Enthalpy of Reaction Calculator
This enthalpy of reaction calculator finds the standard enthalpy change of a reaction from standard enthalpies of formation. The rule is products minus reactants: add up the formation enthalpies of all the products (each multiplied by its coefficient) and subtract the total for the reactants. Enter the two totals and the calculator returns the reaction enthalpy and whether it is exothermic or endothermic. Elements in their standard state have a formation enthalpy of zero. This method, built on Hess\u2019s law, is the standard way to predict reaction energetics from data tables. The formula, a worked example and the assumptions are below.
The formula
Standard reaction enthalpy is delta H_rxn = sum of (formation enthalpies of products) minus sum of (formation enthalpies of reactants), each weighted by the stoichiometric coefficient. Elements in their standard state contribute zero.
Worked example
Forming carbon dioxide from carbon and oxygen: products total minus 393.5 kJ/mol, reactants (elements) total 0, so delta H = minus 393.5 kJ/mol, exothermic. Enter -393.5 and 0 to confirm.
Frequently asked questions
How do I find reaction enthalpy from formation data?
Sum the formation enthalpies of the products, subtract the sum for the reactants, weighting each by its coefficient.
What is the formation enthalpy of an element?
Zero, for an element in its standard state, by definition.
Why products minus reactants?
It follows from Hess\u2019s law: forming products from elements, and un-forming reactants back to elements, gives the overall change.
Who this calculator is for
This calculator is for chemistry students, teachers and lab workers.
What this calculator assumes
- You enter values in the units shown.
- Standard conditions unless stated.
- Results are rounded for display.
Formula and sources
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