Spontaneity

Chemical and physical processes have a natural tendency to occur in one direction under certain conditions.

Spontaneous process

A spontaneous process occurs without the need for a continual input of energy from some external source.

Nonspontaneous process

A nonspontaneous process requires such.

During a spontaneous process

Systems undergoing a spontaneous process may or may not experience a gain or loss of energy, but they will experience a change in the way matter and/or energy is distributed within the system.

Entropy (S) definition

Entropy (S) is a state function that can be related to the number of microstates for a system (the number of ways the system can be arranged) and to the ratio of reversible heat to kelvin temperature.

Entropy interpretation

It may be interpreted as a measure of the dispersal or distribution of matter and/or energy in a system.

Entropy description

It is often described as representing the “disorder” of the system.

Entropy and phase

For a given substance, entropy depends on phase with Ssolid < Sliquid < Sgas.

Entropy and complexity

For different substances in the same physical state at a given temperature, entropy is typically greater for heavier atoms or more complex molecules.

When entropy increases

Entropy increases when a system is heated and when solutions form.

Predicting entropy changes

Using these guidelines, the sign of entropy changes for some chemical reactions and physical changes may be reliably predicted.

Second law of thermodynamics

The second law of thermodynamics states that a spontaneous process increases the entropy of the universe, Suniv > 0.

Nonspontaneous condition

If ΔSuniv < 0, the process is nonspontaneous.

Equilibrium condition

If ΔSuniv = 0, the system is at equilibrium.

Third law of thermodynamics

The third law of thermodynamics establishes the zero for entropy as that of a perfect, pure crystalline solid at 0 K.

Entropy at 0 K

With only one possible microstate, the entropy is zero.

Standard entropy change

We may compute the standard entropy change for a process by using standard entropy values for the reactants and products involved in the process.

Gibbs free energy (G)

Gibbs free energy (G) is a state function defined with regard to system quantities only and may be used to predict the spontaneity of a process.

Spontaneous ΔG

A negative value for ΔG indicates a spontaneous process.

Nonspontaneous ΔG

A positive ΔG indicates a nonspontaneous process.

Equilibrium ΔG

A ΔG of zero indicates that the system is at equilibrium.

Free energy calculation

A number of approaches to the computation of free energy changes are possible.