Gibbs Free Energy Calculator
Calculate Gibbs free energy and predict reaction spontaneity using ΔG = ΔH - TΔS
Negative for exothermic, positive for endothermic
Positive for increasing disorder
Standard temperature: 298.15 K (25°C)
Understanding Gibbs Free Energy
Gibbs free energy (ΔG) is one of the most important concepts in thermodynamics, determining whether a chemical reaction will occur spontaneously. It combines enthalpy (heat content) and entropy (disorder) to predict the thermodynamic favorability of reactions under constant temperature and pressure.
The Gibbs Free Energy Equation
ΔG = ΔH - TΔS
Gibbs-Helmholtz equation
ΔG = Gibbs free energy change (kJ/mol)
Determines spontaneity: negative = spontaneous
ΔH = Enthalpy change (kJ/mol)
Heat absorbed or released: negative for exothermic reactions
T = Temperature (K)
Absolute temperature in Kelvin
ΔS = Entropy change (J/(mol·K))
Change in disorder: positive for increasing randomness
Spontaneity Prediction
ΔG < 0 (Negative)
Spontaneous reaction - Thermodynamically favorable
Reaction proceeds forward without external energy input
Products are more stable than reactants
ΔG = 0 (Zero)
At equilibrium - No net change
Forward and reverse reactions occur at equal rates
ΔG° = -RT ln(K)
ΔG > 0 (Positive)
Non-spontaneous reaction - Thermodynamically unfavorable
Requires external energy input to proceed
Reverse reaction is spontaneous
Practical Example
Formation of Water
2H₂(g) + O₂(g) → 2H₂O(l)
- ΔH = -571.6 kJ/mol (exothermic)
- ΔS = -326.8 J/(mol·K) (decrease in disorder)
- T = 298.15 K (25°C)
ΔG = -571.6 - (298.15 × -326.8 / 1000)
ΔG = -571.6 - (-97.5)
ΔG = -571.6 + 97.5
ΔG = -474.1 kJ/mol
ΔG < 0: The formation of water is spontaneous despite the decrease in entropy!
Temperature Dependence
| ΔH | ΔS | ΔG | Spontaneity |
|---|---|---|---|
| Negative | Positive | Always negative | Always spontaneous |
| Positive | Negative | Always positive | Never spontaneous |
| Negative | Negative | Temperature dependent | Spontaneous at low T |
| Positive | Positive | Temperature dependent | Spontaneous at high T |
Key Concepts
🔥 Enthalpy-Driven
Exothermic reactions (ΔH < 0) tend to be spontaneous
🎲 Entropy-Driven
Reactions increasing disorder (ΔS > 0) are favored
🌡️ Temperature Effect
TΔS term increases with temperature
⚖️ Equilibrium
ΔG° = -RT ln(K) relates G to equilibrium constant
Applications
- ⚗️Chemical Synthesis: Predict which reactions will occur spontaneously and optimize conditions
- 💊Biochemistry: Understand metabolism, ATP hydrolysis, and coupled reactions in cells
- 🔋Electrochemistry: Calculate cell potentials from ΔG = -nFE
- 🏭Industrial Processes: Design efficient chemical plants (Haber, contact processes)
- 🌡️Phase Transitions: Predict melting, boiling, and sublimation temperatures
- 🔬Materials Science: Design new materials with desired thermodynamic properties
⚡Quick Reference
Units:
kJ/mol, J/(mol·K), K
Formula:
ΔG = ΔH - TΔS
Spontaneity:
ΔG < 0: spontaneous
ΔG > 0: non-spontaneous
Standard T:
298.15 K (25°C)
Level:
College Chemistry
🔗Related Calculators
📐Related Formulas
🎯Where It's Used
- ⚗️
Chemical Synthesis
Predicting reaction feasibility
- 💊
Biochemistry
Metabolic pathway analysis
- 🏭
Industrial Chemistry
Process optimization
- 🔋
Electrochemistry
Battery and fuel cell design