Gibbs Free Energy Calculator
Predict if a reaction is spontaneous. Calculates ΔG = ΔH - TΔS and identifies the thermodynamic driving forces.
Negative = Exothermic (Releases Heat)
Positive = More Disorder
Ready to Calculate
Enter ΔH, ΔS, and T to see if the reaction happens.
Understanding Spontaneity
The balance between energy (Enthalpy) and disorder (Entropy).
In thermodynamics, Gibbs Free Energy (G) is the single most important number for determining if a chemical reaction will happen on its own. It combines the system's internal heat content with its level of disorder to predict the direction of the process.
The Master Equation
When is a reaction spontaneous?
| ΔH (Enthalpy) | ΔS (Entropy) | ΔG (Result) | Conclusion |
|---|---|---|---|
| Negative (-) Exothermic | Positive (+) More Disorder | Always Negative | Spontaneous at ALL Temperatures |
| Positive (+) Endothermic | Negative (-) Less Disorder | Always Positive | Non-Spontaneous at ALL Temperatures |
| Negative (-) | Negative (-) | Depends | Spontaneous at LOW Temperatures |
| Positive (+) | Positive (+) | Depends | Spontaneous at HIGH Temperatures |
Exergonic vs Endergonic
Exergonic (ΔG < 0): Determines that energy is released to the surroundings. Similar to exothermic, but includes entropy. These happen spontaneously.
Endergonic (ΔG > 0): Requires energy input. In the body, we power these reactions by coupling them with exergonic ones (like breaking ATP).
Real World: ATP
Your body needs to build complex proteins (an endergonic process, +ΔG). How does it do it?
It uses the breakdown of ATP (-30.5 kJ/mol). As long as the sum of the ΔG values is negative, the coupled reaction proceeds! This is the secret of life.
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Frequently Asked Questions
Does negative ΔG mean the reaction is fast?
No! Spontaneity only tells you if a reaction will happen, not how fast. A reaction can be spontaneous but take a million years (like diamond turning to graphite) if the activation energy is high.
Why do we multiply T times ΔS?
Entropy (ΔS) is energy spread per temperature unit. To compare it directly with Enthalpy (Journal energy), we must multiply by Temperature to get energy units (Joules).
What if ΔG is zero?
The system is at equilibrium. There is no net change occurring in either direction.
Why must Temperature be in Kelvin?
Temperature in thermodynamic equations represents the absolute amount of thermal energy. Celsius/Fahrenheit have arbitrary zero points (freezing water), whereas 0K is absolute zero. Using Celsius would yield incorrect negative energy values.
Can ΔS be negative?
Yes, if the system becomes more ordered (e.g., water freezing into ice, or gas condensing into liquid). Nature generally prefers positive ΔS (disorder).
What is Standard Free Energy (ΔG°)?
It is the free energy change under standard conditions: 1 atm pressure, 1 M concentrations, and usually 25°C (298 K). The actual ΔG changes as concentrations change.
How do I calculate the 'Crossover Temperature'?
Set ΔG to 0 and solve for T. So, 0 = ΔH - TΔS, which means T = ΔH / ΔS. This gives you the exact temperature where the reaction switches from non-spontaneous to spontaneous.
Is ΔH or ΔS more important?
It depends on the temperature! At low temperatures, Enthalpy (ΔH) dominates (energy minimization). At high temperatures, the TΔS term gets huge, so Entropy (ΔS) dominates (disorder maximization).