Page 63 - Physical chemistry understanding our chemical world

30 INTRODUCTION TO PHYSICAL CHEMISTRY

As the value of R (the gas constant) does not vary, we can simplify the equation by
multiplying both sides by R, to obtain

p 1 V 1 p 2 V 2
=
T 1 T 2
which is Equation (1.14).

What causes pressure?

Motion of particles in the gas phase

The question, ‘What is pressure?’ is another odd question, but is not too difficult
to answer.
The constituent particles of a substance each have energy. In practice, the energy
is manifested as kinetic energy – the energy of movement – and explains why all
molecules and atoms move continually as an expression of that kinetic energy. This
energy decreases as the temperature decreases. The particles only stop moving when
◦
cooled to a temperature of absolute zero: 0 K or −273.15 C.
The particles are not free to move throughout a solid substance, but can vibrate
about their mean position. The frequency and amplitude of such vibration increases as
the temperature rises. In a liquid, lateral motion of the particles is possible, with the
motion becoming faster as the temperature increases. We call this energy translational
energy. Furthermore, as the particles acquire energy with increased temperature, so
the interactions (see Chapter 2) between the particles become comparatively smaller,
thereby decreasing the viscosity of the liquid and further facilitating rapid motion of
the particles. When the interactions become negligible (comparatively), the particles
can break free and become gaseous.
And each particle in the gaseous state can move at amazingly high speeds; indeed,
they are often supersonic. For example, an average atom of helium travels at a mean
speed of 1204 m s −1 at 273.15 K. Table 1.4 lists the mean speeds of a few other gas
molecules at 273.15 K. Notice how heavier molecules travel more slowly, so carbon
dioxide has a mean speed of 363 m s −1 at the same temperature. This high speed
of atomic and molecular gases as they move is a manifestation of
The gas particles are their enormous kinetic energy. It would not be possible to travel
widely separated. so fast in a liquid or solid because they are so much denser – we
call them condensed phases.
The separation between each particle in gas is immense, and
Particles of gas travel usually thousands of times greater than the diameter of a single
fast and in straight gas particle. In fact, more than 99 per cent of a gas’s volume
lines, unless they col-
lide. is empty space. The simple calculation in Worked Example 1.5
demonstrates this truth.