Modern Thermodynamics
- Chapter 1
that heat was not an indestructible substance (caloric) that passed from one object to another but was " ..
intense commotion of the parts …."v.
Portrait of Boyle
At constant pressure, the variation of volume with temperature was studied by Jacques
Charles(1746 -1823) who established that:
= f2(p) , f2(p) is some function of the pressure p. (1.4.2)
In 1811 Amedeo Avogadro (1776-1856) announced his hypothesis that under conditions of the
same temperature and pressure, equal volumes of all gases contained equal number of molecules. This
hypothesis greatly helped in explaining the changes in pressure due to chemical reactions in which the
reactants and products were gases. It implied that at constant pressure and temperature, the volume of a
gas is proportional to the amount of the gas. Hence, in accordance with Boyle's law (1.4.1), for N moles
of a gas:
V = N
p (1.4.3)
A comparison of (1.4.1), (1.4.2) and (1.4.3) leads to the conclusion that f1(T) is proportional to T and to
the well-known Law of Ideal Gases:
pV = NRT
in which R is the gas constant. R = 8.31441 J K–1 mol–1 (or Pa m3 K-1 mol-1) = 0.08314 bar L K-1 mol-1 =
0.0821 atm L K–1 mol–1
Portrait of Jacque Charles Portrait of Gay Lussac
As more gases were identified and isolated by the chemists during the eighteenth and nineteenth
centuries, their properties were studied. It was found that many obeyed Boyle's law approximately. For
most gases, this law describes the experimentally observed behavior fairly well for pressures to about 10
atmospheres. As we shall see in the next section, the behavior of gases under a wider range of pressures
can be described by modifications of the ideal gas law that take into consideration the molecular size and
intermolecular forces.
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