Page 65 - Physical chemistry understanding our chemical world

32 INTRODUCTION TO PHYSICAL CHEMISTRY

firstly with each other, and secondly with the internal walls of the
Newton’s first law container they occupy.
states that every action
has an equal but oppo- Just like the walls in a squash court, against which squash balls
continually bounce, the walls of the gas container experience a
site reaction. His sec-
ond law relates the force each time a gas particle collides with them. From Newton’s
force acting on an laws of motion, the force acting on the wall due to this incessant
object to the product of collision of gas particles is equal and opposite to the force applied
its mass multiplied by to it. If it were not so, then the gas particles would not bounce
its acceleration. following a collision, but instead would go through the wall.
We see how each collision between a gas particle and the internal
walls of the container causes the same result as if we had applied a
force to it. If we call the area of the container wall A and give the
The pressure of a symbol F to the sum of the forces of all the particles in the gas, then
gas is a ‘macroscopic’
manifestation of the the pressure p exerted by the gas-particle collisions is given by
‘microscopic’ gas parti- force,F
cles colliding with the pressure,p = (1.15)
internal walls of the area,A
container. In summary, the pressure caused by a container housing a gas is
simply a manifestation of the particles moving fast and colliding
with the container walls.
The surface area inside
a cylinder of radius r
and height h is 2πrh. SAQ 1.11 A cylindrical can contains gas. Its height is
Don’tforgettoinclude 30 cm and its internal diameter is 3 cm. It contains gas
5
the areas of the two at a pressure of 5 × 10 Pa. First calculate the area of the
ends, each of which is cylinder walls (you will need to know that 1 m = 100 cm,
2
4
2
2
πr . so 1 m = 10 cm ), and then calculate the force neces-
sary to generate this pressure.
Aside

A popular misconception says a molecule in the gas phase travels faster than when in
a liquid. In fact, the molecular velocities will be the same in the gas and liquid phases
if the temperatures are the same. Molecules only appear to travel slower in a liquid
because of the large number of collisions between its particles, causing the overall
distance travelled per unit time to be quite short.

Why is it unwise to incinerate an empty can of air
freshener?

The molecular basis of the gas laws

The writing printed on the side of a can of air freshener contains much information.
Firstly, it cites the usual sort of advertising prose, probably saying it’s a better product