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2.3 Equilibrium conditions

Lecture



Consider first a mechanical system, for example, a stone thrown upwards. Thermal processes are absent, i.e.   2.3 Equilibrium conditions S = 0. (The role of free energy is played by mechanical energy.) The equilibrium of such a system can come with a minimum of potential energy, i.e., U = U max . The direction of the spontaneous process is associated with a decrease in potential energy.   2.3 Equilibrium conditions U <0, while increasing the kinetic energy.

If the system * is isolated, then spontaneous processes lead, as already mentioned, to equilibrium, at which

S = S max .

In addition to an isolated system (IP), which does not exchange energy or matter with the environment, there are other systems.

A closed or closed system (CS) does not exchange matter with the environment, but exchanges energy.

An open system (OS) is exchanged with the environment and matter and energy.

Consider the equilibrium conditions of the CS at T = const and P = const. This may be, for example, a flask in a thermostat, a chemical reaction takes place in the flask. The equilibrium conditions of such a system is the Gibbs free energy minimum.

G = E + pV-TS, G = G min , (1)
and the spontaneous flow of the process goes towards   2.3 Equilibrium conditions G <0.
(More on thermodynamic potentials)

As follows from (1)

G = (E + pV) -TS = H-TS, H = E + pV,
where is the H-enthalpy of the system.

Decrease of free energy

  2.3 Equilibrium conditions G =   2.3 Equilibrium conditions H - T   2.3 Equilibrium conditions S (2)
occurs due to a decrease in enthalpy   2.3 Equilibrium conditions H and entropy increases   2.3 Equilibrium conditions S.

Different options are possible here:

  • descending   2.3 Equilibrium conditions H and increase   2.3 Equilibrium conditions S;
  • increase   2.3 Equilibrium conditions H but increasing and   2.3 Equilibrium conditions S, while   2.3 Equilibrium conditions S>   2.3 Equilibrium conditions H / T;
  • descending   2.3 Equilibrium conditions H, entropy simultaneously decreases   2.3 Equilibrium conditions S, if at the same time the entropy decreases faster and condition (2) is not satisfied at all, that is, condition   2.3 Equilibrium conditions G <0 is also not executed.

The process is implemented only in the case when the free energy falls. Changes in enthalpy and entropy should not be considered separately, since this will not give an answer to the question about the direction of the process. Let's look at this process with concrete examples.

We repeat that in a giant factory of natural processes of nature, entropy plays the role of a director who prescribes the type and flow of all transactions, and the law of conservation of energy takes the place of an accountant who balances debit and credit. The change in entropy during a phase transition determines the very possibility of phase transitions. There would be no change in entropy - there would be no our world.


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Synergetics

Terms: Synergetics