June 1988 P2 Q8a
A sample of air exerts a pressure on the walls of its container. Use your knowledge of the kinetic theory to answer the following questions:
(i) The pressure exerted on each part of the walls is the same. What deductions about the motion of the molecules in the air sample can be made from this fact?
(ii) How does the motion of the molecules of the air sample change when the container and the air inside it are heated?
State and explain how the change affects the pressure exerted by the gas.
(iii) How does the expansion of the container affect the change in pressure you describe?
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Answer:
(i) The air molecules are moving in all directions with the same average speed. Their frequency of collisions per unit area of the wall are the same.
(ii) When the container and the air inside it are heated, the average energy of the air molecules increases. This results in the molecules moving faster in every direction, and causes a higher frequency of collisions of the molecules with the walls of the container. Hence, the pressure exerted by the air increases.
(iii) The expansion of the container results in an increase in the space for molecules to move about. Hence, the frequency of collisions of the molecules with the walls of the containers will be slightly less than in (ii) because the average distance travelled before collision is now further. The pressure exerted is thus lesser.
