enthalpy change formulas and interpretation
Reaction enthalpy describes heat absorbed or released at constant pressure.
Stoichiometric coefficients must be included in every summed term.
How to use the enthalpy change calculator
- Choose a model: Select the relationship matching the problem.
- Choose the unknown: Select the quantity to calculate.
- Enter values: Enter all known values with matching units and signs.
- Calculate: Review the result, formula, units, and direction.
Formula and variables
Subtract the total formation enthalpy of reactants from that of products.
ΔH_rxn = ΣΔH_f(products) − ΣΔH_f(reactants)- ΔH_rxn — Reaction enthalpy
- Enthalpy change for the balanced reaction (kJ/mol)
- ΔH_f — Formation enthalpy
- Standard molar formation enthalpy (kJ/mol)
Formation enthalpy example
Products total −393.5 kJ/mol and reactants total 0.
- Products
- −393.5 kJ/mol
- Reactants
- 0 kJ/mol
- ΔH = −393.5 − 0
Result: The reaction enthalpy is −393.5 kJ/mol.
The negative sign indicates an exothermic reaction.
Understanding your results
Interpreting the result
Negative values are exothermic; positive values are endothermic under the stated conditions.
Assumptions
- The selected equation represents the physical system.
- Inputs use a consistent reference direction.
- Values are converted through coherent SI units.
Limitations
- Vector components must be resolved along a common axis.
- External forces or energy losses are not added automatically.
- Results depend on the accuracy of entered measurements.
Common mistakes
- Mixing incompatible units.
- Dropping negative signs that represent direction.
- Using weight where mass is required.
- Entering a zero divisor.
Practical use cases
Physics problems
Check classroom, laboratory, and mechanics calculations.
Practical estimates
Estimate motion, forces, and energy for real systems.
Frequently asked questions
Can a result be negative?
Yes. For directional quantities, the sign indicates direction relative to the chosen positive axis.
Should I use SI units?
The interface can convert supported units, while the formulas are evaluated through coherent SI units.
Sources and review
- SI Brochure, 9th edition — BIPM. Accessed 2026-07-11.
- Special Publication 811 — NIST. Accessed 2026-07-11.
Reviewed 2026-07-11.