Enthalpy of reaction calculator
Enter the coefficients from the balanced equation and the standard enthalpy of formation of each substance, and get the reaction enthalpy ΔH° in kJ/mol and kcal/mol. Or switch to Hess's law and combine known reactions, each multiplied by a factor (use −1 to reverse a step).
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How to use it
Heats of formation
- Balance the equation.
- For each reactant and product, enter its coefficient and its ΔHf° in kJ/mol. Leave unused rows at coefficient 0.
- Read ΔH°; a negative value means the reaction gives off heat.
Hess’s law
- Write the steps that add up to your target reaction.
- Enter each step’s ΔH and the factor you multiply it by: 1 as written, −1 reversed, 2 doubled, 0.5 halved.
- ΔH° = Σ n·ΔHf°(products) − Σ n·ΔHf°(reactants).
- Hess: ΔH = Σ (multiplier × ΔH of step).
- 1 kcal = 4.184 kJ.
Worked example
Burning methane, CH₄ + 2 O₂ → CO₂ + 2 H₂O(l), with common table values ΔHf°(CH₄) = −74.8, ΔHf°(CO₂) = −393.5 and ΔHf°(H₂O, l) = −285.8 kJ/mol:
- products: −393.5 + 2 × (−285.8) = −965.1 kJ/mol
- reactants: −74.8 + 2 × 0 = −74.8 kJ/mol
- ΔH° = −965.1 − (−74.8) = −890.3 kJ/mol, or −212.79 kcal/mol, exothermic.
Hess’s law for C + ½ O₂ → CO: take C + O₂ → CO₂ (ΔH = −393.5) once and the reverse of CO + ½ O₂ → CO₂ (ΔH = −283.0) with multiplier −1. ΔH = −393.5 + 283.0 = −110.5 kJ/mol.
Common mistakes
- Forgetting the coefficients. Two moles of water count twice.
- Wrong sign order. It is products minus reactants, not the other way round.
- Wrong state. Liquid and gaseous water have different ΔHf°; use the one that matches your equation.
Frequently asked questions
How do I calculate the enthalpy of reaction from enthalpies of formation?
Multiply each substance's standard enthalpy of formation by its coefficient in the balanced equation, add them up for the products and for the reactants, and subtract, ΔH° = Σ n·ΔHf°(products) − Σ n·ΔHf°(reactants).
Why is ΔHf° zero for O₂ or H₂?
By definition the standard enthalpy of formation of an element in its standard state (O₂ gas, H₂ gas, graphite, solid iron) is zero. Only compounds and other forms of an element have a non-zero value.
What is Hess's law?
The enthalpy change of a reaction does not depend on the path. If the target reaction is a sum of steps, its ΔH is the sum of the steps' ΔH. Reverse a step and its sign flips (multiplier −1); double it and its ΔH doubles (multiplier 2).
Where do I get the ΔHf° values?
From the data table of your textbook or a thermochemical database. Tables differ slightly, by a few tenths of a kJ/mol, and water must match its state, −285.8 kJ/mol for liquid but about −241.8 kJ/mol for vapour.