Enthalpy of Reaction Calculator
This enthalpy of reaction calculator works out the standard enthalpy change for a chemical reaction using Hess’s law, the method taught in NCEA and university chemistry for predicting whether a reaction releases or absorbs heat. Instead of measuring a reaction directly, you use tabulated standard enthalpies of formation for every substance involved, since these values are well documented for thousands of compounds. You enter two numbers: the sum of the formation enthalpies of all the products, each multiplied by its stoichiometric coefficient, and the same sum for the reactants, both in kilojoules per mole. Any element in its standard state, such as O2 gas or solid carbon, has a formation enthalpy of zero by definition, so it drops out of the sum. The calculator instantly returns the reaction enthalpy, delta H_rxn, worked out as products minus reactants, along with a plain-language label showing whether the reaction is exothermic (negative, heat given off) or endothermic (positive, heat absorbed), plus the reactants total. This saves the manual subtraction and coefficient bookkeeping that trips students up in assignments and exam questions. It suits chemistry students, teachers checking working, and anyone in a lab estimating reaction energetics from published data tables. The default example, forming carbon dioxide from its elements, gives a clearly exothermic result you can use to confirm you are reading the tool correctly before entering your own figures.
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’s 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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