Gen Chem. When the volume of a gas is decreased at constant temperature...

[bnleong]

When the volume of a gas is decreased
at constant temperature, the pressure
increases because the molecules
A. move faster.
B. move slower.
C. become heavier.
D. become lighter.
E. strike a unit area of the container
more often.


The answer is E. I thought that the molecules will move faster since there will be more collisions which is why I picked B. Is this wrong?????

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[stee1erfan]

think of it this way...when you fill up your car tires with air you are expanding the tire to a "newer" size..the molecules of air now have to travel a little further than the "older" size...another way to look at it is...if you standing in the middle of your bedroom and you measure the time it take for you to go back and forth the ends on your bedroom....NOW..lets say you get a renovation done and you expanded your bedroom size..I betcha it will take you longer to go from one end to another..(keeping everything else the same, your physical abilities,) in the case of the above question (Temperature)...Therefore, if you reduce the "size" it will take less time and the molecules will hit the ends more often...i hope this helps..

 

[PreDental88]

I agree with the above poster. Also, B would be incorrect because temperature is held constant, so the kinetics of the molecules are irrelevant.

 

[ZFT]

More collisions do not increase the speed, when two ideal gas molecules collide (perfect plasticity), they bounce off each other. If one speeds up as a result of the collision, then the other must slow down, so the average speed of the combined gas molecules is unchanged.

If two molecules collide and both their speeds increased, then...energy was created!!

 

[namiie]

Only changes in temperature will affect the speed of the particles, so that's why B is wrong. Temperature itself is just a measure of kinetic energy (energy of moving particles).

E is correct because since the container volume has decreased, the gas particles now have less space with which to move around and thus the probability that they collide with the walls of the container (boundaries of that space) is higher.

 

[DQLEUCD]

When the volume of a gas is decreased
at constant temperature, the pressure
increases because the molecules
A. move faster.
B. move slower.
C. become heavier.
D. become lighter.
E. strike a unit area of the container
more often.


The answer is E. I thought that the molecules will move faster since there will be more collisions which is why I picked B. Is this wrong?????

Only way the molecule will move faster is if you increase the T where the avg kinetic energy theory plays in here but T is constant. Also the distance traveled is a lot less now since there is less space to move, you will strike the container more often. Think of it like the octagon, if they octagon was smaller, there will be no where for the fighters to run so there will be more collisions.