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Phys/Chem Questions (1 Viewer)

ichila101

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And this is all due to electronegativities of each element I'm guessing? So for example if they had asked in an exam whether Silicon Dioxide was a Covalent compound or not and the only info I knew at the time was that Covalent compounds occur between non-metals (plus the fact the exam did not provide an electronegativity periodic table), would I be wrong if I said it wasn't a Covalent compound? How would you really know whether something is a Covalent Compound or not if they don't show the electrons sharing between atoms or they don't show the electronegativity table (since you said it can occur between metalloids and metals)?
And this is all due to electronegativities of each element I'm guessing?
Actually it has more to do with the valence shells of the elements (not sure if you have learnt about valence shells) but they are essentially the last shell of an element and because elements that do not 8 electrons in their last shells they are reactive and want to gain or lose electrons in some cases they cant (e.g. with covalent bonds) so they share them which allows them to form covalent bonds

So for example if they had asked in an exam whether Silicon Dioxide was a Covalent compound or not and the only info I knew at the time was that Covalent compounds occur between non-metals (plus the fact the exam did not provide an electronegativity periodic table), would I be wrong if I said it wasn't a Covalent compound?
Yes you would be wrong if you said that it wasnt a covalent compound because it is a covalent compound. Whether or not you had that information I am sure that you would get that question wrong if you had that as your answer


How would you really know whether something is a Covalent Compound or not if they don't show the electrons sharing between atoms or they don't show the electronegativity table (since you said it can occur between metalloids and metals)?
Well I am sure they wont overcomplicate questions and ask that, if they do I am pretty sure that would all work out if you just memorise your notes (which i really dont like especially in science subjects) so there are some ways to do this although for this question it should not be too hard since there are very few mentioned covalent network substances.

Generally that kind of question would be a lot harder to solve using valencies for a yr 9/10 student so you would have to simply use the process of elimination.
For example for Silicon dioxide, it quite obviously cannot be a metallic compound since, as the name suggests, metallic compounds would only include metals

Also it cannot be a ionic compound because neither the silicon element nor the oxygen molecule are completely giving away or gaining electrons from each other.

If you were to explain it in year 12 (you probably wouldnt be asked this type of question though) you could simply describe the bonds that would occur between the elements as shown below
On the right between the sulfur and oxygen element there is clearly a covalent bond; sharing of electrons and on the left side of the sulfur and left oxygen there is what is known as a 'coordinate covalent bond' something that is learnt in year 12 which you dont need to worry about but I just told you to understand that in terms of structure it would be made up of covalent bonds (btw just realised that you may not have learnt lewis dot diagrams and most of this stuff is year 11, tell me if you havent learnt lewis diagrams if you want me to explain but if you already understand them then its all good)


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ichila101

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Right, so essentially Ionic and Metallic compounds aren't molecules due to the fact that for Ionic compounds it's a crystal lattice which involves the atoms joining continuously and Metallic compounds are between the atoms and electrons whereas Covalent compounds tend to just have separate molecules which don't join continuously.

Also, when we melt metals for example; they break the metallic bonds to seperate the atoms and hence create a liquid state. How come when we boil Water for example, the covalent bonds in between don't break and only the intermolecular forces break? How would a liquid turn into a gas if the covalent bonds aren't broken? So my question really is: what factors are influenced by intermolecular forces compared to covalent/ionic/metallic bonds?
Intermolecular forces decide the physical properties of the molecules. When melting a metals, it will change from solid to liquid as you have stated because the metallic bonds have broken, metals do not have any intermolecular forces so the effect of intermolecular forces deciding the physical properties renders meaningless.

Water on the other hand is made up of intermolecular forces which will decide its physical properties one of which includes the boiling point. So if a physical change such as a shift from liquid to gas where to happen this would mean there would be a change in the 'state' of the intermolecular forces and not the intramolecular forces.




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Actually it has more to do with the valence shells of the elements (not sure if you have learnt about valence shells) but they are essentially the last shell of an element and because elements that do not 8 electrons in their last shells they are reactive and want to gain or lose electrons in some cases they cant (e.g. with covalent bonds) so they share them which allows them to form covalent bonds


Yes you would be wrong if you said that it wasnt a covalent compound because it is a covalent compound. Whether or not you had that information I am sure that you would get that question wrong if you had that as your answer



Well I am sure they wont overcomplicate questions and ask that, if they do I am pretty sure that would all work out if you just memorise your notes (which i really dont like especially in science subjects) so there are some ways to do this although for this question it should not be too hard since there are very few mentioned covalent network substances.

Generally that kind of question would be a lot harder to solve using valencies for a yr 9/10 student so you would have to simply use the process of elimination.
For example for Silicon dioxide, it quite obviously cannot be a metallic compound since, as the name suggests, metallic compounds would only include metals

Also it cannot be a ionic compound because neither the silicon element nor the oxygen molecule are completely giving away or gaining electrons from each other.

If you were to explain it in year 12 (you probably wouldnt be asked this type of question though) you could simply describe the bonds that would occur between the elements as shown below
On the right between the sulfur and oxygen element there is clearly a covalent bond; sharing of electrons and on the left side of the sulfur and left oxygen there is what is known as a 'coordinate covalent bond' something that is learnt in year 12 which you dont need to worry about but I just told you to understand that in terms of structure it would be made up of covalent bonds (btw just realised that you may not have learnt lewis dot diagrams and most of this stuff is year 11, tell me if you havent learnt lewis diagrams if you want me to explain but if you already understand them then its all good)


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Right, thanks. I’ve learnt about Lewis Dot Diagrams and valence shells. But you said Silicon Dioxide cannot be Ionic because they’re not giving away electrons and such. But if a basic question just asked whether Silicon Dioxide was Ionic compound or Covalent, I would automatically assume it’s Ionic because Silicon is a semi-metal and I’ve been taught that Covalent molecules only occur between non-metals.
 

ichila101

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Right, thanks. I’ve learnt about Lewis Dot Diagrams and valence shells. But you said Silicon Dioxide cannot be Ionic because they’re not giving away electrons and such. But if a basic question just asked whether Silicon Dioxide was Ionic compound or Covalent, I would automatically assume it’s Ionic because Silicon is a semi-metal and I’ve been taught that Covalent molecules only occur between non-metals.
Okay, fair enough but as long as you can prove to your teacher that by definition (or by recording what they said or something or some weird textbook) that covalent compounds can only occur between non-metals, you should be able to get it marked as correct, otherwise it would be wrong

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There is also another question I have: H2O is a covalent molecular substance and is held together by Strong covalent bonds and weak intermolecular forces. Yet when we boil it, the intermolecular forces are gone but the covalent bonds still hold together the Hydrogen and Oxygen atoms. Whereas say for an Ionic or Metallic structure, when we boil/melt it - the ionic/metallic bonds also diminish resulting in the liquid/gaseous state. Even for Covalent network substances, once we melt it; the covalent bonds are gone and hence it's in a liquid form. I don't understand how covalent molecular substances can become liquid/gases if the Covalent bonds remain intact even after boiling/melting the substance.

Also regarding the solubility of these substances. We know that when a Covalent Molecular substance dissolves in water, the atoms don't break apart from each other but rather stays together (i.e. if DiIodine dissolved, the Iodine molecules would still remain intact and not split apart). Whereas for Ionic compounds, once they dissolve in water; the ions are split apart and remain free as charged ions.

My question is, say we have a soluble tablet (used as medication) dissolved in water, would the atoms being split apart (Ionic) have the same affect as the atoms being stuck together (Covalent molecular)? Do Scientists want the atoms to stick together so that the 'healing' properties of the tablet actually impacts the body or would the atoms being split apart have the same impact?
 
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ichila101

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Anyone got answers to Q5 and Q6?
some smart science uni students might help (maybe), but Ill make do for now
5) How does cutting of an object work in the molecular level? We know that metals don't break since the metallic structures prevent it to (Cations slide across a cushion of delocalised electrons). But say we had a metal-cutting band saw or something and managed to cut a piece of metal in half, how does that work? What would happen to the bonds? What caused them to break?
I cant exactly recall metal being cut, however I do recall 1000 degrees Celcius knives on youtube cutting metal and that is simply because the metal is subject to high temperatures which melts it

6) Similar to Q5, Ionic compounds tend to be brittle. How come? Aren't their bonds strong due to the opposite charges attracting each other? I understand when a force is applied the ions displace and repel each other and that causes to break the structure, but why does that happen in the first place? Why is it so easy to displace the Cation and Anions if their bonds are so strong? How is a metallic structure's bonding stronger than one of an Ionic structure? Metallic structures are Cations surrounded by a sea of delocalised Electrons, wouldn't it be easier for them to vibrate more and break apart whilst Ionic bonds are more tightly held together by the Electrostatic attraction between the Cation and Anions, hence harder for it to vibrate and break (yet somehow Ionic compounds are brittle)?
Brittle doesn't mean its weak it simply means it has a higher chance of breaking more easily however something can still be strong yet brittle. For example, the more calcium I drink, the stronger my bones get however the stronger my bones get the more brittle they get, its kind of like the opposite of flexible.

Yeah ionic bonds are stronger than metallic bonds, what do you mean by displace? can you give an example if possible
 

ichila101

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There is also another question I have: H2O is a covalent molecular substance and is held together by Strong covalent bonds and weak intermolecular forces. Yet when we boil it, the intermolecular forces are gone but the covalent bonds still hold together the Hydrogen and Oxygen atoms. Whereas say for an Ionic or Metallic structure, when we boil/melt it - the ionic/metallic bonds also diminish resulting in the liquid/gaseous state. Even for Covalent network substances, once we melt it; the covalent bonds are gone and hence it's in a liquid form. I don't understand how covalent molecular substances can become liquid/gases if the Covalent bonds remain intact even after boiling/melting the substance.

Also regarding the solubility of these substances. We know that when a Covalent Molecular substance dissolves in water, the atoms don't break apart from each other but rather stays together (i.e. if DiIodine dissolved, the Iodine molecules would still remain intact and not split apart). Whereas for Ionic compounds, once they dissolve in water; the ions are split apart and remain free as charged ions.

My question is, say we have a soluble tablet (used as medication) dissolved in water, would the atoms being split apart (Ionic) have the same affect as the atoms being stuck together (Covalent molecular)? Do Scientists want the atoms to stick together so that the 'healing' properties of the tablet actually impacts the body or would the atoms being split apart have the same impact?
I think I mentioned this earlier not fully sure but anyways intermolecular bonds make up the physical properties of the compound, and because covalent compounds have intermolecular bonds those structures break and not the intramolecular ones. However ionic, metallic and covalent network all do not have any intermolecular bonds so their physical properties are created by their intramolecular bonds which is why this break instead

They are completely different structures (since one is made up of ionic and the other is made up of covalent) hence they will have different impacts.
 

ichila101

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There is also another question I have: H2O is a covalent molecular substance and is held together by Strong covalent bonds and weak intermolecular forces. Yet when we boil it, the intermolecular forces are gone but the covalent bonds still hold together the Hydrogen and Oxygen atoms. Whereas say for an Ionic or Metallic structure, when we boil/melt it - the ionic/metallic bonds also diminish resulting in the liquid/gaseous state. Even for Covalent network substances, once we melt it; the covalent bonds are gone and hence it's in a liquid form. I don't understand how covalent molecular substances can become liquid/gases if the Covalent bonds remain intact even after boiling/melting the substance.

Also regarding the solubility of these substances. We know that when a Covalent Molecular substance dissolves in water, the atoms don't break apart from each other but rather stays together (i.e. if DiIodine dissolved, the Iodine molecules would still remain intact and not split apart). Whereas for Ionic compounds, once they dissolve in water; the ions are split apart and remain free as charged ions.

My question is, say we have a soluble tablet (used as medication) dissolved in water, would the atoms being split apart (Ionic) have the same affect as the atoms being stuck together (Covalent molecular)? Do Scientists want the atoms to stick together so that the 'healing' properties of the tablet actually impacts the body or would the atoms being split apart have the same impact?
I think I mentioned this earlier not fully sure but anyways intermolecular bonds make up the physical properties of the compound, and because covalent compounds have intermolecular bonds those structures break and not the intramolecular ones. However ionic, metallic and covalent network all do not have any intermolecular bonds so their physical properties are created by their intramolecular bonds which is why this break instead

They are completely different structures (since one is made up of ionic and the other is made up of covalent) hence they will have different impacts.
 

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I think I mentioned this earlier not fully sure but anyways intermolecular bonds make up the physical properties of the compound, and because covalent compounds have intermolecular bonds those structures break and not the intramolecular ones. However ionic, metallic and covalent network all do not have any intermolecular bonds so their physical properties are created by their intramolecular bonds which is why this break instead
So is there way to break the Covalent bonds (intramolecular bonds) for Covalent molecular substances? If so, what would the outcome of it look like?

They are completely different structures (since one is made up of ionic and the other is made up of covalent) hence they will have different impacts.
Right, but what I'm trying to say is; would an Ionic substance (tablet) dissolved in water (with the result of free ions) have the same impact on the body as it would if the tablet was just swallowed instead (ionic bonds hold the opposing ions together) without any water?
 
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ichila101

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Right, but what I'm trying to say is; would an Ionic substance (tablet) dissolved in water (with the result of free ions) have the same impact on the body as it would if the tablet was just swallowed instead (ionic bonds hold the opposing ions together) without any water?
Yes, if both are the exact same (i.e. tablets) and all the water is consumed rhey will both have the exact same impact on the body, the only difference is it will be easier to swallow the one that is dissolved

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some smart science uni students might help (maybe), but Ill make do for now

I cant exactly recall metal being cut, however I do recall 1000 degrees Celcius knives on youtube cutting metal and that is simply because the metal is subject to high temperatures which melts it
Oh, so the 'cutting' of metal is essentially just applying a lot of heat energy (through friction) and then melting it apart?

Brittle doesn't mean its weak it simply means it has a higher chance of breaking more easily however something can still be strong yet brittle. For example, the more calcium I drink, the stronger my bones get however the stronger my bones get the more brittle they get, its kind of like the opposite of flexible.

Yeah ionic bonds are stronger than metallic bonds, what do you mean by displace? can you give an example if possible
Ahh right, I confused the definition of brittleness. Makes more sense with the definition below:

A textbook I was going through stated this "If the ionic crystal is hit with an external force, the electrostatic attractions between the Cations and Anions can break (leading them to displace), allowing ions of like charge to line up and repel. Thus the crystal will fracture". I guess this explains its brittleness?
 
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ichila101

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Oh, so the 'cutting' of metal is essentially just applying a lot of heat energy (through friction) and then melting it apart?



Ahh right, I confused the definition of brittleness. Makes more sense with the definition below:

A textbook I was going through stated this "If the ionic crystal is hit with an external force, the electrostatic attractions between the Cations and Anions can break (leading them to displace), allowing ions of like charge to line up and repel. Thus the crystal will fracture". I guess this explains its brittleness?
Yeah kind of, brittleness is a comparative term so its simply the measure of how easy it is for the ionic crystal to displace. If there is a high chance of it displacing then it is really brittle however if there is a low chance of it displacing then it isnt really that brittle

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Yeah kind of, brittleness is a comparative term so its simply the measure of how easy it is for the ionic crystal to displace. If there is a high chance of it displacing then it is really brittle however if there is a low chance of it displacing then it isnt really that brittle

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Do you know whether Covalent compounds (Covalent Molecular & Covalent Network substances) are brittle or not? If they are, how does that work (in terms of molecular level)?
 

ichila101

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Do you know whether Covalent compounds (Covalent Molecular & Covalent Network substances) are brittle or not? If they are, how does that work (in terms of molecular level)?
I dont think you can give covalent compounds a level of brittleness since thet just vary way to much. Like covalent compounds like H2O and CH4 are not brittle at all because they arent even solid they are liquids and gases respectively. The case is the same with covalent network, if we compared graphite with diamond, we would say graphite could break really easily though this certainly isnt the case for diamond

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I dont think you can give covalent compounds a level of brittleness since thet just vary way to much. Like covalent compounds like H2O and CH4 are not brittle at all because they arent even solid they are liquids and gases respectively. The case is the same with covalent network, if we compared graphite with diamond, we would say graphite could break really easily though this certainly isnt the case for diamond

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Yeah, but let's say when we freeze the above mentioned compounds. Ice is quite brittle (frozen water).
 

ichila101

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Yeah, but let's say when we freeze the above mentioned compounds. Ice is quite brittle (frozen water).
Sorry yeah in that case ice is pretty brittle, however noble gases for example cannot be frozen so they do not have a brittle property.
 

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Sorry yeah in that case ice is pretty brittle, however noble gases for example cannot be frozen so they do not have a brittle property.
So since Ice is brittle, what's the reasoning behind that in the molecular level? I get the intermolecular forces are weak, but the Covalent bonds are strong. To break the structure, do the intermolecular forces only need to be broken? Is there any reasoning like the Ionic compounds (where same charges repel each other and break the crystal)?

If Covalent bonds don't need to be broken for the structure to change, then what purpose do they serve by holding the atoms together? For example, you said that a solid tablet (let's use NaCl for simplicity) would have the same effect as the Na+ and Cl- ions which are free in the water (dissolved state). So relating back to that same example, if we were to have H2O (in liquid state) where the Covalent Bonds are broken and the Hydrogen and Oxygen atoms are roaming free in that liquid structure, wouldn't it be the same as drinking normal water (where Covalent bonds hold the Hydrogen and Oxygen atoms together)?

Also, will we learn why Noble gases cannot be frozen in Yrs 11/12? I saw something related to it being unpolarised and other terms I'm unfamiliar with.
 
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