A level: Kinetic Theory Questions


(a) The molecular theory model of an ideal gas leads to the derivation of the equation

pV = 1/3 Nm

Explain what each symbol in the equation represents.

p: pressure and V: volume

N: number of molecules

m: mass of one molecule/particle/atom

: mean square speed (NOT 'mean speed squared' - that is something entirely different!)

(4 marks)

(b) One assumption used in the derivation of the equation stated in part (a) is that molecules are in state of random motion.

(i) Explain what is meant by random motion.

When molecules are in state of random motion they have a range of speeds and they have no preferred direction of movement

(ii) State two more assumptions used in this derivation.

Any two of the following points:

  • elastic collisions
  • intermolecular forces are negligible (except during collisions)
  • volume of molecules negligible (compared to volume of container)
  • time of collisions negligible (compared to time between collisions)
  • all molecules identical
  • laws of statistics apply or large number of molecules
  • Newtonian laws apply

(3 marks)

(c) Describe how the motion of gas molecules can be used to explain the pressure exerted by a gas on the walls of its container.

Molecules are in constant random motion. When the molecules collide (with the walls) the walls exert a force on the molecules causing a change in their momentum. The molecules exert an (equal and opposite) force (on the walls)creating pressure (as pressiure = force/area)

(4 marks)

(Total 11 marks)