Enter Reaction Data
Use standard enthalpies of formation and balanced coefficients for each species.
Example Data Table
| Reaction | Reactant Sum (kJ) | Product Sum (kJ) | ΔHrxn (kJ) | Type |
|---|---|---|---|---|
| CH4 + 2O2 → CO2 + 2H2O(l) | -74.8 | -965.1 | -890.3 | Exothermic |
| N2 + O2 → 2NO | 0.0 | 180.5 | 180.5 | Endothermic |
| H2 + 1/2O2 → H2O(g) | 0.0 | -241.8 | -241.8 | Exothermic |
Formula Used
The calculator uses Hess's law with standard enthalpies of formation. It multiplies each species formation enthalpy by its stoichiometric coefficient, sums products, sums reactants, then subtracts reactants from products.
ΔHrxn = Σ(νproducts × ΔHf,products) − Σ(νreactants × ΔHf,reactants)
Total heat = ΔHrxn × reaction extent
Negative values mean heat is released. Positive values mean heat is absorbed. Standard elemental forms usually have zero formation enthalpy under standard reference conditions.
How to Use This Calculator
- Enter a reaction name for easy identification.
- Set the reaction extent in moles for total heat estimation.
- Add each reactant with its balanced coefficient and standard formation enthalpy.
- Add each product using the same balanced reaction basis.
- Submit the form to see the result above the calculator.
- Review the contribution table to validate each term.
- Download the values as CSV or PDF for reporting.
FAQs
1. What does heat of reaction mean?
It is the enthalpy change when a balanced chemical reaction occurs. A negative result means the process releases heat, while a positive result means it absorbs heat.
2. Which data should I enter here?
Enter stoichiometric coefficients from the balanced equation and standard enthalpies of formation for each reactant and product species, usually at 25°C and 1 atm.
3. Can I use fractional coefficients?
Yes. Fractional coefficients are acceptable if your balanced equation uses them consistently. The reaction extent then scales the total heat from that exact stoichiometric basis.
4. Why do elemental species often use zero?
Elements in their standard reference states usually have zero standard enthalpy of formation. Examples include O2(g), N2(g), H2(g), and graphite carbon.
5. What is reaction extent in this tool?
Reaction extent scales the enthalpy from one stoichiometric reaction event to the actual number of reaction moles you want to analyze.
6. Is this the same as calorimetry?
Not exactly. This tool estimates enthalpy from tabulated formation data using Hess's law. Calorimetry measures heat experimentally from temperature change and system properties.
7. Why is my answer different from literature values?
Differences can come from phase selection, rounding, temperature, pressure, incomplete balancing, or using nonstandard enthalpy values for species in different physical states.
8. Can I export the results for reports?
Yes. After calculation, use the CSV export for spreadsheet work or the PDF button for a quick printable summary of your entered data and results.