Enter your Calculation Method, then fill in your reactants and products with their coefficients and custom enthalpy values (kJ/mol) to calculate the Enthalpy Change (ΔH) — plus get a breakdown of your reaction type, energy description, and total reactant and product enthalpies.
Enthalpy Change (ΔH)
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Reaction Type
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Energy Description
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Total Reactant Enthalpy
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Total Product Enthalpy
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Enthalpy (H) is a thermodynamic property that measures the total heat content of a system. It represents the sum of internal energy and the product of pressure and volume. Enthalpy change (ΔH) indicates the heat absorbed or released during a chemical reaction at constant pressure.
A reaction is endothermic if ΔH is positive (heat is absorbed from surroundings) and exothermic if ΔH is negative (heat is released to surroundings). Endothermic reactions feel cold while exothermic reactions feel warm or hot.
Standard formation enthalpy (ΔHf°) is the enthalpy change when one mole of a compound forms from its elements in standard states. Heat of reaction (ΔHrxn) is the enthalpy change for any chemical reaction, calculated using formation enthalpies of reactants and products.
Use the formula: ΔHrxn = Σ(ΔHf° products) - Σ(ΔHf° reactants). Multiply each formation enthalpy by its stoichiometric coefficient, sum all products, sum all reactants, then subtract reactants from products.
Key values include: H2O(l) = -285.8 kJ/mol, CO2(g) = -393.5 kJ/mol, NH3(g) = -46.1 kJ/mol, and CH4(g) = -74.8 kJ/mol. Elements in their standard states have ΔHf° = 0 kJ/mol.
By definition, elements in their most stable form under standard conditions (25°C, 1 atm) have zero formation enthalpy because they are already in their reference state and require no energy to 'form' from themselves.
Yes, this calculator works for all types of reactions including combustion. For combustion reactions, you'll typically see large negative ΔH values because combustion reactions are highly exothermic, releasing significant amounts of energy.