Calculator Inputs
Example Data Table
| Example | ΔH (kJ/mol) | ΔS (J/mol·K) | T (K) | ΔG (kJ/mol) | Conclusion |
|---|---|---|---|---|---|
| Reaction A | -100 | -150 | 298 | -55.30 | Spontaneous at this temperature |
| Reaction B | 50 | 200 | 400 | -30.00 | Spontaneous at higher temperature |
| Reaction C | 80 | -120 | 298 | 115.76 | Nonspontaneous |
| Reaction D | -40 | 120 | 298 | -75.76 | Spontaneous at all temperatures |
Formula Used
Gibbs free energy relation:
ΔG = ΔH − TΔS
Where ΔG is Gibbs free energy, ΔH is enthalpy change, T is absolute temperature in Kelvin, and ΔS is entropy change.
If ΔG < 0, the process is thermodynamically spontaneous. If ΔG > 0, it is nonspontaneous. If ΔG = 0, the system is at equilibrium.
Extra interpretation rules
- ΔH < 0 and ΔS > 0 usually favor spontaneity at all temperatures.
- ΔH > 0 and ΔS < 0 usually oppose spontaneity at all temperatures.
- ΔH < 0 and ΔS < 0 can be spontaneous at lower temperatures.
- ΔH > 0 and ΔS > 0 can be spontaneous at higher temperatures.
How to Use This Calculator
- Enter a reaction name for easier report exports.
- Provide enthalpy change and choose the correct unit.
- Provide entropy change and choose the correct unit.
- Enter the working temperature and select Kelvin or Celsius.
- Set the graph start, end, and step values.
- Press the calculate button to view ΔG and spontaneity.
- Review the threshold temperature and graph crossover point.
- Use the CSV or PDF buttons to export the results.
Frequently Asked Questions
1. What does spontaneity mean here?
It means thermodynamic favorability under the entered conditions. A negative Gibbs free energy indicates the process can proceed without external energy input, though reaction speed may still be slow.
2. Why must temperature be in Kelvin?
The Gibbs equation uses absolute temperature. Kelvin starts at absolute zero, so the entropy term remains physically meaningful and consistent with thermodynamic derivations.
3. Can a reaction become spontaneous only at high temperature?
Yes. When both ΔH and ΔS are positive, the entropy benefit grows with temperature. Above the crossover temperature, TΔS can exceed ΔH and make ΔG negative.
4. What happens when ΔG equals zero?
The system is at thermodynamic equilibrium for that condition. The forward and reverse tendencies balance, and there is no net driving force in either direction.
5. Does spontaneity guarantee a fast reaction?
No. Spontaneity concerns thermodynamics, not kinetics. A reaction may have negative ΔG and still proceed slowly if the activation energy barrier is large.
6. Why do unit choices matter so much?
ΔH and TΔS must use compatible energy units. If enthalpy is entered in kilojoules and entropy in joules, incorrect conversion can shift ΔG by a thousandfold.
7. What does the threshold temperature show?
It is the temperature where ΔG becomes zero, calculated from ΔH/ΔS when entropy is nonzero. It marks the crossover between spontaneous and nonspontaneous behavior.
8. Can I use this for classroom examples?
Yes. The page includes a worked data table, formula notes, graphing, and export tools, making it useful for homework checks, lecture demos, and lab discussions.