Phys/Chem Questions (1 Viewer)

[_Anonymous]

Got a few questions I was wondering on.

1) How does the aluminium oxide layer prevent oxidation and why is Aluminium foil good for food?

2) How does the wall exert a force let alone an equal one on a ball? Isn't it F=Ma and the Mass are different for each? How can a stationary object exert a force across? I understand Newton's third law states there's an opposite and equal reaction, but how can a stationary object exert a force when something hits it?

3) Is CH2 the same as C6H12? If so, how? Don't they have different numbers of atoms and hence create different compounds?

4) How does the ball slow down when thrown? Say ball going at 10ms^2 acceleration and mass 50g, how much would air slow it down if air resistance force was 500N? Does the acceleration of the ball decrease or does the opposite "air resistance" force have to be greater than the force of the ball for the ball to decelerate? Also, say there's a day with little to no wind and the force of Air Resistance is around 10N, if the force of a projectile is 500N; why will it eventually slow down and stop? If there's not enough of an opposite force which is strong enough to stop it, why would the ball still eventually slow down and stop? Obviously there wont be a day where the AR force is 10N, but this is just a scenario.

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?

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)?

7) We were learning about Nuclear Fusion and how 4 Hydrogen atoms would create 1 Helium Atom. The equation looked something like this:

4H ---> He

but since Hydrogen has an Atomic number of 1 and a Mass number of 1, how would having 4 of them only give to create Helium? How does having 1 Proton and having 4 of those create Helium which only has 2 Protons (why not 4 Protons)?

8) What's the difference between intermolecular forces and the metallic/ionic/covalent bonding? Why do Covalent molecules have intermolecular forces between them but Covalent network substances have strong covalent bonds? What's the difference?

9) Also another question: how do semi-metals and non-metals combine to form a covalent network substance? For example Silicon Dioxide is a Covalent Network Structure (I thought Covalent meant between non-metal and non-metal)?

 

[Sp3ctre]

1. The aluminium oxide (Al2O3) is inert and therefore prevents the aluminium from undergoing a redox reaction with another substance as the Al2O3 is there to provide a barrier for the aluminium. Aluminium oxide is also impervious to oxygen and water and therefore aluminium foil is good for corrosion resistance of the food. Along with some other obvious properties of aluminium, it is light, non-toxic, malleable and ductile, so it can be made into foil easily and does not affect the food.

 

[Sp3ctre]

3. CH2 (methene) doesn't exist, if its treated as an empirical formula then it's the same as C6H12 since it's just simplified as a ratio of atoms, but alkenes require a double bond between two carbon atoms, but there is only one carbon atom in this case

 

[Sp3ctre]

4. As long as the ball is moving, there will be friction between the ball and the air molecules which will decrease the velocity of the ball. The velocity of the wind does not have to be greater than the ball for it to decelerate. Even in a case where there is no wind at all, there are still air molecules that will collide with the ball which will decelerate it.

 

[_Anonymous]

3. CH2 (methene) doesn't exist, if its treated as an empirical formula then it's the same as C6H12 since it's just simplified as a ratio of atoms, but alkenes require a double bond between two carbon atoms, but there is only one carbon atom in this case

So a compound with the same ratio of atoms would essentially be the same as the most simple ratio of the same compound? So meaning the more atoms there are, regardless it would be the same. I had previously thought that when there are more or less atoms, even with the same ratio there would be different properties of that compound.

 

[_Anonymous]

4. As long as the ball is moving, there will be friction between the ball and the air molecules which will decrease the velocity of the ball. The velocity of the wind does not have to be greater than the ball for it to decelerate. Even in a case where there is no wind at all, there are still air molecules that will collide with the ball which will decelerate it.

But since friction is a force, if the force is smaller than the actual force of the projectile; how would the projectile slow down? I had thought that you would need a greater force than the projectile to actually make an impact (i.e. slow it down). Since 0 net force means object travels at rest or constant velocity, there would need to be a larger opposite force to slow an object down, isn't it essentially the same in this projectile/ball scenario?

 

[Sp3ctre]

So a compound with the same ratio of atoms would essentially be the same as the most simple ratio of the same compound? So meaning the more atoms there are, regardless it would be the same. I had previously thought that when there are more or less atoms, even with the same ratio there would be different properties of that compound.

If you're talking about something like CH4 in terms of being the molecular formula of the compound, then no. CH4, C2H8, C3H12 are all different (C2H8 and C3H12 don't exist though as a side-note), but if CH4 is treated as an empirical formula then yes it's the same as C2H8 and C3H12.

 

[Sp3ctre]

But since friction is a force, if the force is smaller than the actual force of the projectile; how would the projectile slow down? I had thought that you would need a greater force than the projectile to actually make an impact (i.e. slow it down). Since 0 net force means object travels at rest or constant velocity, there would need to be a larger opposite force to slow an object down, isn't it essentially the same in this projectile/ball scenario?

Not sure if I understand what you're trying to say or maybe you're overthinking it. Might have to let someone else clear this question up since physics isn't my strongest subject anyway

 

[Sp3ctre]

7. Pretty sure this is beyond the scope of the course (like some of your other questions), but I happened to research this during my HSC too, I don't know the specifics but iirc the 4 hydrogen atoms do not fuse immediately, there is more than just one single reaction and between the reactions radioactive decay occurs too

 

[ichila101]

Source: https://www.quora.com/What-is-the-process-to-convert-hydrogen-to-helium

From what I can understand from this explanation, protons are just the word that is being used instead of a hydrogen nuclei, which is essentially a proton (since it has no electrons or neutrons but just one proton).

In terms of equations to understand what the guy is saying the following happens:
(See image, disclaimer: havent done this or learnt this so it may not be right but yeah, better than nothing)

 

[ichila101]

Part 2

 

[ichila101]

For question 8, intermolecular forces, as the name suggests, are forces that occur between molecules some of which include hydrogen bonding, dispersion forces and dipole-dipole forces. In contrast, metal/ionic/covalent bonding are all considered to be intramolecular forces because they occur within the molecule. The reason why covalent molecules have intermolecular forces between them is because that is the very definition of an intermolecular force, (i.e. forces that are found between molecules) however this is not the case for covalent network substances because they aren't separate molecules rather one large, macromolecules hence only intramolecules which include covalent bonds are only applicable in them

 

[ichila101]

But since friction is a force, if the force is smaller than the actual force of the projectile; how would the projectile slow down? I had thought that you would need a greater force than the projectile to actually make an impact (i.e. slow it down). Since 0 net force means object travels at rest or constant velocity, there would need to be a larger opposite force to slow an object down, isn't it essentially the same in this projectile/ball scenario?

You are overthinking it, if the opposing force is greater than the force of the projectile then it will actually start to make the projectile move in the opposite direction. For example, There are two people who are pushing each other with 100% of their 'force output' one of them weighs 50 kilograms and the other weighs 30 kilograms, both can accelerate their push at the same speed. The 50 kg person will quite clearly have an enormous advantage in pushing the 30 kilogram guy but the fact still remains that he has to push him which will slow him down. Now if we put a 60 kilogram guy instead of the 30 kilogram guy the 50 kg guy will be getting pushed back

 

[_Anonymous]

For question 8, intermolecular forces, as the name suggests, are forces that occur between molecules some of which include hydrogen bonding, dispersion forces and dipole-dipole forces. In contrast, metal/ionic/covalent bonding are all considered to be intramolecular forces because they occur within the molecule. The reason why covalent molecules have intermolecular forces between them is because that is the very definition of an intermolecular force, (i.e. forces that are found between molecules) however this is not the case for covalent network substances because they aren't separate molecules rather one large, macromolecules hence only intramolecules which include covalent bonds are only applicable in them

How about for Ionic and Metallic compounds? They don't have intermolecular forces but rather Ionic and Metallic bonds between them. How come they don't have any intermolecular forces? For example, NaCl has Ionic bonding due to the strong electrostatic attraction between oppositely charged ions, therefore they're stuck together - yet they don't have an intermolecular force. Or is Sodium Chloride not a molecule?

 

[_Anonymous]

If you're talking about something like CH4 in terms of being the molecular formula of the compound, then no. CH4, C2H8, C3H12 are all different (C2H8 and C3H12 don't exist though as a side-note), but if CH4 is treated as an empirical formula then yes it's the same as C2H8 and C3H12.

Let's say those compounds did exist. They would all have the same properties (same melting/boiling point, electrical/heat conductivity, etc.)?

 

[ichila101]

How about for Ionic and Metallic compounds?

They work the same way as covalent bonds working as intramolecular bonds

How about for Ionic and Metallic compounds? They don't have intermolecular forces but rather Ionic and Metallic bonds between them. How come they don't have any intermolecular forces? For example, NaCl has Ionic bonding due to the strong electrostatic attraction between oppositely charged ions, therefore they're stuck together - yet they don't have an intermolecular force.

Yes they dont have intermolecular forces. This is because the bonds that they have are iirc not between any molecules but rather between atoms or an atom and a molecule which in both cases count as intramolecular forces and not intermolecular forces. Your example also shows this, Na and Cl are both elements and not molecules hence they cannot be considered to be intermolecular since a intermolecular force by definition is any force between molecules

This is also true for metallic bonds, their forces are not occuring between molecules but rather between positively charged elements (i.e. metals) in a 'sea' of electrons. Since it does not occur between molecules it cannot be considered to be a intermolecular force



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[ichila101]

Let's say those compounds did exist. They would all have the same properties (same melting/boiling point, electrical/heat conductivity, etc.)?

Yes they would all have different properties

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[ichila101]

Also another question: how do semi-metals and non-metals combine to form a covalent network substance? For example Silicon Dioxide is a Covalent Network Structure (I thought Covalent meant between non-metal and non-metal)?

Covalent bonds can be defined as bonds that occur between elements where the elements simply share electrons to complete their respective valence shells. This is why it is called 'co' (meaning jointly) and 'valent' (referring to the valence shell).

Although examples of non-metal and non-metal are very commonly used as they are simpler to understand, this does not mean that covalent bonds cannot occur between metalloids or metals as well

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[_Anonymous]

Covalent bonds can be defined as bonds that occur between elements where the elements simply share electrons to complete their respective valence shells. This is why it is called 'co' (meaning jointly) and 'valent' (referring to the valence shell).

Although examples of non-metal and non-metal are very commonly used as they are simpler to understand, this does not mean that covalent bonds cannot occur between metalloids or metals as well

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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)?

 

[_Anonymous]

They work the same way as covalent bonds working as intramolecular bonds


Yes they dont have intermolecular forces. This is because the bonds that they have are iirc not between any molecules but rather between atoms or an atom and a molecule which in both cases count as intramolecular forces and not intermolecular forces. Your example also shows this, Na and Cl are both elements and not molecules hence they cannot be considered to be intermolecular since a intermolecular force by definition is any force between molecules

This is also true for metallic bonds, their forces are not occuring between molecules but rather between positively charged elements (i.e. metals) in a 'sea' of electrons. Since it does not occur between molecules it cannot be considered to be a intermolecular force



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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?