Page 115 - Physical chemistry understanding our chemical world

82 ENERGY AND THE FIRST LAW OF THERMODYNAMICS

Internal energy

Initially Finally
(before reaction) (after reaction)
Figure 3.2 In an endothermic process, the final product has more energy than the initial starting
materials. Energy has been taken in

Worked Example 3.2 What is the change in internal energy dur-
A process is endo- ing sweating?
thermic if the final
state has more energy
The definition of U in Equation (3.1) is U = U (final) − U (initial) ,so
than does the ini- the value of U (evaporation) is obtained as U (water, g) − U (water, l) .We
tial state. The word
already know that the final state of the water is more energetic than
derives from the Greek
roots thermo (mean- its initial state, so the value of U is positive. We say such a process
is endothermic.
ing ‘energy’ or ‘tem-
perature’) and endo
(meaning ‘inside’ or We feel cooler when sweating because the skin loses energy
‘within’). An endother- by transferring it to the water on its surface, which then evapo-
mic process takes in rates. This process of water evaporation (sweating) is endothermic
energy. because energy passes from the skin to the water, and a body
containing less energy has a lower temperature, which is why we
feel cooler.

Aside

Heat is absorbed from the surroundings while a liquid evaporates. This heat does not
change the temperature of the liquid because the energy absorbed equates exactly to
the energy needed to break intermolecular forces in the liquid (see Chapter 2). Without
these forces the liquid would, in fact, be a gas.
At constant temperature, the heat absorbed during evaporation is often called the
latent heat of evaporation. This choice of words arises from the way evaporation occurs
without heating of the liquid; ‘latent’ is Latin for ‘hidden’, since the energy added to is
not ‘seen’ as a temperature rise.