Page 113 - Physical chemistry understanding our chemical world

80 ENERGY AND THE FIRST LAW OF THERMODYNAMICS

From Equation (3.1), which defines the changes to internal energy,
We will use the word
U for the process in Equation (3.2) is U (condensation) = U (water, l)
‘process’ here to mean
any physical chemistry − U (water, g) .
requiring a change in As we saw in Chapter 2, the simplest way of telling whether some-
energy. thing gains energy is to ascertain whether its temperature goes up.
The temperature of the skin does increase greatly (so it feels hot); its
energy increases following the condensation reaction. Conversely,
the temperature of the water decreases – indeed, its temperature decreases to below its
boiling temperature, so it condenses. The water has lost energy. In summary, we see
how energy is transferred, with energy passing from the steam to the skin.
When energy passes from one body to another, we say the
The word ‘exother- process is thermodynamic. The condensation of water is a ther-
mic’ comes from two modynamic process, with the energy of the water being lower fol-
Greek roots: thermo, lowing condensation. Stated another way, the precursor steam had
meaning ‘energy’ or more energy than the liquid water product, so U (final) is lower than
‘temperature’, and exo U (initial) . Figure 3.1 represents this situation visually, and clearly
meaning ‘outside’ or shows how the change in internal energy U during steam con-
‘beyond’. An exother- densation is negative. We say the change in U is exothermic.
mic process therefore The energy lost by the steam passes to the skin, which therefore
gives out energy.
gains energy. We experience this excess energy as burning: with the
skin being an insulator, the energy from the steam remains within
the skin and causes damaging thermal processes. Nerve endings in the skin report the
damage to the brain, which leads to the experience of pain.
But none of the energy is lost during condensation, so exactly the same amount of
energy is given out by the steam as is given to the skin. (In saying this, we assume
no other thermodynamic processes occur, such as warming of the surrounding air.
Even if other thermodynamic processes do occur, we can still say confidently that no
energy is lost. It’s just more difficult to act as an ‘energy auditor’, and thereby follow
where it goes.)

Internal energy

Initially Finally
(before reaction) (after reaction)

Figure 3.1 In an exothermic process, the final product has less energy than the initial starting
materials. Energy has been given out