IBDP Physics 3.2 – Modelling a gas: IB Style Question Bank- HL Paper 1

Question

Two ideal gases X and Y are at the same temperature. The mass of a particle of gas X is larger than the mass of a particle of gas Y. Which is correct about the average kinetic energy and the average speed of the particles in gases X and Y?

Ans:

Average kinetic energy

Average speed

A

larger for Y

larger for Y

B

same

larger for Y

C

same

same

D

larger for Y

same

Answer/Explanation

Ans: B

Average speed   
Hence lighter particle will have higher Average Speed.
The Kinetic energy associated with each degree of freedom has the same average value (i.e., ), and this average value depends on temperature. Hence it is independent of mass of particle.

Question

What is not an assumption of the kinetic model of an ideal gas?

A  Attractive forces between molecules are negligible.

B Collision duration is negligible compared with time between collisions.

C Molecules suffer negligible momentum change during wall collisions.

 D  Molecular volume is negligible compared with gas volume.

Answer/Explanation

Ans: C

The basic assumptions of kinetic theory are :

  • All gases are made of molecules moving randomly in all directions.
  • The size of a molecule is much smaller than the average separation between the molecules.
  • The molecules exert no force on each other or on the walls of the container except during collision.
  • All collisions between two molecules or between a molecule and a wall are perfectly elastic. Also, the time spent during a collision is negligibly small.
  • The molecules obey Newton’s laws of motion.
  • When a gas is left for sufficient time, it comes to a steady state. The density and the distribution of molecules with different velocities are independent of position, direction and time. This assumption may be justified if the number of molecules is very large.