Page 35 - Physical chemistry understanding our chemical world

2 INTRODUCTION TO PHYSICAL CHEMISTRY

was at the same temperature as we were. We feel warmer firstly because the radiator
is warmer than us, and secondly because some of the heat energy leaves the radiator
and we absorb it. A transfer of energy occurs and, therefore, a change. Without the
cause,the effect of feeling warmer could not have followed.
We are always at the mercy of events as they occur around
A variable is an exper- us. The physical chemist could do nothing if nothing happened;
imental parameter we chemists look at changes. We say a physical chemist alters vari-
can change or ‘tweak’. ables, such as pressure or temperature. Typically, a chemist causes
one variable to change and looks at the resultant response, if any.
Even a lack of a response is a form of result, for it shows us what is and what is not
a variable.

Why does water get hot in a kettle?
The fearsome-looking
word ‘physicochemi- Physicochemical relationships
cal’ means ‘relating to
physical chemistry’. Putting water into an electric kettle does not cause the water to get
hot. The water stays cold until we turn on the power to the kettle
element, which converts electrical energy from the mains into heat energy. The heat
energy from the kettle element is then absorbed by the water, which gets hot as a
direct consequence.
The temperature of the water does not increase much if a small amount of elec-
trical energy is consumed; conversely, the water gets hotter if a greater amount of
energy is consumed and thereafter passed to the water. A physi-
In words, the symbols cal chemist says a ‘relationship’ exists (in this case) between heat
T = f(energy) means ‘T input and temperature, i.e. the temperature of the water depends on
is a function of energy’. the amount of energy consumed.
Note how variables are Mathematically, we demonstrate the existence of a relationship
usually printed in italic by writing T = f (energy), where T is temperature and the small
type.
f means ‘is a function of’.
So the concept of variables is more powerful than just changing
parameters; nor do physical chemists merely vary one parameter and see what happens
to others. They search for ‘physicochemical’ relationships between the variables.

Are these two colours complementary?

Qualitative and quantitative measurements

We often hear this question, either at the clothes shop or at a paint merchant. Either
someone wants to know if the pink colour of a sweatshirt matches the mauve of a
skirt, or perhaps a decorator needs to know if two shades of green will match when
painted on opposing bedroom walls.