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Avogadro wrote:Since I need the practice before Regionals, I'll put one forward:

Consider a material with a Face-Centered Cubic (FCC) crystal structure and an atomic radius of 300 pm.
a. What is the atomic packing factor? Show work.
b. Which plane has the greatest density? Calculate the density of this plane.
I'll be honest this is mostly because I don't know how to do the second part

just the effective volume of all the atoms contained in the unit cell (using the atomic radius) divided by the volume of the unit cell given by 16*sqrt(2)*R^3. There are 4 atoms contained by the unit cell, so the APF is 0.74.

Planar density is given by the number of atoms contained within the plane divided by the area of that plane. For face-centered cubic, there are only a couple planes that I can think of which have a significant number of atoms contained. These include the (100) plane and the (110) plane. There are 2 atoms contained in the (100) plane and 2 atoms in the (110) plane. Because the (110) plane is diagonal, it has a larger area. Therefore, the (100) plane has the highest planar density.

Tom_MS wrote:

Avogadro wrote:Since I need the practice before Regionals, I'll put one forward:

Consider a material with a Face-Centered Cubic (FCC) crystal structure and an atomic radius of 300 pm.
a. What is the atomic packing factor? Show work.
b. Which plane has the greatest density? Calculate the density of this plane.
I'll be honest this is mostly because I don't know how to do the second part

a.

The atomic packing factor is (Click to Show Hidden Text)

just the effective volume of all the atoms contained in the unit cell (using the atomic radius) divided by the volume of the unit cell given by 16*sqrt(2)*R^3. There are 4 atoms contained by the unit cell, so the APF is 0.74.

b.

I'll explain it but this page gives a great visual for planar density in general: [url]http://web.utk.edu/~prack/mse201/chp7.pdf[/url] (Click to Show Hidden Text)

Planar density is given by the number of atoms contained within the plane divided by the area of that plane. For face-centered cubic, there are only a couple planes that I can think of which have a significant number of atoms contained. These include the (100) plane and the (110) plane. There are 2 atoms contained in the (100) plane and 2 atoms in the (110) plane. Because the (110) plane is diagonal, it has a larger area. Therefore, the (100) plane has the highest planar density.

Avogadro wrote:I'll also revive this thread!

A new polymer is being tested for use as a a lightweight rain jacket.
a. Given that the empirical formula of this compound is (completely made up, not real) CH3N, what types of attractive forces would you expect molecules of this polymer to exhibit?
b. In a laboratory, water is dropped onto a 1 cm x 1 cm square of the material. Water is added to the droplet in order to determine the maximum and minimum contact angles. Because the two angles are very similar, the contact angle is simplified to be 60°. Would you recommend this material for its intended use? Explain.
c. Name the test used above to determine contact angle, and give another test that can be used to achieve the same purpose.

hearthstone224 wrote:

Avogadro wrote:I'll also revive this thread!

A new polymer is being tested for use as a a lightweight rain jacket.
a. Given that the empirical formula of this compound is (completely made up, not real) CH3N, what types of attractive forces would you expect molecules of this polymer to exhibit?
b. In a laboratory, water is dropped onto a 1 cm x 1 cm square of the material. Water is added to the droplet in order to determine the maximum and minimum contact angles. Because the two angles are very similar, the contact angle is simplified to be 60°. Would you recommend this material for its intended use? Explain.
c. Name the test used above to determine contact angle, and give another test that can be used to achieve the same purpose.

Um, I'm really not quite sure but here goes:

a) I think there would be covalent bonding in between the carbon and the nitrogen, but also maybe some hydrogen bonding occuring between the nitrogen and the hydrogen.

b) Not completely, since I would expect an angle of 90 degrees or more to be ideal for use on a rain jacket. The whole purpose of the rain jacket is to repel rain, not absorb it. The contact angle is the wetability, so if the angle is about 60 I think that would be alright but preferrably something with an angle of 90 degrees or more.

c) Contact angle hystersis? Another test I can think of would be the Wilhelmy plate method which measures the tensile forces of the droplet in which it is put onto.

Avogadro wrote:

hearthstone224 wrote:

Avogadro wrote:I'll also revive this thread!

A new polymer is being tested for use as a a lightweight rain jacket.
a. Given that the empirical formula of this compound is (completely made up, not real) CH3N, what types of attractive forces would you expect molecules of this polymer to exhibit?
b. In a laboratory, water is dropped onto a 1 cm x 1 cm square of the material. Water is added to the droplet in order to determine the maximum and minimum contact angles. Because the two angles are very similar, the contact angle is simplified to be 60°. Would you recommend this material for its intended use? Explain.
c. Name the test used above to determine contact angle, and give another test that can be used to achieve the same purpose.

Um, I'm really not quite sure but here goes:

a) I think there would be covalent bonding in between the carbon and the nitrogen, but also maybe some hydrogen bonding occuring between the nitrogen and the hydrogen.

b) Not completely, since I would expect an angle of 90 degrees or more to be ideal for use on a rain jacket. The whole purpose of the rain jacket is to repel rain, not absorb it. The contact angle is the wetability, so if the angle is about 60 I think that would be alright but preferrably something with an angle of 90 degrees or more.

c) Contact angle hystersis? Another test I can think of would be the Wilhelmy plate method which measures the tensile forces of the droplet in which it is put onto.

a. Looks pretty good to me, maybe mention van der Waals somewhere in there.
b. I'll accept this too, but note that if they're specifically testing the polymer for this one use, if it's not repelling (contact angle >90°) it's probably not worth it for them to produce. c. The second part is good, but the first part is a little bit off. What you gave is the term for the difference in the maximum and receding angles. I'll accept this since you seem to get what I'm trying to go for. But the method itself is known as the Sessile Drop Method.

Tom_MS wrote:

hearthstone224 wrote:

Tom_MS wrote:What is the name of the process by which a clay ceramic hardens under firing? Describe the microstructural changes of the ceramic during this process.

I googled some info but I'm not sure what name you are referring to- most of the sites I found just called the process "firing".

I found that the water molecules inside the clay are being evaporated out and therefore when that happens the structure cannot return back to normal and its sort of "locked in place", and this change is called the ceramic change.

You're right about the water evaporating, but that is more in the drying process than the firing process. This is a difficult question, but I thought that it might be on a test under the manufacturing techniques section of the rules. If you're using the Callister textbook, its in the ceramic applications and processing section. If you're using something else, the name of the process is vitrification.

Tesel wrote: How is vitrification different from sintering? I'm a little confused.