Calculate the standard enthalpy of reaction for any chemical equation using Hess's Law and standard enthalpies of formation. Our free online calculator instantly determines whether your reaction is exothermic or endothermic with detailed step-by-step results.
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Learn how to calculate enthalpy of reaction with these practical examples using Hess's Law:
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)
N2(g) + 3 H2(g) → 2 NH3(g)
CaCO3(s) → CaO(s) + CO2(g)
The enthalpy of reaction (ΔH°rxn) is the change in enthalpy that occurs during a chemical reaction at constant pressure. It represents the heat absorbed or released during the reaction. A negative ΔH°rxn indicates an exothermic reaction (heat is released), while a positive value indicates an endothermic reaction (heat is absorbed).
Hess's Law states that the enthalpy change of a reaction is independent of the pathway taken. To calculate ΔH°rxn: ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants). Multiply each compound's standard enthalpy of formation by its stoichiometric coefficient, sum all products, sum all reactants, then subtract reactants from products.
Exothermic reactions release energy (heat) to the surroundings and have negative ΔH°rxn values (e.g., combustion of methane: -890.3 kJ/mol). Endothermic reactions absorb energy from the surroundings and have positive ΔH°rxn values (e.g., decomposition of calcium carbonate: +178.3 kJ/mol).
Standard enthalpy of formation (ΔH°f) is the enthalpy change when one mole of a compound is formed from its elements in their standard states at 25°C (298 K) and 1 atm pressure. By definition, elements in their standard state have ΔH°f = 0 kJ/mol (e.g., O2(g), N2(g), C(graphite)).
By convention, the standard enthalpy of formation of an element in its most stable form (standard state) at 25°C and 1 atm is defined as zero. This provides a reference point for measuring the enthalpies of formation of all compounds. For example, O2(g), H2(g), N2(g), and C(graphite) all have ΔH°f = 0 kJ/mol.
Standard enthalpies of formation can be found in chemistry textbooks, NIST (National Institute of Standards and Technology) databases, CRC Handbook of Chemistry and Physics, or online chemistry databases. Most general chemistry courses provide tables of common ΔH°f values for reference.
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