Solving the Mystery of Ideal Gases

[siewwen168]

hello,i have question to ask here.

1. which of the following is not correct for
an ideal gas?
A. the average kinetic energy of the gas
increases with temperature
B. the force of repulsion between
molecules is proportional to the
distance between them
C. the gas molecules have mass but
negligible volume.
D. there are intermolecular forces
between molecules

my answer is D,but the answer given is B,why?

 

[dextercioby]

Why did you choose D ?

Daniel.

 

[siewwen168]

because i thought D is the wrong answer,ideal gas don't have intermolecular forces between molecules

 

[Gokul43201]

If there were no forces between the molecules, why would it take any work to compress an ideal gas ?

The notion that an ideal gas has no intermolecular forces is common (even Wiki gets this wrong). But at the same time it is accepted that the molecules undergo elastic collisions. These two statements are in direct contradiction with each other. When molecules collide, they do experience forces. However, when the molecules are separated, the force between them is neglected.

 

[Gokul43201]

Also, if you think D is wrong (that in fact, there are no forces), how can you think that B is correct (that the force is linear is the distance) ?

 

[nikhaar jain]

i think if u read it from any compettitive exam book than it is just a fault ,otherwise it is very true that there r no intermolecular force between molecules of real gas

 

[Gokul43201]

On the contrary there very much are intermolecular forces between real gases.

 

[siewwen168]

thanks for your explanation..but which one is the most suitable answer?or no answer at all?

 

[Gokul43201]

B is the correct answer (meaning that it is the only incorrect statement).

The force is not linear in the separation, but that does not mean there is no force. The force vs. separation curve is zero everywhere except very near the origin, where it has a sharp spike.