Enthalpy of Reaction Calculator
The standard enthalpy of reaction is the heat exchanged when a reaction runs at constant pressure under standard conditions. It is found from the standard enthalpies of formation of the species involved: add up the formation enthalpies of the products, each multiplied by its coefficient, and subtract the same sum for the reactants. A negative result means the reaction is exothermic. This calculator takes the coefficient-weighted total formation enthalpy of products and of reactants, then reports the reaction enthalpy and whether it is exothermic or endothermic. Use values from the NIST WebBook.
Enthalpy of reaction formula
delta H reaction = sum(coef * dHf products) - sum(coef * dHf reactants)
dHf of an element in its standard state = 0
delta H < 0 means exothermic, delta H > 0 means endothermic
Enter the coefficient-weighted total formation enthalpy for products and for reactants. Elements in their standard state contribute zero, so they may be omitted from the reactant sum.
Reaction enthalpy context
- Standard enthalpies of formation are defined at 1 bar and usually 298.15 K.
- Each species is weighted by its stoichiometric coefficient before summing.
- A negative delta H indicates an exothermic reaction that releases heat.
- Elements in their reference state have a formation enthalpy of zero.
- This approach is an application of Hess's law using formation enthalpies.
Enthalpy of reaction: frequently asked questions
How do I calculate the enthalpy of reaction?
Subtract the total standard enthalpy of formation of the reactants from that of the products: delta H reaction = sum of products minus sum of reactants, each multiplied by its stoichiometric coefficient. Standard enthalpies of formation come from tables such as the NIST WebBook.
What is a standard enthalpy of formation?
The standard enthalpy of formation is the enthalpy change when one mole of a compound forms from its elements in their standard states at 1 bar and a specified temperature, usually 298.15 K. Elements in their standard state have a formation enthalpy of zero.
What does a negative enthalpy of reaction mean?
A negative enthalpy of reaction means the reaction releases heat to the surroundings and is exothermic. A positive value means the reaction absorbs heat and is endothermic. The sign follows the convention that energy leaving the system is negative.
Why multiply by stoichiometric coefficients?
Each compound's formation enthalpy is per mole, so you must weight it by how many moles appear in the balanced equation. Two moles of a product contribute twice its formation enthalpy. This calculator lets you enter the summed, coefficient-weighted totals.
Where do I find enthalpy of formation values?
Standard enthalpies of formation are tabulated in the NIST Chemistry WebBook and other reference databases. Always use values at a consistent temperature and standard pressure, typically 298.15 K and 1 bar, for all species in the reaction.
Official sources
- NIST Chemistry WebBook: standard enthalpies of formation.
- IUPAC Gold Book: enthalpy of reaction and formation terminology.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.