Entropy and Gibbs Free Energy in Chemistry 2

Entropy (ΔS)

A measure of the disorder or randomness of a system, quantifying how energy is dispersed and how many microstates are possible.

SI Unit of Entropy

J/mol·K

Nature of Entropy

Greater disorder results in higher entropy, and more ways to arrange particles also leads to higher entropy.

Second Law of Thermodynamics

For any spontaneous process, the total entropy of the universe increases: ΔS{universe} = ΔS{system} + ΔS_{surroundings} > 0.

Entropy change of surroundings

Linked to heat exchange: ΔS{surroundings} = rac{-q{system}}{T} (when temperature is constant).

Qualitative Entropy Trends

Entropy increases when a substance changes from solid to liquid to gas, a solute dissolves in a solvent, the number of gas molecules increases during a reaction, or temperature increases.

Third Law of Thermodynamics

The entropy of a perfect crystal at absolute zero (0 K) is zero: S = 0 at T = 0 K.

Standard Molar Entropy (S°)

Entropy values of substances under standard conditions (25°C, 1 atm), found on reference tables.

Standard Entropy Change (ΔS°rxn)

ΔS°rxn = Σ S°(products) - Σ S°(reactants).

Gibbs Free Energy (ΔG)

Measures the maximum amount of useful work a system can perform at constant pressure and temperature, combining enthalpy and entropy to predict spontaneity.

Gibbs Free Energy Equation

ΔG = ΔH - TΔS, where ΔG is Gibbs free energy change, ΔH is enthalpy change, T is absolute temperature in Kelvin, and ΔS is entropy change.

Standard Free Energy Change (ΔG°)

Used under standard conditions: 1 atm, 25°C (298 K), 1 M for solutions.

Calculating ΔG°

From standard enthalpy and entropy values: ΔG° = ΔH° - TΔS°, using kJ for ΔH and J for ΔS.

Standard Free Energies of Formation

ΔG°{rxn} = Σ ΔGf°(products) - Σ ΔG_f°(reactants).

Gibbs Free Energy and Equilibrium

The relationship between ΔG and the equilibrium constant (K) is given by ΔG° = -RT ln K.

R in Gibbs Free Energy Equation

R = 8.314 J/mol·K.

Temperature in Gibbs Free Energy Equation

T is the temperature in Kelvin.

Equilibrium Constant (K)

K is the equilibrium constant.