Thermodynamics: Entropy and Spontaneity

Thermodynamics

• The fusion of ice into liquid water is an example of a process with a positive enthalpy change (∆H = 6.01 kJ), but the sign of ∆H alone cannot predict spontaneity.

Entropy

• Entropy (S) is a measure of the disorder or randomness of a system.
• The greater the disorder, the greater the entropy.
• Solid state has less entropy than liquid state, and liquid state has less entropy than gaseous state.
• An increase in entropy (∆S > 0) indicates an increase in randomness.
• Examples of processes with increasing entropy include melting, vaporization, and mixing.

Entropy Changes in the System

• The entropy change of the system (∆Ssys) can be positive or negative.
• For an equilibrium process, ∆Suniv = ∆Ssys + ∆Ssurr = 0.
• If ∆Suniv is negative, the process is not spontaneous in the direction described.

Third Law of Thermodynamics

• The entropy of a pure crystalline substance is zero at absolute zero (0 K).
• As temperature increases, entropy increases.
• The standard entropy (S°) of a substance is the entropy at a temperature of 298 K.

Gibbs Free Energy

• Gibbs free energy (G) combines enthalpy and entropy to determine spontaneity.
• The change in free energy (∆G) is given by: ∆G = ∆H - T∆S.
• ∆G < 0 indicates a spontaneous process, ∆G > 0 indicates a non-spontaneous process, and ∆G = 0 indicates equilibrium.

Spontaneity and Equilibrium

• A spontaneous process is characterized by: unidirectional, time-independent, reaches equilibrium, and accompanied by an increase in entropy.
• Examples of spontaneous processes include a waterfall running downhill, heat flowing from a hotter object to a colder one, and a gas expanding in an evacuated bulb.

Standard Free-Energy Changes

• The standard free-energy of reaction (∆Gorxn) is the free-energy change for a reaction under standard-state conditions.
• ∆H = +ve indicates an endothermic reaction, and ∆H = -ve indicates an exothermic reaction.