Role of Gibbs free energy in spontaneous process

To?examine?how?well?two?common?processes—the?power?plant?turbine?and?the?Joule?Thompson?process—meet?the?criteria?for?spontaneous?processes,?they?have?been?selected?for?this?discussion.

What is a spontaneous process?

In layman's terms, a "spontaneous process" is a process that happens on its own without assistance from outside sources. The definition frequently prompts inquiries about how a process can proceed without outside assistance. Indeed, it is feasible; the adiabatic process is one instance. An outside intervention does not occur in an adiabatic process. The process is brought about by internal energy supply intervention. According to thermodynamics, a spontaneous process happens inside the system without the need for outside input. A more technical definition would be the time evolution of a system that results in the release of free energy and a decrease in energy state to one that is closer to thermodynamic equilibrium and more thermodynamically stable.

Explanation

Gibbs free energy is the energy that does the thermodynamic work. Gibbs free energy predicts the potential of a gas to perform work at constant pressure. The direction of a process depends on the free energy change of the process?Gibbs free energy change is expressed by equation,dG = dH - TdSdG is free energy change, dH is enthalpy ( total energy) change, T is temperature and dS is the entropy change.

dG = 0

When the change in free energy is zero, dH = TdS

This means the total energy dH is neutralized by the product of T and dS. when this happens there is no energy for the process to move in either direction, forward or reverse.

dG < 0

When dG < 0 or negative, dH - TdS is < 0?In other words, the term TdS is greater > dH.

The most stable state of a process is to move towards equilibrium. When dG is < 0 or dH - TdS < 0, the process while trying to achieve equilibrium (dG =0) spontaneously moves forward.

dG > 0

This means dH - TdS > 0. In such cases, the process to achieve equilibrium (dG = 0) reverses.

Power plant turbine

It works between two enthalpy points (1) boiler and (2) condenser adiabatically and isentropically with dH = delta W assuming no entropy losses. This means dH = 0 and TdS =0 both are zero Therefore, the free energy change dG = 0.

Therefore, a power plant turbine has no energy to run spontaneously. It is idealized as a reversible process.

Joule Thompson process

Joule Thompson process is an isenthalpic process. The enthalpy at upstream H1 is equal to enthalpy at downstream. H1 = H2. While the total energy remains constant it is the switch from potential energy to kinetic energy as a real gas expands against its intermolecular forces produces cooling.

Coming back to Gibbs equation

dG = 0 - Tds. since dH = 0 (isenthalpic) This makes dG negative and therefore Joule Thompson process a spontaneous forward moving process.

It may be further noted in the Joule-Thompson process it is the entropy gain upstream when the gas expands makes the process spontaneous and irreversible