Page 43 - Physical chemistry understanding our chemical world

10 INTRODUCTION TO PHYSICAL CHEMISTRY

generally easier to state the corollary, ‘a body has a higher temper-
‘Corollary’ means a
deduction following on ature if it has more energy, and a lower temperature if it has less
from another, related, energy’.
fact or series of facts. We have been rather glib so far when using words such as
‘heat’ and ‘temperature’, and will be more careful in future. Heat
is merely one way by which we experience energy. Everything contains energy in
various amounts, although the exact quantity of the energy is not only unknown
but unknowable.
Much of the time, we, as physical chemists, will be thinking
about energy and the way energetic changes accompany chemical
The word ‘thermo- changes (i.e. atoms, ions or whole groups of atoms combine, or
chemistry’ has two
add, or are being lost, from molecules). While the total energy
roots: thermo,mean-
ing ‘temperature or cannot be known, we can readily determine the changes that occur
energy’, and chem- in tandem with chemical changes. We sometimes give the name
istry, the science of thermochemistry to this aspect of physical chemistry.
the combination of In practice, the concept of temperature is most useful when deter-
chemicals. We see mining whether two bodies are in thermal equilibrium. Firstly, we
how ‘thermochemistry’ need to appreciate how these equilibrium processes are always
studies the energy and dynamic, which, stated another way, indicates that a body simulta-
temperature changes
accompanying chemi- neously emits and absorbs energy, with these respective amounts
cal changes. of energy being equal and opposite. Furthermore, if two bodies
participate in a thermal equilibrium then we say that the energy
emitted by the first body is absorbed by the second; and the first
body also absorbs a similar amount of energy to that emitted by
A body in ‘dynamic equi- the second body.
librium’ with another
exchanges energy with Temperature is most conveniently visualized in terms of the
it, yet without any net senses: we say something is hotter or is colder. The first ther-
change. mometer for studying changes in temperature was devised in 1631
by the Frenchman Jean Rey, and comprised a length of water in a
glass tube, much like our current-day mercury-in-glass thermometers but on a much
bigger scale. The controlled variable in this thermometer was temperature T , and the
observed variable was the length l of the water in the glass tube.
Rey’s thermometer was not particularly effective because the density of water is so
low, meaning that the volume of the tube had to be large. And the tube size caused
an additional problem. While the water expanded with temperature (as required for
the thermometer to be effective), so did the glass encapsulating it. In consequence
of both water and glass expanding, although the water expanded in a straightforward
way with increasing temperature, the visible magnitude of the expansion was not in
direct proportion to the temperature rise.
Although we could suggest that a relationship existed between
the length l and the temperature T (saying one is a function of the
Scientists use the word other), we could not straightforwardly ascertain the exact nature
‘ideal’ to mean obeying of the function. In an ideal thermometer, we write the mathemat-
the laws of science.
ical relationship, l = f(T ). Because Rey’s thermometer contained