyethene, etc etc - when you melt them you do NOT break covalent bonds only weak intermolecular forces- so the strength of the covalent bonds in the molecules does not affect mp

ionic compounds- when you melt them you break down the electrostatic attraction between the positive and negative ions (i.e. ionic bonds)- note makes no sense to talk about intermolecular forces in ionic compounds because they don't exist as molecules!
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LoveAmore
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(Original post by BDavies1)
giant covalent substances- (essentially only diamond graphite and SiO2)- to melt them you need to break covalent bonds so melting point is affected by the strength of the covalent bonds

all other covalent substances- H2O, CO2, C6H12O6, polyethene, etc etc - when you melt them you do NOT break covalent bonds only weak intermolecular forces- so the strength of the covalent bonds in the molecules does not affect mp

ionic compounds- when you melt them you break down the electrostatic attraction between the positive and negative ions (i.e. ionic bonds)- note makes no sense to talk about intermolecular forces in ionic compounds because they don't exist as molecules!
thank you so much, this really helps
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scimus63
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I think to begin with you should think about the type of structure, if it is giant or molecular. Giant structures have high mp/bp due to lots of strong bonds and huge masses due to their structure.
In small covalent molecules the inter -molecular bonding plays a large part in many physical properties such as melting and boiling points, viscosity. Hydrogen bonding in particular can raise the melting and boiling points well above what you might expect, as an obvious example think about a tiny molecule like water which has a massively elevated melting and boiling point, this is entirely due to the H-bonding between the "tiny" water molecules. If you have lots of strong intermolecular bonding present, for example in large covalent molecules such as polymers or proteins then intermolecular bonding will have a large affect on the mp/bp.
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LoveAmore
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(Original post by scimus63)
I think to begin with you should think about the type of structure, if it is giant or molecular. Giant structures have high mp/bp due to lots of strong bonds and huge masses due to their structure.
In small covalent molecules the inter -molecular bonding plays a large part in many physical properties such as melting and boiling points, viscosity. Hydrogen bonding in particular can raise the melting and boiling points well above what you might expect, as an obvious example think about a tiny molecule like water which has a massively elevated melting and boiling point, this is entirely due to the H-bonding between the "tiny" water molecules. If you have lots of strong intermolecular bonding present, for example in large covalent molecules such as polymers or proteins then intermolecular bonding will have a large affect on the mp/bp.
thanks!
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Qxi.xli
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EliteBoy8
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(Original post by LoveAmore)
I'm kind of confused about what influences melting/boiling point, whether it's the bonds or intermolecular forces. My GCSE chem revision guide says the covalent bonds but I was always taught that it was due to the intermolecular forces
From my understanding, it’s the bonds that has an impact on melting/boiling point and the intermolecular forces has an affect on the strength. For example, graphite has a high melting point but it is soft because it has weak forces between the bonds (London Forces) and these can slide over each other. Hope this helps.
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LoveAmore
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(Original post by EliteBoy8)
From my understanding, it’s the bonds that has an impact on melting/boiling point and the intermolecular forces has an affect on the strength. For example, graphite has a high melting point but it is soft because it has weak forces between the bonds (London Forces) and these can slide over each other. Hope this helps.
Thank you
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