Page 150 - Essentials of physical chemistry

112 Essentials of Physical Chemistry

It is worth memorizing some of the numbers related to water because water is so important to our
bodies and covers about 70% of the surface of the Earth. A rough estimate would be that
DH vap (H 2 O) ffi 44 kJ=mol 10:52 kcal=mol. Recently, the author met a student from a past class
who won a bet for a steak dinner from his boss when he correctly quoted the property of water under
discussion in an industrial project; now that is a benefit of education! The property in question was
(DH vap =g) for water and we see that to a good approximation it is

43990 J=mol 1 cal
¼ 583:6 ffi 584 cal=g:
18:01528 g=mol 4:184 J
A related fact is the value of the heat of fusion to melt ice per gram as

6010 J=mol 1 cal
¼ 79:73 ffi 80 cal=g. Since you only need to multiply by 1000 to
18:01528 g=mol 4:184 J
get calories=kilogram (a mixed set of units) and then multiply by 0.4536 kg=lb (453.6 g=lb) you can
convert to calories=pound. While this seems like trivia, chemical engineers worry over such unit
conversions all the time and do not have the luxury of having all SI units. Several years ago, a Mars
Lander project crashed because part of a computer program was in feet while another part was in
meters. We need to know the SI units and other units, which occur in everyday life as well. Consider
the British thermal unit, the btu.

1 btu the heat energy required to raise the temperature of 1 lb (Av.) of water 18F while
1 cal the heat energy required to raise the temperature of 1 g of water 18C.

100 C

In calories this would be (453:6 g) (1 F) ¼ 252 cal.

180 F

Verbally ‘‘1 btu ¼ 252’’ (cal), should be easy to remember. Using another memory device
(a calorie is worth many jewels!) we can convert a btu to joules as well:
1 btu ¼ 252 cal ¼ (252 cal)(4:184 J=cal) ¼ 1054:368 J:

PHASE EQUILIBRIA
One of Gibbs’ most enduring contributions was the study of phases. This is very important in
metallurgical engineering where complicated phase diagrams are common in the study and formu-
lation of alloys. Other examples occur in the study of as many as eight slightly different phases of
ice, all solids but not exactly the same. Other solids have been studied intensively, such as copper–
lithium–aluminum alloys for aircraft construction and solid state electronic devices formed from
doped silicon. It should be stated here that while a gas phase can only be a gas and liquids easily
become homogeneous solution mixtures, solid phases often have more than one crystal structure,
which vary with temperatures below the melting point. We leave those complicated cases of alloys
to advanced courses and here we want to just discuss the overview of a diagram of three state
phases: solid, liquid, and gas. We see in Figure 6.4, a generic phase diagram with boundary lines
between the phases and on the left an unusual diagram for water. Water is a very unusual substance
in many ways. For most materials the generic shape diagram on the right prevails in which the
boundary line between solid and liquid ‘‘leans’’ to the higher temperature side and most materials
generally follow this rule. Most materials can be ‘‘squeezed’’ from a liquid into a solid at a given
constant temperature by the application of pressure. In the phase diagram for water, we see a very
unusual situation where application of pressure to ice at a temperature below the melting point
allows the solid to melt into a liquid. Thus, glaciers can ‘‘flow’’ like rivers since their heavy mass
provides great pressure on any obstacle in the path of the glacier, pressure melts the ice to a liquid,
which runs around the object and then the water refreezes.