Enthalpy of Reaction Calculator
Find reaction enthalpy from formation enthalpies.
Results update as you type.
How it works
The enthalpy of reaction, ΔH, tells you whether a reaction releases or absorbs heat and how much, and this calculator finds it from standard enthalpies of formation. You enter the summed formation enthalpies of the products and of the reactants (each species multiplied by its coefficient in the balanced equation), and it returns the reaction enthalpy and whether the reaction is exothermic or endothermic. The method relies on Hess's law: the overall enthalpy change equals the total formation enthalpy of the products minus that of the reactants. Standard enthalpies of formation, tabulated for many substances, are the heat change when one mole of a compound forms from its elements in their standard states; elements in their standard state have a formation enthalpy of zero. A negative ΔH means the products are more stable and energy is released as heat, so the reaction is exothermic, like combustion. A positive ΔH means heat is absorbed and the reaction is endothermic. This is one of the most useful thermochemistry calculations, letting you predict energy changes without measuring them directly. Be sure to multiply each substance's formation enthalpy by its coefficient before summing. Enter the product and reactant formation enthalpy sums to find ΔH.
ΔH = Σ ΔHf(products) − Σ ΔHf(reactants)
By Hess's law, the reaction enthalpy equals the sum of the standard formation enthalpies of the products minus the sum for the reactants, each multiplied by its coefficient. A negative result means the reaction releases heat (exothermic); a positive result means it absorbs heat (endothermic).
Source:Hess's law (thermochemistry)
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Worked example
How to use this calculator
- 1
Enter your values
Enter sum of product δhf and sum of reactant δhf.
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Adjust the options
Fine-tune any options to match your situation.
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Read your result
Reaction enthalpy (ΔH) updates instantly as you type — no button to press.
Frequently asked questions
How do you calculate the enthalpy of reaction?
Use Hess's law: subtract the summed standard formation enthalpies of the reactants from those of the products, with each substance multiplied by its coefficient. If products total −500 kJ and reactants −200 kJ, then ΔH is −300 kJ. A negative result is exothermic (releases heat) and a positive one is endothermic (absorbs heat).
What is the difference between exothermic and endothermic?
An exothermic reaction releases heat to the surroundings and has a negative ΔH, meaning the products hold less energy than the reactants. An endothermic reaction absorbs heat and has a positive ΔH, with products holding more energy. Combustion is exothermic; melting ice or many decomposition reactions are endothermic. The sign of ΔH tells you which.
What is an enthalpy of formation?
The standard enthalpy of formation is the heat change when one mole of a compound forms from its constituent elements in their standard states, at standard conditions. Values are tabulated for many substances. Elements in their standard state, like O₂ gas, have a formation enthalpy of zero by definition, which is why they can be omitted from the calculation.
What is Hess's law?
Hess's law states that the total enthalpy change of a reaction is the same regardless of the pathway taken, because enthalpy is a state function. This lets you calculate a reaction's ΔH from formation enthalpies or by combining other reactions, rather than measuring it directly. It is the basis for the product-minus-reactant formation enthalpy method.
Why multiply by the coefficients?
Formation enthalpies are given per mole, but a balanced equation may involve several moles of a substance. Multiplying each formation enthalpy by its coefficient accounts for the actual amounts reacting or forming, so the totals reflect the whole equation. Skipping this step gives an incorrect ΔH, so always balance the equation and apply the coefficients first.