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Entropy (S)

Measure of disorder or randomness

Increase in entropy

More disorder, more possible arrangements

Decrease in entropy

Less disorder, fewer arrangements

Second law of thermodynamics

Entropy of the universe increases in a spontaneous process

ΔS > 0

Entropy increases (more disorder)

ΔS < 0

Entropy decreases (less disorder)

State with highest entropy

Gas

State with lowest entropy

Solid

Gibbs free energy (ΔG)

Determines spontaneity of a reaction

Gibbs equation

ΔG = ΔH - TΔS

ΔG < 0

Spontaneous reaction

ΔG > 0

Non-spontaneous reaction

ΔG = 0

System at equilibrium

Effect of ΔH and ΔS (both favorable)

ΔH < 0 and ΔS > 0 → always spontaneous

Effect of ΔH and ΔS (both unfavorable)

ΔH > 0 and ΔS < 0 → never spontaneous

Effect of temperature on spontaneity

Depends when ΔH and ΔS have same sign

Exothermic reaction

Releases heat (ΔH < 0)

Endothermic reaction

Absorbs heat (ΔH > 0)

Phase change entropy trend

Solid < Liquid < Gas

Dissolving a solid

Usually increases entropy

Gas expansion

Increases entropy

Spontaneous process

Occurs without continuous external input

At equilibrium

ΔG = 0

Relationship between ΔG and equilibrium constant

ΔG° = -RT ln K

Large K (K >> 1)

Products favored, ΔG° negative

Small K (K << 1)

Reactants favored, ΔG° positive

Temperature (T)

Always in Kelvin for calculations