Enthalpy Calculator
The enthalpy calculator determines the heat absorbed or released in a chemical reaction using Hess's Law and standard enthalpies of formation. Enthalpy change (delta H) quantifies the thermal energy exchanged between a chemical system and its surroundings at constant pressure. Negative delta H indicates an exothermic reaction that releases heat; positive delta H indicates an endothermic reaction that absorbs heat. This tool accepts up to four products and four reactants with their stoichiometric coefficients and standard enthalpies of formation, then applies delta H_rxn = sum(products) - sum(reactants) to compute the reaction enthalpy. Used in calorimetry, combustion analysis, HVAC design, and pharmaceutical manufacturing.
Enthalpy formula (Hess's Law)
delta H_rxn = sum(n_i * delta H_f(products)) - sum(n_j * delta H_f(reactants))
Elements in standard state: delta H_f = 0 kJ/mol
Example: CH4 + 2O2 to CO2 + 2H2O
delta H = (1*(-393.5) + 2*(-241.8)) - (1*(-74.8) + 2*0) = -802.3 kJ/mol
Standard enthalpies of formation (kJ/mol)
- CO2(g): -393.5
- H2O(g): -241.8; H2O(l): -285.8
- CH4(g) methane: -74.8
- NH3(g): -46.1
- HCl(g): -92.3
Enthalpy: frequently asked questions
What is enthalpy?
Enthalpy (H) is a thermodynamic quantity equal to the internal energy of a system plus the product of its pressure and volume: H = U + PV. At constant pressure (most lab and industrial conditions), the enthalpy change equals the heat exchanged: delta H = q_p. It is measured in kJ/mol or kJ.
What is the enthalpy of formation?
The standard enthalpy of formation (delta H_f) is the enthalpy change when one mole of a compound is formed from its elements in their standard states at 298 K and 1 atm. By definition, elements in their standard state have delta H_f = 0. NIST and IUPAC publish extensive tables of these values.
How do you calculate the enthalpy of a reaction?
Hess's Law: delta H_rxn = sum of delta H_f(products) - sum of delta H_f(reactants). Each term is multiplied by its stoichiometric coefficient. This works because enthalpy is a state function - the path doesn't matter, only the initial and final states.
What is an exothermic vs endothermic reaction?
Exothermic reactions release heat (delta H negative): combustion, neutralization, metal oxidation. Endothermic reactions absorb heat (delta H positive): melting ice, photosynthesis, dissolving ammonium nitrate in water. The sign of delta H is always from the system's perspective.
What is bond enthalpy and how is it used?
Bond enthalpy (or bond energy) is the average energy required to break one mole of a particular bond in the gas phase. Approximate enthalpy of reaction: delta H = sum of bonds broken (positive, energy in) - sum of bonds formed (negative, energy out). This gives estimates when exact thermochemical data is unavailable.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.