🔥 Heat of Reaction Calculator
ΔH°rxn = Σ[n·ΔHf°(products)] − Σ[n·ΔHf°(reactants)]
Products
Reactants
ΔH°rxn
How to Use This Calculator
This tool calculates the standard enthalpy of reaction (ΔH°rxn) from the standard enthalpies of formation (ΔHf°) of each reactant and product. Look up ΔHf° values in a thermodynamic data table, enter each one with its stoichiometric coefficient, and click Calculate.
In the Products section, enter the stoichiometric coefficient (n) and ΔHf° in kJ/mol for each product. Click "+ Add product" if you have more than one product. For example, for the combustion of methane the products are CO₂ (ΔHf° = -393.5 kJ/mol, n=1) and H₂O (ΔHf° = -285.8 kJ/mol, n=2).
In the Reactants section, enter n and ΔHf° for each reactant. Elements in their standard state (O₂, H₂, C as graphite) have ΔHf° = 0 by definition. For methane combustion: CH₄ (ΔHf° = -74.8 kJ/mol, n=1) and O₂ (ΔHf° = 0, n=2).
Click Calculate ΔHrxn. The result shows ΔH°rxn along with the sum for products and the sum for reactants separately.
Check the sign: a negative ΔH means the reaction releases heat (exothermic). A positive ΔH means it absorbs heat (endothermic).
The Hess's Law Formula
n is the stoichiometric coefficient from the balanced equation. ΔHf° is the standard enthalpy of formation in kJ/mol, defined as the enthalpy change when exactly 1 mole of the compound forms from its constituent elements in their standard states at 298 K and 1 bar pressure.
Worked Examples
Where This Calculation Comes Up
ΔH°rxn calculations sit at the core of thermochemistry and appear on every general chemistry exam. In practice, Hess's Law is used when you cannot measure the enthalpy of a reaction directly. The formation of carbon monoxide from carbon and oxygen cannot be measured cleanly in a calorimeter because some CO₂ always forms. Instead, you measure the combustion of C to CO₂ (ΔH = -393.5 kJ/mol) and the combustion of CO to CO₂ (ΔH = -283.0 kJ/mol), then combine them algebraically to get the formation enthalpy of CO: -393.5 − (-283.0) = -110.5 kJ/mol.
In engineering, ΔHrxn tells you how much heat a reactor releases or consumes per mole of reaction. The industrial oxidation of SO₂ to SO₃ in sulfuric acid manufacture releases about 99 kJ per mole. That heat must be managed through heat exchangers to maintain the optimal catalyst temperature of around 450 °C. Getting the enthalpy calculation right is part of designing a safe and economical process. In food chemistry, the same calculation gives you the caloric content of nutrients by using combustion enthalpies for carbohydrates, fats, and proteins.
Frequently Asked Questions
What is the heat of reaction?
ΔH°rxn = Σ[n×ΔHf°(products)] − Σ[n×ΔHf°(reactants)]. It is the enthalpy change for a reaction under standard conditions.
What are standard enthalpies of formation?
ΔHf° is the heat change when 1 mole of a compound forms from its elements in standard states. By definition, ΔHf° = 0 for pure elements.
What does negative ΔHrxn mean?
Negative ΔH means the reaction is exothermic — it releases heat to the surroundings.
What does positive ΔHrxn mean?
Positive ΔH means the reaction is endothermic — it absorbs heat from the surroundings.
What is Hess's Law?
Hess's Law states that the total enthalpy change for a reaction is independent of the path taken. This allows us to calculate ΔH from tabulated ΔHf° values.