Explore enthalpy change across real laboratory situations today. Switch between heat, calorimetry, formation, and bonds. Check units, verify steps, then download your report easily.
Choose a method, enter values, then compute ΔH. Inputs are arranged in a responsive grid: three columns on large screens, two on medium, and one on mobile.
For reaction methods, results correspond to the stoichiometry you enter. Use the scale factor to model multiple reaction “runs” or different extents.
If your sign looks reversed, confirm whether your inputs describe heat absorbed by the system or released to the surroundings.
These sample cases show typical inputs and the expected sign. Values are illustrative.
| Case | Method | Inputs | ΔH (kJ) | Interpretation |
|---|---|---|---|---|
| Heating water | m·cₚ·ΔT | m=0.50 kg, cₚ=4184 J/kg·K, ΔT=10 K | +20.92 | Endothermic heating |
| Calorimeter reading | qₚ | qₚ = −15.0 kJ | −15.0 | Exothermic release |
| Methane combustion | Formation enthalpies | ΣP≈(1×−393.5), ΣR≈(1×−74.8)+(2×0) | ≈−318.7 | Exothermic reaction |
| Gas-phase estimate | Bond energies | Broken: 4×C–H, 2×O=O; Formed: 2×C=O, 2×O–H | Typically negative | Approximate trend |
Enthalpy change (ΔH) measures heat exchanged by a system at constant pressure. In many laboratory and engineering settings, pressure remains close to atmospheric, so ΔH tracks “heat in” or “heat out” more directly than internal energy. Positive ΔH usually indicates heat absorbed by the reacting or warming material, while negative ΔH indicates heat released to the surroundings.
Common reporting uses kJ for processes and kJ/mol for reaction stoichiometry. Small temperature changes in liquids often produce tens of kJ per kilogram, while combustion reactions can reach hundreds of kJ per mole of fuel. This calculator displays results in kJ, J, cal, kcal, or Btu so you can align outputs with laboratory notes or industrial reporting formats.
If you directly measure heat transfer at constant pressure, you can set ΔH equal to qp. For example, a calorimeter might report −15.0 kJ during a reaction. Entering qp = −15.0 kJ yields ΔH = −15.0 kJ, indicating exothermic behavior. This approach is best when your heat measurement already includes calibration corrections.
For heating and cooling, ΔH ≈ m·cp·ΔT is widely used. Typical liquid water cp is about 4184 J/(kg·K); many oils range near 1700–2200 J/(kg·K), and common metals can be 350–900 J/(kg·K). A 0.50 kg water sample warmed by 10 K absorbs about 20.92 kJ.
When you have standard formation enthalpies (ΔHf°), compute reaction enthalpy as ΣνΔHf°(products) − ΣνΔHf°(reactants). For methane combustion, illustrative values include ΔHf°[CO2(g)] ≈ −393.5 kJ/mol, ΔHf°[CH4(g)] ≈ −74.8 kJ/mol, and ΔHf°[O2(g)] = 0. With stoichiometric coefficients, the trend is strongly negative.
Bond energies estimate ΔH by subtracting energy released forming bonds from energy required breaking bonds: ΣE(broken) − ΣE(formed). Average bond energies vary by molecular environment, so this method can differ from formation-data results. It is most informative for quick screening, gas-phase estimates, and checking whether a reaction is plausibly exothermic or endothermic.
The sign reflects the system perspective. If the system releases heat, ΔH becomes negative. If you model multiple reaction “runs,” apply a scale factor: a scale of 2 doubles the computed ΔH. This is useful when comparing per-run results to batch totals, or converting from per-mole calculations to a chosen reaction extent.
Keep units consistent, especially when mixing J and kJ. Use temperature differences (ΔT) rather than absolute temperature. For reaction methods, verify coefficients and include only species in the balanced equation. If results look unusual, re-check sign conventions, confirm reference-state assumptions, and compare two methods to identify input mistakes.
No. Heat is energy transferred; ΔH is a state-function change. At constant pressure, heat transferred to the system equals ΔH, which is why qp can be used.
Liquid water stores heat efficiently. Near room temperature, its specific heat is close to 4184 J/(kg·K), making it a common calibration reference for simple calorimetry estimates.
Yes. A temperature difference of 10 °C equals 10 K, so ΔT can be entered in either scale as long as it is a difference, not an absolute temperature.
It multiplies the final ΔH. Use it to model multiple identical trials, different reaction extents, or batch totals when your inputs represent a single baseline scenario.
Bond energies are averages and depend on molecular environment. Formation enthalpies are reference-state values for specific species, so they usually produce more accurate reaction enthalpies.
For elements in their standard state, ΔHf° is defined as 0 kJ/mol. Enter 0 for species such as O2(g) or N2(g) in standard conditions.
Positive ΔH indicates the system absorbs heat (endothermic). You may observe cooling of surroundings or require external heating to maintain temperature during the process.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.