Enthalpy is the heat content of a system at constant pressure. Negative ΔH means a reaction releases heat (exothermic), like burning fuel; positive means it absorbs heat (endothermic), like cold packs.
How to use the Enthalpy Calculator
- Method
- Change in internal energy ΔU
- Pressure p
- Change in volume ΔV
- Reactant 1 coefficient
- Reactant 1 ΔH_f (kJ/mol)
- Reactant 2 coefficient
- Reactant 2 ΔH_f (kJ/mol)
- Reactant 3 coefficient
- Reactant 3 ΔH_f (kJ/mol)
- Product 1 coefficient
- Product 1 ΔH_f (kJ/mol)
- Product 2 coefficient
- Product 2 ΔH_f (kJ/mol)
- Product 3 coefficient
- Product 3 ΔH_f (kJ/mol)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the method. For reactions, enter the stoichiometric coefficient and standard enthalpy of formation (kJ/mol) of each species; leave unused coefficients at 0.
Tips
- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.
Common mistakes to avoid
- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).
Inputs explained
| Field | What to enter |
|---|---|
| Method | Choose one: ΔH = ΔU + pΔV, Enthalpy of reaction from ΔH_f |
| Change in internal energy ΔU | Units: J, kJ, MJ, kWh, cal, kcal … |
| Pressure p | Units: kPa, Pa, bar, atm, psi, MPa |
| Change in volume ΔV | Units: L, m³, mL, ft³ |
| Reactant 1 coefficient | A number |
| Reactant 1 ΔH_f (kJ/mol) | A number |
| Reactant 2 coefficient | A number |
| Reactant 2 ΔH_f (kJ/mol) | A number |
| Reactant 3 coefficient | A number |
| Reactant 3 ΔH_f (kJ/mol) | A number |
| Product 1 coefficient | A number |
| Product 1 ΔH_f (kJ/mol) | A number |
| Product 2 coefficient | A number |
| Product 2 ΔH_f (kJ/mol) | A number |
| Product 3 coefficient | A number |
| Product 3 ΔH_f (kJ/mol) | A number |
Example values
How the main result changes when Change in internal energy ΔU changes and everything else stays at the example values above:
| Change in internal energy ΔU (J) | Enthalpy change ΔH (kJ) |
|---|---|
| 5 | -890.3 |
| 7.5 | -890.3 |
| 10 | -890.3 |
| 12.5 | -890.3 |
| 15 | -890.3 |
| 20 | -890.3 |
Formula
ΔH = ΔU + p·ΔV · ΔH_rxn = Σν·ΔH_f(products) − Σν·ΔH_f(reactants)Worked example
Inputs — Method: Enthalpy of reaction from ΔH_f; Reactant 1 coefficient: 1; Reactant 1 ΔH_f (kJ/mol): -74.8; Reactant 2 coefficient: 2; Reactant 2 ΔH_f (kJ/mol): 0; Reactant 3 coefficient: 0; Reactant 3 ΔH_f (kJ/mol): 0; Product 1 coefficient: 1; Product 1 ΔH_f (kJ/mol): -393.5; Product 2 coefficient: 2; Product 2 ΔH_f (kJ/mol): -285.8; Product 3 coefficient: 0; Product 3 ΔH_f (kJ/mol): 0.
Results — Enthalpy change ΔH = -890.3 kJ; Process = Exothermic (releases heat).
Key terms
- Enthalpy of formation
- ΔH to form 1 mol of a compound from its elements.
Frequently asked questions
What does a negative ΔH mean?
The reaction releases heat.
Why is the ΔH_f of O₂ zero?
Elements in their standard state are the reference.
What is the example?
Methane combustion: CH₄ + 2O₂ → CO₂ + 2H₂O(l), about −890 kJ/mol.
Is enthalpy the same as energy?
Enthalpy includes the pV work at constant pressure.
What formula does the Enthalpy Calculator use?
ΔH = ΔU + p·ΔV · ΔH_rxn = Σν·ΔH_f(products) − Σν·ΔH_f(reactants).