Enthalpy Calculator
Free enthalpy calculator using calorimetry. Enter mass, specific heat and temperature change to find the heat q and the molar enthalpy change ΔH of a reaction.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
Sign convention: a temperature rise means the reaction released heat (exothermic, ΔH negative). Leave moles blank or 0 to see only the heat q.
How the Enthalpy Calculator Works
This calculator finds the heat (q) released or absorbed in a reaction using calorimetry, then converts it to the molar enthalpy change (ΔH). Enter the mass of the solution, its specific heat, the measured temperature change (ΔT), and optionally the moles of reactant.
The Formula
The heat exchanged with the solution is:
q = m × c × ΔT
where m is mass in grams, c is specific heat in J/g·°C, and ΔT is the temperature change in °C. The reaction’s enthalpy change has the opposite sign, because heat gained by the water was released by the reaction:
ΔH_rxn = −q, and molar ΔH = −q ÷ moles
A temperature rise (positive ΔT) means the reaction is exothermic and ΔH is negative.
Worked Example
A reaction warms 100 g of solution (c = 4.18 J/g·°C) by 6.5 °C, using 0.05 mol of reactant:
- q = 100 × 4.18 × 6.5 = 2,717 J (2.717 kJ) gained by the water
- ΔH_rxn = −2.717 kJ (exothermic, since temperature rose)
- Molar ΔH = −2.717 kJ ÷ 0.05 mol = −54.3 kJ/mol
Sign Conventions
| Observation | ΔT | Reaction type | Sign of ΔH |
|---|---|---|---|
| Solution gets warmer | positive | Exothermic | negative |
| Solution gets cooler | negative | Endothermic | positive |
| No temperature change | zero | Thermally neutral | zero |
Notes on Accuracy
- The method assumes no heat loss to the surroundings or the calorimeter, which slightly underestimates real enthalpy changes.
- Use the specific heat of the actual solution; 4.18 J/g·°C is a good estimate for dilute aqueous solutions.
- For per-mole results, use the moles of the limiting reactant.
- This calorimetry approach gives the reaction enthalpy directly; it does not use tabulated standard formation enthalpies.
Frequently asked questions
How do you calculate enthalpy change from calorimetry?+
Measure the temperature change of the solution and use q = m × c × ΔT, where m is the solution mass, c is its specific heat and ΔT is the temperature change. The reaction's heat is the negative of the heat gained by the solution. Divide by the moles of reactant to get the molar enthalpy change ΔH in kJ/mol.
What is the formula for enthalpy?+
In calorimetry, the heat exchanged is q = m × c × ΔT. The enthalpy change of the reaction at constant pressure equals this heat with the opposite sign: ΔH_rxn = -q. To express it per mole, divide by the moles of the limiting reactant. The standard unit is kilojoules per mole.
Why is exothermic enthalpy negative?+
Enthalpy change is defined from the system's point of view. An exothermic reaction releases heat to the surroundings, so the system loses energy and ΔH is negative. If the temperature of the solution rises, heat flowed out of the reaction into the water, which is the signature of an exothermic process.
What specific heat should I use?+
For dilute aqueous solutions, the specific heat of water, 4.18 J per gram per degree Celsius, is the standard approximation. For other solvents or metals, look up the substance's specific heat. Using the wrong value scales your heat and enthalpy results proportionally.
What is the difference between heat q and enthalpy ΔH?+
Heat q is the energy that flows during the experiment, measured from the temperature change. Enthalpy change ΔH is the heat exchanged at constant pressure, which is how most lab reactions run. They have the same magnitude here but opposite signs, because q is measured for the surroundings while ΔH describes the reacting system.