Enthalpy of Reaction Calculator (Hess's Law)
Compute the standard enthalpy of a reaction from the heats of formation of its products and reactants using Hess's law.
Exothermic (releases heat)
- 1
Sum of product ΔHf° terms
-393.5 − 571.6 = -965.1 kJ/mol - 2
Sum of reactant ΔHf° terms
-74.8 + 0 = -74.8 kJ/mol - 3
Reaction enthalpy ΔH°rxn = products − reactants
-965.1 − -74.8 = -890.30Hess's law: ΔH°rxn = Σ(n·ΔHf° products) − Σ(m·ΔHf° reactants).
How does this calculator work?
Reaction enthalpy from Hess's law equals the total standard heat of formation of the products minus that of the reactants. Enter each species coefficient times its ΔHf° in kJ/mol; the calculator sums both sides and subtracts. A negative result is exothermic, a positive result is endothermic.
Formula
How this is calculated
Hess's law states that the enthalpy change of a reaction depends only on the initial and final states, not the path taken. This calculator applies the standard form ΔH°rxn = Σ(n·ΔHf° products) − Σ(m·ΔHf° reactants), where ΔHf° is the standard enthalpy of formation and n, m are the stoichiometric coefficients.
Enter one number per species in each box: each entry should already be the coefficient multiplied by that species' standard heat of formation (in kJ/mol). The tool sums the product entries, sums the reactant entries, and subtracts the reactant total from the product total. Values may be separated by commas or newlines.
By convention, an element in its standard state (such as O₂, N₂, or graphite) has ΔHf° = 0, so enter 0 for those. A negative ΔH°rxn means the reaction is exothermic (releases heat), while a positive value means it is endothermic (absorbs heat). All values are assumed to be at standard conditions (298.15 K, 1 bar) and expressed in kJ/mol.
Frequently asked questions
A negative ΔH°rxn indicates an exothermic reaction that releases heat; a positive value indicates an endothermic reaction that absorbs heat.
The standard enthalpy of formation of any element in its most stable standard state (e.g. O₂, H₂, graphite) is zero, so enter 0 for those species.
Yes. Each entry should be the stoichiometric coefficient multiplied by that species ΔHf° (in kJ/mol). For example, 2 mol of CO₂ at −393.5 kJ/mol is entered as −787.0.
Also known as
TG we-Calculate Editorial Team. (2026). Enthalpy of Reaction Calculator (Hess's Law) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/enthalpy-of-reaction-calculator
TG we-Calculate Editorial Team. "Enthalpy of Reaction Calculator (Hess's Law)." TG we-Calculate. 2026. https://we-calculate.com/calculator/enthalpy-of-reaction-calculator.
TG we-Calculate Editorial Team, "Enthalpy of Reaction Calculator (Hess's Law)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/enthalpy-of-reaction-calculator
@misc{wecalculate_enthalpy_of_reaction_calculator, title = {Enthalpy of Reaction Calculator (Hess's Law)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/enthalpy-of-reaction-calculator}}, year = {2026}, note = {TG we-Calculate} }
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