Page 93 - Physical chemistry understanding our chemical world

60 INTRODUCING INTERACTIONS AND BONDS

All matter seeks to minimize its energy and entropy; see Chapter 4. This concept
explains, for example, why a ball rolls down a hill, and only stops when it reaches
its position of lowest potential energy. These interparticle interactions form for a
similar reason.
When we say that two atoms interact, we mean that the outer electrons on the two
atoms ‘respond’ to each other. The electrons within the inner orbitals are buried too
deeply within the atom to be available for interactions or bonding. We indicate this
situation by saying the electrons that interact reside within the ‘frontier’ orbitals.
And this interaction always occurs in such as way as to minimize the energy. We
could describe the interaction schematically by

A + B −−→ product + energy (2.5)

where A and B are particles of gas which interact when their frontier orbitals are
sufficiently close to form a ‘product’ of some kind; the product is generally a molecule
or association complex. (A less na¨ ıve view should also accommodate changes in
entropy; see Chapter 4.)
We saw earlier (on p. 33) that measuring the temperature is the
Energy is liberated simplest macroscopic test for an increased energy content. There-
when bonds and inter- fore, we understand that the tyre becomes warmer during inflation
actions form. because interactions form between the particles with the concurrent
release of energy (Equation (2.5)).

How does a fridge cooler work?

Introduction to the energetics of bond formation

At the heart of a fridge’s cooling mechanism is a large flask containing volatile
organic liquids, such as alkanes that have been partially fluorinated and/or chlorinated,
which are often known as halons or chlorofluorocarbons (CFCs). We place this flask
behind the fridge cabinet, and connect it to the fridge interior with a thin-walled pipe.
The CFCs circulate continually between the fridge interior and the rear, through a
heat exchanger.
Now imagine placing a chunk of cheese in the refrigerator. We need to cool the
◦
cheese from its original temperature to, say, 5 C. Because the cheese is warmer
than the fridge interior, energy in the form of heat transfers from
the cheese to the fridge, as a consequence of the zeroth law of
Converting the liquid thermodynamics (see p. 8). This energy passes ultimately to the
CFC to a gas (i.e. boil-
ing) is analogous to volatile CFCs in the cooling system.
The CFC is initially a liquid because of intermolecular interac-
putting energy into a
kettle, and watching tions (of the London dispersion type). Imagine that the interactions
◦
the water boil off as involves 4 kJ of energy but cooling the cheese to 5 C we liberate
steam. about 6 kJ of energy: it should be clear that more energy is liber-
ated than is needed to overcome the induced dipoles. We say that