Enthalpy Calculator
Calculate reaction enthalpy using Hess Law from formation enthalpies, or find enthalpy change from temperature and heat capacity.
Type of enthalpy calculation
Stoichiometric coefficient
Standard enthalpy of formation
Stoichiometric coefficient
Standard enthalpy of formation
Stoichiometric coefficient
Standard enthalpy of formation
Stoichiometric coefficient
Standard enthalpy of formation
Molar heat capacity
Change in temperature
Amount in moles
What is enthalpy and Hess Law?
Enthalpy (H) is the total heat content of a system. Hess Law states that the enthalpy change of a reaction is the same regardless of the pathway taken - it's a state function. You can sum enthalpy changes of multiple steps to get the total reaction enthalpy. ΔHrxn = Σ ΔHf(products) - Σ ΔHf(reactants).
What is a negative (exothermic) vs positive (endothermic) enthalpy?
Exothermic reactions release heat (negative ΔH) - products have lower enthalpy than reactants. Burning methane is exothermic (-890 kJ/mol). Endothermic reactions absorb heat (positive ΔH) - products have higher enthalpy. Photosynthesis is endothermic (+2800 kJ/mol).
How do I calculate enthalpy change with temperature?
Use the formula ΔH = n × Cp × ΔT, where n = moles, Cp = molar heat capacity (J/mol·K), and ΔT = temperature change (K or °C, same magnitude). For water: ΔH = 1 mol × 75.3 J/mol·K × 10 K = 753 J = 0.753 kJ.
What are standard enthalpies of formation?
Standard enthalpy of formation (ΔHf°) is the enthalpy change when 1 mole forms from its elements in their standard states. Elemental forms in standard states have ΔHf° = 0. For example: ΔHf°(CO2) = -393.5 kJ/mol, ΔHf°(H2O) = -285.8 kJ/mol, ΔHf°(CH4) = -74.8 kJ/mol.