Gibbs free energy (ΔG) determines whether a reaction is spontaneous. It combines enthalpy and entropy to predict if a reaction will occur without external energy input.
ΔG = ΔH - TΔS
Free energy change determines spontaneity
Units: J/mol or kJ/mol
Meaning: Energy available to do work
✓ ΔG < 0 → Spontaneous (exergonic)
• ΔG = 0 → Equilibrium
✗ ΔG > 0 → Non-spontaneous (endergonic)
Units: J/mol or kJ/mol
Meaning: Heat absorbed or released
ΔH < 0 = exothermic (releases heat)
Units: Kelvin (K) — MUST use Kelvin!
Conversion: K = °C + 273.15
Units: J/(mol·K)
Meaning: Change in disorder
⚠️ Often in J/(mol·K) while ΔH is in kJ/mol — convert units!
| ΔH | ΔS | ΔG | Spontaneity |
|---|---|---|---|
| Negative (-) | Positive (+) | Always negative | Always spontaneous at all T |
| Negative (-) | Negative (-) | Negative at low T | Spontaneous at low temperature |
| Positive (+) | Positive (+) | Negative at high T | Spontaneous at high temperature |
| Positive (+) | Negative (-) | Always positive | Never spontaneous |
ΔH and TΔS must have same units. Convert ΔS to kJ/(mol·K):
ΔS = -120 J/(mol·K) × (1 kJ / 1000 J) = -0.120 kJ/(mol·K)
TΔS = 298 K × (-0.120 kJ/(mol·K)) = -35.8 kJ/mol
ΔG = ΔH - TΔS = -75 - (-35.8) = -39.2 kJ/mol
Answer: ΔG = -39.2 kJ/mol (negative)
✓ Spontaneous! Exothermic overcomes negative entropy.
ΔH is often in kJ/mol while ΔS is in J/(mol·K). Convert ΔS to kJ/(mol·K) or ΔH to J/mol!
It's ΔG = ΔH - TΔS, not ΔG = ΔH + TΔS. The minus is crucial!
Temperature MUST be in Kelvin. Convert °C to K by adding 273.15.
ΔG < 0 means thermodynamically favorable, NOT necessarily fast. Kinetics is separate!
ΔG = ΔH - TΔS. It predicts reaction spontaneity. ΔG < 0 means spontaneous, ΔG > 0 means non-spontaneous.
Negative ΔG means the reaction is thermodynamically favorable (spontaneous) and can occur without external energy input.
Temperature affects the TΔS term. High T favors positive ΔS reactions, low T favors negative ΔH reactions.
ΔG° = -RT ln K. When ΔG = 0, the system is at equilibrium. ΔG° relates to the equilibrium constant K.
ΔH and TΔS must have matching units (both kJ/mol or both J/mol). ΔS is often given in J/(mol·K), requiring conversion.