Page 72 - Physical chemistry understanding our chemical world

PHYSICAL AND MOLECULAR INTERACTIONS 39

molecules generally draw themselves closer, which itself makes
Translational motion
the interaction stronger. We see a simple analogy with everyday
magnets: once two magnets are brought close enough to induce an is movement through
space, rather than a
interaction, we feel the attractive force dragging them closer still.
vibration about a mean
As soon as the particles of a gas attract, the inertia of the
point or a rotation
aggregate species increases, thereby slowing down all translational about an axis.
motion. And slower particles, such as these aggregates, are an eas-
ier target for further collisions than fast-moving gas atoms and
Formation of an ag-
molecules. The same principle explains why it is impossible to
gregate facilitates fur-
catch someone who is running very fast during a playground game
ther coalescence (even-
of ‘tig’. Only when the runners tire and slow down can they tually forming a con-
be caught. In practice, as soon as an aggregate forms, we find densed phase). We say
that other gas particles soon adhere to it, causing eventual coa- ‘nucleation’ occurs.
lescence and the formation of a droplet of liquid. We say that
nucleation occurs.
With the same reasoning as that above, we can force the molecules of ammonia
still closer together by applying a yet larger pressure, to form a denser state such as
a solid.

Why does steam condense in a cold bathroom?

Elastic and inelastic collisions

In the previous example, we looked at the interactions induced when changing the
external pressure, forcing the molecules into close proximity. We look here at the
effects of changing the temperature.
A bathroom mirror is usually colder than the temperature of the steam rising from
a hot bath. Each molecule of steam (gaseous water) has an enormous energy, which
comes ultimately from the boiler that heats the water. The particles of steam would
remain as liquid if they had less energy. In practice, particles evaporate from the
bath to form energetic molecules of steam. We see this energy as kinetic energy, so
the particles move fast (see p. 30). The typical speeds at which gas particles move
make it inevitable that steam molecules will collide with the mirror.
We say such a collision is elastic if no energy transfers during the No energy is exchang-
collision between the gas particle and the mirror; but if energy does ed during an ‘elastic’
transfer – and it usually does – we say the collision is inelastic. collision, but energy is
exchanged during an
The energy transferred during an inelastic collision passes from
‘inelastic’ collision.
the hot molecule of steam to the cooler mirror. This energy flows
in this direction because the steam initially possessed more energy
per molecule than the mirror as a consequence of its higher temperature. It is merely a
manifestation of the minus-oneth law of thermodynamics, as discussed in Chapter 1.
But there are consequences to the collisions being inelastic: the molecules of steam
have less energy following the collision because some of their energy has transferred.