Page 112 - Physical chemistry understanding our chemical world

INTRODUCTION TO THERMODYNAMICS: INTERNAL ENERGY 79

Chemists need to understand the physical chemistry underlying
Placing the Greek letter
these changes in chemical energy. We generally prefer to write in
(Delta) before the
shorthand, so we don’t say ‘changes in internal energy’ nor the
symbol for a parameter
shorter phrase ‘changes in U’, but say instead ‘ U’. But we need
such as U indicates
to be careful: the symbol does not just mean ‘change in’. We the change in U while
define it more precisely with Equation (3.1): passing from an initial
to a final state. We
U (overall) = U (final state) − U (initial state) (3.1) define the change in a
parameter X as X =
X (final state) − X (initial state) .
where the phrases ‘initial state’ and ‘final state’ can refer to a single
chemical or to a mixture of chemicals as they react. This way, U
has both a magnitude and a sign.

Aside

In some texts, Equation (3.1) is assumed rather than defined, so we have to work out
which are the final and initial states each time, and remember which comes first in
expressions like Equation (3.1). In other texts, the final state is written as a subscript
and the initial state as a superscript. The value of U for melting ice cream would
(final state) (melted ice cream)
be written as U ,i.e. U . It may even be abbreviated to
(initial state) (ice cream before melting)
l
U ,where s = solid and l = liquid.
s
To further complicate matters, other books employ yet another notation. They retain
the sub- and super-scripts, but place them before the variable, so the last expression in
l
the previous paragraph would be written as U.
s
We will not use any of these notation styles in this book.

Why is skin scalded by steam?

Exothermic reactions

The ‘condensation’
Water in the form of a gas is called ‘steam’. Two things hap- reaction is one of the
pen concurrently when human skin comes into close contact with simplest forms of a
steam – it could happen, for example, when we get too close to ‘phase change’, which
a boiling kettle. Firstly, the flesh in contact with the steam gets we discuss in greater
burnt and hurts. Secondly, steam converts from its gaseous form depth in Chapter 5.
to become liquid water. We say it condenses. We summarize the
condensation reaction thus:
(3.2)
H 2 O (g) −−→ H 2 O (l)