Gen Chem Question

[Sublimation]

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HEy guys,

I was going through examkrackers today and i hit this question.

A force is applied to a container of gas reducing its volume by half. The temperature of the gas:

A) Decrease
B) Increase
C) Remains Constant
D) The temperature change depends on the amount of force applied.

 

[PooyaH]

Would it be C or D? I'm leaning more towards D, cuz the force is the pressure applied to the container. If the pressure has been increased by half, then the temp would remain the same. If the pressure has been increased by a factor of 4, then the temp would increase!

 

[Omenino]

I say C, temp is unchanged....

 

[PooyaH]

I say C, temp is unchanged....

Actually now I'm certain it's D lol Temp. depends on volume and pressure of a system!

 

[iwannabadentist]

Actually now I'm certain it's D lol Temp. depends on volume and pressure of a system!

I would have to say its C because in a closed container V and P always act antagonistically. You cant change the volume of a closed container without changing the pressure by the same amount right? I'm guessing the only way to increase the temperature would be to either add heat or take out some moles of gas and the only way to decrease the temperature would be to add some moles of gas or by taking out heat (put it in a bucket of ice?)

 

[Sublimation]

lol. nope guys, initially i thought it would be C. But then i reasoned that D is def. true. But then i thought temp and volume are directly proportional Charles law. And i reasoned that if the volume decreased by half then the temperature must have decreased. I know that one way to force a gas into a solid or liquid is increase the hell out of the pressure at a constant temp. However, the answer is that the temperature will increase B. this is the explanation.

The force does work on the gas which means the internal energy of the gas is increased. Sice the inter. energy of the gas is increased the moles remains the same, temprature, which is average kenetic energy per mole, also increases.

 

[Omenino]

yea ur rite

Actually now I'm certain it's D lol Temp. depends on volume and pressure of a system!

 

[Sublimation]

It is said that one way to cool a gas is to increase the volume. WHen a gas expands the volume of the container it is transferring its KE to the walls of the container to do work therefore loosing energy and decreasing the temp.

 

[iwannabadentist]

lol. nope guys, initially i thought it would be C. But then i reasoned that D is def. true. But then i thought temp and volume are directly proportional Charles law. And i reasoned that if the volume decreased by half then the temperature must have decreased. I know that one way to force a gas into a solid or liquid is increase the hell out of the pressure at a constant temp. However, the answer is that the temperature will increase B. this is the explanation.

The force does work on the gas which means the internal energy of the gas is increased. Sice the inter. energy of the gas is increased the moles remains the same, temprature, which is average kenetic energy per mole, also increases.

hmm i still find that kind of odd because yah you increase the internal energy but at the same time you are increasing the pressure right? I'm thinking that if the correct answer is B, then they want you to assume its NOT a closed system?

 

[PooyaH]

hmm i still find that kind of odd because yah you increase the internal energy but at the same time you are decreasing the pressure right? I'm thinking that if the correct answer is B, then they want you to assume its NOT a closed system?

How do you decrease the pressure by applying force? Just think of the applied force as a piston! When you push the piston down, would you increase the pressure of the container or decrease? of course you'd increase the pressure!!!

 

[iwannabadentist]

^^
Edited!

 

[PooyaH]

^^
Edited!

Oh LOL

 

[Sublimation]

hmm i still find that kind of odd because yah you increase the internal energy but at the same time you are decreasing the pressure right? I'm thinking that if the correct answer is B, then they want you to assume its NOT a closed system?

From wat i understand is that temperature and pressure are both related to the KE of the molecules. Pressure is KE per volume and Temp is KE per mole. So increasing the pressure will dec the volume and subsequently cause an increase in the temp.

 

[PooyaH]

It makes sense for the temp. to increase, because you're increasing the pressure, but you're decreasing the volume at the same time! It only makes sense for the temp. to increase only if the pressure increases by a larger factor than the volume would decrease!

 

[iwannabadentist]

It makes sense for the temp. to increase, because you're increasing the pressure, but you're decreasing the volume at the same time! It only makes sense for the temp. to increase only if the pressure increases by a larger factor than the volume would decrease!

i agree with the 2nd sentence...but im kinda confused abt when you say "you're increasing the pressure, but you're decreasing the volume at the same time!" wouldnt that cause the temp to remain constant? 😕

 

[PooyaH]

i agree with the 2nd sentence...but im kinda confused abt when you say "you're increasing the pressure, but you're decreasing the volume at the same time!" wouldnt that cause the temp to remain constant? 😕

Yes it would! But that doesn't mean pressure would always increase by the same factor that the volume would decrease!!! Sometimes the pressure quadruples as the volume decrease by half, and sometimes the pressure is quartered as the volume is increased by half!

 

[iwannabadentist]

Yes it would! But that doesn't mean pressure would always increase by the same factor that the volume would decrease!!! Sometimes the pressure quadruples as the volume decrease by half, and sometimes the pressure is quartered as the volume is increased by half!

so in order for the exam giver to choose either a increase in decrease in temperature, they would have to provide us with information about what happens to pressure right? if thats the case, then this question is very badly worded

 

[PooyaH]

so in order for the exam giver to choose either a increase in decrease in temperature, they would have to provide us with information about what happens to pressure right? if thats the case, then this question is very badly worded

haha yeah pretty much! Don't you need to be by the pool relaxing since your test is tmrw!? LOL

 

[iwannabadentist]

haha yeah pretty much! Don't you need to be by the pool relaxing since your test is tmrw!? LOL

LOL yah i wish...i can't relax im soo stressed. Believe it or not i dreamt about ochem last night

 

[HowAboutDAT]

i originally guessed C also, but I guess if you think about the particles it can make sense too. Increased pressure means the particles are moving at a higher rate and thus bouncing off one another at a higher rate, causing an increase in temperature... Do you guys agree?

 

[Sublimation]

If i had my exam tomorrow i would sit back and chill. But man there is so much that i forgot for Gen chem lol.

 

[PooyaH]

LOL yah i wish...i can't relax im soo stressed. Believe it or not i dreamt about ochem last night

hahahaha just relax! Trust me it won't be as bad as you think it'd be! you'll be surprised by how easy and basic some of the questions are. As of now there's nothing you can do...I remember I used the phrase " F it" a million times the day of my test LOL cuz there's really nothing you can do and you've studied as much as you could! But I'm sure you'll be fine!

 

[Sublimation]

LOL yah i wish...i can't relax im soo stressed. Believe it or not i dreamt about ochem last night

LMAO🤣

 

[PooyaH]

i originally guessed C also, but I guess if you think about the particles it can make sense too. Increased pressure means the particles are moving at a higher rate and thus bouncing off one another at a higher rate, causing an increase in temperature... Do you guys agree?

Yes that would def. make sense, but then you need to exclude the effects of volume and pressure on the system! Now if you can ignore those 2, then the answer makes sense! Or maybe this question is reffering to real gases and not ideal gases!!!

 

[nze82]

I'm thinking D. Temperature is a measure of the average kinetic energy of the gas particles. When force is applied to the container, this energy is eventually passed on to gas particles, changing their average kinetic energy, which will in turn change the temperature.

 

[Sublimation]

Or maybe this question is reffering to real gases and not ideal gases!!!

Ya, i think so too.

 

[doc toothache]

V1p1/t1= v2p2/t2

 

[tdkyun]

I know of a good example for this. Think about a can of air. When you use it what happens to the can? It cools right? As air is released out of the can, pressure goes down and temperature drops. So if pressure goes up, temperature also goes up.

 

[doc toothache]

I know of a good example for this. Think about a can of air. When you use it what happens to the can? It cools right? As air is released out of the can, pressure goes down and temperature drops. So if pressure goes up, temperature also goes up.

Nice try. However, in the case of "can of air" we are dealing with an open system. The change in temperature that you may be seeing is more a function of the evaporation of the propellant than of the change in pressure.

 

[tdkyun]

Nice try. However, in the case of "can of air" we are dealing with an open system. The change in temperature that you may be seeing is more a function of the evaporation of the propellant than of the change in pressure.

I should have worded that differently. It's an easy way to remember the effect pressure has on temperature.

 

[Sublimation]

http://www.youtube.com/watch?v=v8QqvDD5dNY

Hope this clears things up 🙂