Chem Lab C

Test your knowledge of various Science Olympiad events.
Tesel
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Re: Chem Lab B/C

Post by Tesel » February 14th, 2018, 4:46 pm

What is the specific heat capacity of an ideal gas? Express this value using variables from this equation: PV=m/(mm)RT. Good luck ;)
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Re: Chem Lab B/C

Post by dmis » February 18th, 2018, 9:13 pm

This question was on an invitational test: and I am pretty confident though my answer was marked wrong. What do you think:

As a substance undergoes a change from the solid to the liquid phase at constant temperature the average
kinetic energy of its molecules
a) decreases
b) increases
c) remains the same

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Re: Chem Lab B/C

Post by g-omal » February 19th, 2018, 2:13 pm

C. Temperature is constant during a phase change and is proportional to average kinetic energy of the particles.
dmis wrote:This question was on an invitational test: and I am pretty confident though my answer was marked wrong. What do you think:

As a substance undergoes a change from the solid to the liquid phase at constant temperature the average
kinetic energy of its molecules
a) decreases
b) increases
c) remains the same

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Re: Chem Lab B/C

Post by g-omal » February 19th, 2018, 2:33 pm

Assuming we're talking about a monatomic ideal gas, you can use Cv = 3/2R and Cp = Cv + R. These come by setting U=3/2nRT equal to Q=nCvT in an isovolumetric process. Going way back, the 3/2 comes from some pretty neat degrees of freedom stuff, but I'm not exactly sure how far I should be going down this deriving rabbit hole.
Tesel wrote:What is the specific heat capacity of an ideal gas? Express this value using variables from this equation: PV=m/(mm)RT. Good luck ;)

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Re: Chem Lab C

Post by Tesel » February 19th, 2018, 6:10 pm

I'd say that's far enough, I should have worded it better, but the main thing was Cv=3/2R.
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Re: Chem Lab C

Post by g-omal » February 20th, 2018, 12:58 pm

Using band theory, explain how you can tell if a material is an insulator, conductor, or semiconductor. (PS: I'm not looking for "if it has a sea of electrons..." or "if it is a metal...") :)

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Re: Chem Lab B/C

Post by dmis » February 23rd, 2018, 9:47 am

g-omal wrote:
C. Temperature is constant during a phase change and is proportional to average kinetic energy of the particles.
dmis wrote:This question was on an invitational test: and I am pretty confident though my answer was marked wrong. What do you think:

As a substance undergoes a change from the solid to the liquid phase at constant temperature the average
kinetic energy of its molecules
a) decreases
b) increases
c) remains the same
I would agree. The key is marked B however, so either they are wrong or we both are.

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Re: Chem Lab C

Post by JavaScriptCoder » April 8th, 2018, 1:06 pm

oof this question marathon is even deader, I'll restart this:

(Source: USNCO 2015, Local #11)

A 2.0 mL sample of a colorless solution, when treated
with a few drops of 2 M hydrochloric acid, forms a white
precipitate which dissolves when the solution is heated to
boiling. The original solution could have contained
which of the following cations?
I. 0.1 M Ag+
II. 0.1 M Pb2+

(A) I only
(B) II only
(C) Either I or II
(D) Neither I nor II
Here to help!~

IMSA Class of '22
Events: Astronomy, Protein Modeling, (formerly: Wright Stuff, Chem Lab, Circuit Lab, Thermodynamics)

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Re: Chem Lab B/C

Post by UTF-8 U+6211 U+662F » April 8th, 2018, 1:22 pm

dmis wrote:
g-omal wrote:
C. Temperature is constant during a phase change and is proportional to average kinetic energy of the particles.
dmis wrote:This question was on an invitational test: and I am pretty confident though my answer was marked wrong. What do you think:

As a substance undergoes a change from the solid to the liquid phase at constant temperature the average
kinetic energy of its molecules
a) decreases
b) increases
c) remains the same
I would agree. The key is marked B however, so either they are wrong or we both are.
The answer is B because the molecules are more free: this is what distinguishes the liquid phase from the solid phase. Note that temperature is NOT proportional to the average kinetic energy of a particle. Its real definition is slightly more complicated and is related to the properties of a gas thermometer and the zeroth law of thermodynamics.

See this chart:
[img]http://www.splung.com/heat/images/latentheat/phasechange.png[/img]
Some energy is used to overcome the latent heat of fusion and goes into increasing the average kinetic energy of the molecules.
(Continue with your question marathoning)

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