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Introduction to Physical Chemistry 9

Conclusion of the Kinetic Theory of Gases
The kinetic theory of gases is a good approximation, used to explain
the behaviour of real gases. The theory has to be modified at very high
pressures and in the presence of van der Waals forces, and for polar
molecules where stronger intermolecular forces of attraction are
involved.

THE GENERAL GAS EQUATION
Consider a gas at temperature TI, pressure p1 and volume V1, and
another gas at temperature T2 and pressure p2. The volume of the
latter gas can be determined easily using the equation:

A useful way of remembering this is 'peas and Vegetables go on the
Table'! In fact, given any five of the above variables, the sixth can be
evaluated using the equation. This will form the basis of the worked
example at the end of this chapter.

Standard State
The standard state of a body is the most stable state of that body at
25°C and 1 bar pressure (its symbol is ", e.g. E", AH", AS", defined
later in Chapters 2,3 and 6 respectively).

I Standard State-Most Stable State-S.S.-25 "C and 1 bar pressure. 1

IMPORTANCE OF UNITS IN PHYSICAL CHEMISTRY

In physical chemistry questions, the International system of units (SI)
should be used. In any problem, one of the first steps is to convert all
units to SI; note especially that temperature must always be given in
kelvins, never degrees centigrade:
1 Remember: T(K) = T("C) + 273 I

e.g. 25 "C is equal to 298 K since (25 + 273) K = 298 K. Table 1.2 lists
(a) the basic SI units, (b) the derived SI units, and (c) some examples of
non-SI units, which are commonly used in physical chemistry.