General Chemistry Question. PLEASE HELP!!!

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clc8503

clc8503

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I'm having trouble with this particular concept of General Chemistry. I will appreciate any help I can get.

Thanks!

Consider the following gas molecules: CH4 CF2Cl2 CHCl3 CH3Cl CH2F2

(a) At 25.0 °C, which would be moving the fastest?


(b) At 25.0 °C, which molecule would have the most energy?


(c) If the above gas molecules were all moving at he same speed, put them in order of increasing temperature.

< < < <


Thank you so much!
 
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clc8503 said:
I'm having trouble with this particular concept of General Chemistry. I will appreciate any help I can get.

Thanks!

Consider the following gas molecules: CH4 CF2Cl2 CHCl3 CH3Cl CH2F2

(a) At 25.0 °C, which would be moving the fastest?


(b) At 25.0 °C, which molecule would have the most energy?


(c) If the above gas molecules were all moving at he same speed, put them in order of increasing temperature.

< < < <


Thank you so much!
Click to expand...

How sad is it that my brain is fried that I can't answer this question? Don't worry CLC...someone will be along soon whose mind is in a better place than mine is right now...(too much studying)..sorry. :luck:
 
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Fastest...Wouldn't you just look at the weight first? Lighter -> move Faster

Kinetic energy..if at the same temp they should all have the same kinetic energy, unless there is some rule I have forgotten

The last part is kind of fun. They are moving at the same speed. It is kind of a logic/reason exercise. If you are factoring the weight as the main reason, then you could think that a reallly heavy molecule moving at the same speed as the really light molecule will be at a higher temperature. Just have to reason them out like that.

If there is another way please tell me..I just eyeballed it really fast.
 
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If they are all moving at the same speed than temp. relative to others can be determined simply by comparing molecular weights. Heavier - higher temp, Lighter - lower temp.
 
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hmm..the only reason I didn't think weight played as big a role as intermolecular forces is because they are not ideal gasses...so graham's law of effusion/diffusion can't apply...or can it?
 
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MossPoh said:
Fastest...Wouldn't you just look at the weight first? Lighter -> move Faster

Kinetic energy..if at the same temp they should all have the same kinetic energy, unless there is some rule I have forgotten

The last part is kind of fun. They are moving at the same speed. It is kind of a logic/reason exercise. If you are factoring the weight as the main reason, then you could think that a reallly heavy molecule moving at the same speed as the really light molecule will be at a higher temperature. Just have to reason them out like that.

If there is another way please tell me..I just eyeballed it really fast.
Click to expand...

RIGHT ON THE MARK.

Lighter = faster at the same temperature, because the same temperature means that they all have the same kinetic energy. If they all have the same 0.5mv2, then small m is balanced out by larger v.

And I so agree that the last question is very clever. If they all have the same speed, then the heaviest one has the greatest kinetic energy. Temperature measures average kinetic energy, so higher kinetic energy equates to a higher temperature. The ranking of highest temperature to lowest temperature for part C follows heaviest to lightest.
 
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vicinihil said:
hmm..the only reason I didn't think weight played as big a role as intermolecular forces is because they are not ideal gasses...so graham's law of effusion/diffusion can't apply...or can it?
Click to expand...

Because the difference in molecular mass between the compounds (16 to 121)is so drastic, the trend can be predicted using Graham's Law. I also assume at the MCAT-level, the deviations from ideality are ignored unless addressed in the passage and/or question. In this particular example, four of the five compounds are liquids at room temperature, so while you are right to consider intermolecular forces, the temperature and/or pressure of the system necessary for the compounds to exist as gases would be so high (temperature) or low (pressure) that Graham's law likely won't have a significant deviation.

You need to consider intermolecular forces and the subsequent deviations when the molecular masses are close, at which time Graham's Law breaks down.
 
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vicinihil said:
so would intermolecular forces ever oplay a role?
Click to expand...


I would say no. Since we are dealing with a gas, and molecules in a gas state don’t come close enough frequently enough to have intermolecular forces apply. So you shouldn’t be looking at London Dispersion, or polarity. Someone may correct me if I’m wrong.
 
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