Page 110 - Physical chemistry understanding our chemical world

3

Energy and the first law

of thermodynamics

Introduction

In this chapter we look at the way energy may be converted from one form to another,
by breaking and forming bonds and interactions. We also look at ways of measuring
these energy changes.
While the change in internal energy U is relatively easy to visualize, chemists
generally concentrate on the net energy H, where H is the enthalpy. H relates to
changes in U after adjusting for pressure–volume expansion work, e.g. against the
atmosphere and after transfer of energy q into and out from the reaction environment.
Finally, we look at indirect ways of measuring these energies. Both internal energy
and enthalpy are state functions, so energy cycles may be constructed according to
Hess’s law; we look also at Born–Haber cycles for systems in which ionization
processes occur.

3.1 Introduction to thermodynamics:
internal energy

Why does the mouth get cold when eating ice cream?

Energy

Eating ice cream soon causes the mouth to get cold, possibly to
Ice cream melts as it
the extent of making it feel quite uncomfortable. The mouth of a warms in the mouth
◦
normal, healthy adult has a temperature of about 37 C, and the ice and surpasses its nor-
◦
cream has a maximum temperature of 0 C, although it is likely to mal melting tempera-
◦
be in the range −5to −10 C if it recently came from the freezer. ture; see Chapter 5.
A large difference in temperature exists, so energy transfers from
the mouth to the ice cream, causing it to melt.