Page 128 - Physical chemistry understanding our chemical world

INTRODUCTION TO THERMODYNAMICS: INTERNAL ENERGY 95

capacity C of both the calorimeter itself and the surrounding water, then we can
readily calculate the change in energy U accompanying the reaction.

Whydoesitalwaystake4 mintoboilanegg properly?

Thermochemistry

Most people prefer their eggs to be lightly boiled, with the yellow yolk still liquid
and the albumen solid and white. We say the egg white has been ‘denatured’. The
variation in egg size is not great. An average egg contains essentially a constant
amount of yolk and albumen, so the energy necessary to heat both the yolk and
albumen (and to denature the albumen) is, more or less, the same for any egg.
If the energy required to cook an egg is the same per egg, then the simplest way
to cook the egg perfectly every time is to ensure carefully that the same amount of
energy is absorbed. Most people find that the simplest way to do this is to immerse
an egg in boiling water (so the amount of energy entering the egg per unit time is
constant), and then to say, ‘total energy = energy per second × number of seconds’.
In practice, it seems that most people prefer an egg immersed in boiling water for
about 240 s, or 4 min.
This simple example introduces the topic of thermochemistry.
In a physical chemist’s laboratory, we generally perform a similar Thermochemistry is
type of experiment but in reverse, placing a sample in the calorime- the branch of thermo-
ter and measuring the energy released rather than absorbed.The dynamics concerned
most commonly performed calorimetry experiment is combustion with the way energy
inside a bomb calorimeter (Figure 3.6). We place the sample in the is transferred, released
or consumed during a
calorimeter and surround it with oxygen gas at high pressure, then
chemical reaction.
seal the calorimeter securely to prevent its internal contents leaking
away, i.e. we maintain a constant volume. An electrical spark then
ignites the sample, burning it completely. A fearsome amount of energy is liberated
in consequence of the ignition, which is why we call this calorimeter a ‘bomb’.
The overall heat capacity of the calorimeter is a simple function of the amount of
steel the bomb comprises and the amount of water surrounding it. If the mass is m
and the heat capacity is C, then the overall heat capacity is expressed by

C (overall) = (m (steel) × C m (steel) ) + (m (water) × C m (water) ) (3.8)

If the amount of compound burnt in the calorimeter is n, and
C (overall) is the heat
remembering that no work is done, then a combination of Equa- capacity of the reac-
tions (3.7) and (3.8) suggests that the change in internal energy tion mixture and the
occurring during combustion is given by calorimeter.

C (overall) T
U m (combustion) =− (3.9)
n