PHYSICAL CHEMISTRY IN BRIEF

CHAP. 1: BASIC TERMS [CONTENTS] 28

1.3 Thermodynamic quantities.

Observation of any system allows us to determine a number of its properties. The proper-

ties in which we are interested from the thermodynamic point of view are calledthermody-

namic quantities, or, briefly, quantities. Typical thermodynamic quantities are temperature,

pressure, volume, enthalpy and entropy. Neither heat nor work rank among thermodynamic

quantities.

Note: Terms such as thermodynamic function, thermodynamic variable, state quantity

(i.e. a quantity determining the state of a system, see1.4), state function, or state variable

are used as synonyms of the term thermodynamic quantity.

1.3.1 Intensive and extensive thermodynamic quantities

Let us consider a homogeneous system without any external force fields present. We distinguish

betweenextensiveandintensivethermodynamic quantities of a system. Intensive quantities

are those whose values do not change when the system is divided into smaller sub-systems.

Extensive quantities are those whose values are proportional to the amount of substance of

the system at a fixed temperature and pressure (see Figure 1.1). Temperature, pressure, and

composition expressed by mole fractions are typical intensive quantities. Volume, mass and the

number of particles are typical extensive quantities.

Note:Some quantities, e.g. the system surface, are neither extensive nor intensive.

Every extensive quantity may be converted into an intensive one if we relate it to a certain

constant mass of the system. We then obtain specific or molar quantities (see3.2.5). For every

extensive quantityX and the respectivemolar and specificquantitiesXmandxwe may

write

X = n Xm, (1.1)
X = m x , (1.2)
wherenis the amount of substance andmis the mass of the system.
S Symbols: We will use the subscript mto denote molar quantities and small letters to denote
specific quantities.