Thermodynamic potentials

Internal energy.

For the reversible process:

and

; .

If there is no heat exchanging one can receive from the first law of TD:

,

or (there is no heat exchanging).

When the volume is constant then:

,

and (the volume is constant).

The heat capacity at constant volume is:

Free energy.

Work done by the system for reversible isothermal process may be written:

The function of state

is called as free energy of a body.

So for reversible isothermal process:

(the temperature is constant).

Free energy at constant temperature plays the same role as an internal energy in adiabatic processes.

For irreversible isothermal process:

and .

It means that the decrease of free energy define the maximum work can be done by the system in isothermal process.

;

Let us consider the irreversible process at constant volume and temperature:

then if ,

( , )

This process is accompanied by increasing the free energy.

The free energy reaches the minimal value at equilibrium ( , ).

Enthalpy.

The heat received by the system at constant pressure can be represented as:

The function

is called enthalpy or thermal function.

In this case

( )

;

For molar heat capacity at constant pressure:

The enthalpy at constant pressure plays the same role as an internal energy at constant volume.

The Gibbs potential.

It is the function of state that defined by the expression:

.

;

At constant pressure and temperature:

then if ,

( , )

This process is accompanied by increasing the Gibbs potential.

The Gibbs potential reaches the minimal value at equilibrium ( , ).

Thermal potential	Properties
Internal energy	in adiabatic process; at
Free energy	in reversible isothermal process; in equilibrium at ,
Enthalpy	at
Gibbs potential	in equilibrium at ,

Liquid state

The molecules at liquid state are disposed closely. The interaction energy is compare to the kinetic energy of the molecules. There is short range ordering of the molecules in that state. There is long range ordering of the particles in crystals. Some liquids have the long range ordering – they are the liquid crystals.

The theory of the liquid state was developed by Y.I.Frenkel. "Settled and nomadic" behavior of molecules in liquids. Amorphous solid – it is a supercooled liquid (glass, bitumen, resin).

Surface tension

- it is the radius of molecular interaction.

- it is some distances between molecules.

The transition of molecule from liquid state to gas increases its potential energy and correspondingly decreases its kinetic energy. Due to evaporation liquid is cooled down.

The "tendency" of liquid to contract its border surface with gas leads to the force of surface tension.

- it is a force per unit length.

, because there are two interfaces.

- it is an increment of border area.

- it is an increment of free energy in isothermal process.

So

;

S – it is a surface of the border area!

;