# What kind of system is required for dynamic equilibrium to be set up?

Most websites and books mention a closed system, but technically isn't an isolated system required? If energy can be transferred, then the position of of the equilibrium will continuously change and no equilibrium will be set up, right?

• Who cares about closedness? An equilibrium is possible in any system. – Ivan Neretin Apr 23 '19 at 18:08
• Do you know what dynamic means? You're talking about static not dynamic situation. – Mithoron Apr 23 '19 at 19:31
• @Mithoron he used the term correctly. – A.K. Apr 23 '19 at 23:18

If energy can be transferred, it does not mean it is transferred.

If there is e.g. zero net thermal energy transfer, a system is in thermal equilibrium with the neighborhood, even if it is not isolated.

As Ivan says, an equilibrium is possible even in an open system, if net transfer of particular matter kinds is zero.

The thermodynamic equilibrium consists of thermal, radiative, chemical and mechanical equilibrium.

Note that thermodynamic equilibrium happens at following conditions:

• For a completely isolated system, the entropy $$S$$ is maximum at thermodynamic equilibrium.
• For a system with controlled constant temperature and volume, the Helmholtz free energy $$A$$ is minimum at thermodynamic equilibrium.
• For a system with controlled constant temperature and pressure, the Gibbs free energy $$G$$ is minimum at thermodynamic equilibrium.

\begin{align} G&=H-TS \\ \Delta G&=\Delta H-T\Delta S \\ \end{align}

Total entropy of the not isolated system and it's surrounding increases in time.

Entropy of the system alone may decrease with time, if the Gibbs energy of system decreases, as this latter decrease also means $$\Delta S_{\rm surrounding} + \Delta S_{\rm system} \gt 0$$

• What I found after discussing this with my friend is that Gibbs free energy = Enthalpy change - (Temperature * Entropy change) has to equal to 0 for equilibrium to be established. In a closed system this is possible, provided your temperature is kept constant (as entropy increases with time). Is that what your answer is saying as well or is it different? – d_g Apr 24 '19 at 5:41
• Gibbs free energy change = .. G=H-TS DeltaG=DeltaH-T.DeltaS – Poutnik Apr 24 '19 at 5:45
• See the update. – Poutnik Apr 24 '19 at 6:23

If energy can be transferred, then the position of of the equilibrium will continuously change and no equilibrium will be set up, right?

When a system is at equilibrium, its state does not change. So there can't be a net transfer of energy into the system or out of the system, that would change its state. There also can't be a net transfer of matter into the system or out of the system, that would also change its state.

An open system allows energy and matter exchange with the surrounding. However, once the system and the surrounding together have reached equilibrium, the net transfer of matter will be zero (and the net transfer of radiation), and there will be no heat transferred between system and surrounding, and not work done by the system on the surrounding or the other way around.

So it is not that you have to prevent energy transfer, but that the processes at equilibrium will not show a net transfer of energy even though it is possible and probably happened as the processes approached equilibrium.

What kind of system is required for dynamic equilibrium to be set up?

There are no requirements, any kind of system is fine, including the universe (we're not at equilibrium yet, though).

I don't know German, but my perception is that German readily forms new words by making compound words out of what would phrases in English. So in English adjectives get thrown around. You have to learn the lingo.

A closed system is one in which matter isn't being exchanged with the outside environment, but energy can be.

An isolated system is one in which neither matter or energy is being exchanged with the outside environment.

An open system allows both the exchange of matter and energy.

A closed system is required for the dynamic equilibrium to set up.