Calculator
Example Data Table
| Reaction | ΔG | ΔS | Temperature | Formula | ΔH Result |
|---|---|---|---|---|---|
| Example A | -237.13 kJ/mol | -0.163 kJ/mol·K | 298.15 K | ΔH = ΔG + TΔS | -285.72 kJ/mol |
| Example B | 12.5 kJ/mol | 0.045 kJ/mol·K | 310 K | ΔH = 12.5 + 310 × 0.045 | 26.45 kJ/mol |
| Example C | -40.2 kJ/mol | 85 J/mol·K | 350 K | ΔH = -40.2 + 350 × 0.085 | -10.45 kJ/mol |
Formula Used
Gibbs relation: ΔG = ΔH − TΔS
Rearranged formula: ΔH = ΔG + TΔS
Total enthalpy: Total ΔH = ΔH per mole × moles
Uncertainty estimate: σH = √(σG² + (TσS)² + (SσT)²)
Temperature must be in kelvin. Entropy must match energy per mole per kelvin.
How to Use This Calculator
- Enter the Gibbs free energy change for the reaction.
- Choose the matching energy unit.
- Enter entropy change with its correct unit.
- Enter temperature and select its scale.
- Add moles when total enthalpy is needed.
- Add uncertainties for a basic error estimate.
- Press Calculate to show results above the form.
- Use CSV or PDF buttons to export results.
Article
Understanding Enthalpy From Gibbs Energy
Enthalpy, Gibbs free energy, and entropy describe energy changes in a system. They are common in physics, physical chemistry, and thermal science. This calculator finds enthalpy change when Gibbs free energy, entropy change, and temperature are known. The relation is useful because direct heat measurement is not always available. Many laboratory tables report ΔG and ΔS. With those values, ΔH can be estimated quickly.
Why the Formula Matters
The core equation is ΔG = ΔH − TΔS. Rearranging gives ΔH = ΔG + TΔS. This shows that enthalpy has two parts. The first part is free energy. The second part is the thermal entropy contribution. Temperature controls how strongly entropy affects the final answer. Higher temperature makes the TΔS term larger.
Using Correct Units
Unit agreement is critical. Temperature must be in kelvin. Entropy must use energy per mole per kelvin. Gibbs free energy and enthalpy must use energy per mole. This page converts joules, kilojoules, calories, and kilocalories. It also converts Celsius and Fahrenheit to kelvin. This helps reduce common calculation errors.
Interpreting Results
A positive ΔH means the process is endothermic. It absorbs heat from surroundings. A negative ΔH means the process is exothermic. It releases heat. Gibbs free energy gives a different idea. A negative ΔG suggests spontaneity under the entered conditions. A positive ΔG suggests the process is not spontaneous there.
Advanced Options
The calculator also estimates total enthalpy for a chosen mole amount. This is helpful for batch reactions. Uncertainty fields give a simple propagated error estimate. The method assumes independent uncertainty sources. Results can be exported as CSV or PDF. That makes the tool useful for reports, worksheets, and lab records.
FAQs
1. What does this calculator find?
It finds enthalpy change from Gibbs free energy, entropy change, and temperature. It uses the rearranged Gibbs relation, ΔH = ΔG + TΔS.
2. Why must temperature be in kelvin?
The Gibbs equation uses absolute temperature. Kelvin starts at absolute zero, so it gives the correct thermal entropy contribution.
3. What does positive enthalpy mean?
Positive enthalpy means the process is endothermic. The system absorbs heat from its surroundings during the change.
4. What does negative enthalpy mean?
Negative enthalpy means the process is exothermic. The system releases heat to its surroundings during the change.
5. Can I enter entropy in J/mol·K?
Yes. The calculator converts J/mol·K into kJ/mol·K internally before applying the formula.
6. Is Gibbs free energy the same as enthalpy?
No. Gibbs free energy includes enthalpy and entropy effects. Enthalpy mainly describes heat change at constant pressure.
7. What is the TΔS term?
TΔS is the temperature multiplied by entropy change. It measures the entropy contribution to the energy balance.
8. Why use the uncertainty fields?
They estimate how measurement errors may affect enthalpy. The result is a simple propagated uncertainty for quick reporting.