Page 132 - Physical chemistry understanding our chemical world

ENTHALPY 99

Hess’s law is a restatement if the first law of thermodynamics. We do not need to
measure an energy change directly but can, in practice, divide the reaction into several
constituent parts. These parts need not be realizable, so we can actually calculate the
energy change for a reaction that is impossible to perform in the laboratory. The only
stipulation is for all chemical reactions to balance.
The importance of Hess’s law lies in its ability to access information about a
reaction that may be difficult (or impossible) to obtain experimentally, by looking at
a series of other, related reactions.

3.2 Enthalpy

How does a whistling kettle work?

Pressure–volume work

The word ‘work’ in the question above could confuse. In common parlance, we say
a kettle works or does not work, meaning it either functions as a kettle or is useless.
But following the example in the previous section, we now realize how the word
‘work’ has a carefully defined thermodynamic meaning. ‘Operate’ would be a better
choice in this context. In fact, a kettle does not perform any work at all, since it has
no moving parts and does not itself move.
In a modern, automatic kettle, an electric heater warms the water inside the ket-
◦
tle – we call it the ‘element’. The electric circuit stops when the water reaches 100 C
because a temperature-sensing bimetallic strip is triggered. But the energy for a more
old-fashioned, whistling kettle comes from a gas or a coal hob. The water boils on
heating and converts to form copious amounts of gas (steam), which passes through
a small valve in the kettle lid to form a shrill note, much like in a football ref-
eree’s whistle.
The whistle functions because boiling is accompanied by a change in volume, so
the steam has to leave the kettle. And the volume change is large: the volume per
3
mole of liquid water is 18 cm (about the size of a small plum) but the volume of a
mole of gaseous water (steam) is huge.

SAQ 3.5 Assuming steam to be an ideal gas, use the ideal-gas equation
(Equation (1.13)) to prove that 1 mol of steam at 100 C (373 K) and
◦
5
3
O
standard pressure (p = 10 Pa) has a volume of is 0.031 m .
3
3
A volume of 0.031 m corresponds to 31 dm , so the water increases its volume by
a factor of almost 2000 when boiled to generate steam. This staggering result helps
us realize just how great the increase in pressure is inside the kettle when water boils.
3
The volume inside a typical kettle is no more than 2 dm . To avoid a rapid build
up of pressure within the kettle (which could cause an explosion), the steam seeks
to leave the kettle, exiting through the small aperture in the whistle. All the vapour
passes through this valve just like a referee blowing ‘time’ after a game. And the