Page 111 - Physical chemistry understanding our chemical world

78 ENERGY AND THE FIRST LAW OF THERMODYNAMICS

The evidence for such a transfer of energy between the mouth and the ice cream is
the change in temperature, itself a response to the minus-oneth law of thermodynamics
(p. 7), which says heat travels from hot to cold. Furthermore, the zeroth law (p. 8)
tells us energy will continue to transfer from the mouth (the hotter object) to the
ice cream (the colder) until they are at the same temperature, i.e. when they are in
thermal equilibrium.

Internal energy U

Absolutely everything possesses energy. We cannot ‘see’ this energy directly, nor do
we experience it except under certain conditions. It appears to be invisible because it
is effectively ‘locked’ within a species. We call the energy possessed by the object
the ‘internal energy’, and give it the symbol U.
The internal energy U is defined as the total energy of a body’s
We cannot know how components. Unfortunately, there is no way of telling how much
much energy a body energy is locked away. In consequence, the experimentalist can
or system has ‘locked’ only look at changes in U.
within it. Experimen- The energy is ‘locked up’ within a body or species in three prin-
tally, we can only study cipal ways (or ‘modes’). First, energy is locked within the atomic
changes in the internal nuclei. The only way to release it is to split the nucleus, as hap-
energy, U.
pens in atomic weapons and nuclear power stations to yield nuclear
energy. The changes in energy caused by splitting nuclei are mas-
sive. We will briefly mention nuclear energy in Chapter 8, but the
The energy E locked topic will not be discussed otherwise. It is too rare for most physical
into the atomic nucleus chemists to consider further.
is related to its mass This second way in which energy is locked away is within chem-
m and the speed of ical bonds. We call this form of energy the chemical energy, which
light c, according to is the subject of this chapter. Chemical energies are smaller than
the Einstein equation, nuclear energies.
2
E = mc .
And third, energy is possessed by virtue of the potential energy,
and the translational, vibrational, rotational energy states of the
atoms and bonds within the substance, be it atomic, molecular or ionic. The energy
within each of these states is quantized, and will be discussed in greater detail in
Chapter 9 within the subject of spectroscopy. These energies are normally much
smaller than the energies of chemical bonds.
As thermodynamicists, we generally study the second of these
Strictly, the bonds modes of energy change, following the breaking and formation of
are held together with bonds (which are held together with electrons), although we occa-
‘outer-shell’ electrons. sionally consider potential energy. The magnitude of the chemical
energy will change during a reaction, i.e. while altering the number
and/or nature of the bonds in a chemical. We give the name calorimetry to the study
of energy changes occurring during bond changes.