Key Terms
Classic example
Diamond converting to graphite at ambient pressure is thermodynamically spontaneous — but the rate is so slow it is effe
Another example
Radioactive decay of U-238 is spontaneous; it just takes 4+ billion years. Tc-99m is also spontaneous but decays in hour
Pattern
Spontaneous processes move toward greater, more uniform distribution of matter and/or energy.
Phase changes
S_solid < S_liquid < S_gas
So
Melting, vaporization, sublimation — delta-S > 0 Freezing, condensation, deposition — delta-S < 0
Temperature changes
Entropy increases with temperature for any substance. Higher temperature = broader kinetic energy distribution = more mi
Dissolution
Solid dissolving in liquid — delta-S > 0. Particles gain freedom of motion and interact with solvent; matter and energy
Gas-phase reactions
Count moles of gas on each side. Net increase in moles of gas = delta-S > 0.
Example (ice melting)
At -10 C (263.15 K), delta-S_univ is negative — melting is nonspontaneous. At +10 C (283.15 K), delta-S_univ is positive
Reasoning
At 0 K, no thermal motion, every particle is in one fixed position, only one possible microstate (W = 1). S = k ln(1) =
Standard entropy S-degrees
Entropy for one mole of a substance at 1 bar and 298.15 K. Units: J/(mol·K).
Standard entropy change for a reaction
Delta-S-degrees = sum[nu * S-degrees(products)] - sum[nu * S-degrees(reactants)]
Note
Gases have much higher S-degrees than liquids or solids. This table is reference — you will not memorize it, but you nee
Example
Decomposition of zinc sulfide alone is nonspontaneous (delta-G > 0); coupling it with oxidation of sulfur (delta-G stron