PHYSICAL CHEMISTRY IN BRIEF

CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 68

3.2 Definition of fundamental thermodynamic quantities

T,p,V,n,U,Sas defined in the preceding sections represent basic thermodynamic functions.

In this section we will define further basic thermodynamic functions: enthalpy, Helmholtz

energy, Gibbs energy, heat capacities and fugacity.

3.2.1 Enthalpy.

EnthalpyHis a function of state defined by the relation

H=U+pV. (3.9)

U Main unit:J.

Note:Similarly as internal energy, enthalpy is defined up to the additive constant.

If a system exchanges only heat and reversible volume work with its surroundings during a

thermodynamic process [see4.1.1], we have

dH= ̄dQ+Vdp. (3.10)

If this process is isobaric (dp= 0), the change in the enthalpy of a system is equal to the heat

supplied to the system

∆H=Q , [p]. (3.11)

Example

A system containing 5 moles of an ideal gas was heated from temperatureT 1 = 300 K to

temperatureT 2 = 400 K. The internal energy of the gas increased by∆U= 800 J. How did the

enthalpy of the system change?