Hess's Law Calculator
Combine known reactions to find the enthalpy change of a target reaction.
Known Reactions
How Hess's Law Works
Hess's Law states that the total enthalpy change for a chemical reaction is independent of the pathway taken. This means you can combine known thermochemical equations — multiplying them by coefficients and reversing them as needed — to determine the enthalpy change of a target reaction that may be difficult or impossible to measure directly.
This calculator sets up a system of linear equations where each known reaction contributes a row of species coefficients. The solver then finds the linear combination of known reactions that produces the target reaction, using a least-squares approach with Gaussian elimination. The resulting coefficients are multiplied by each known ΔH and summed to give the target enthalpy.
FAQ: Common Questions
Use Unicode subscripts (₂, ₃, etc.) or just type regular numbers: H2O works the same as H₂O.
A negative coefficient means the known reaction must be reversed. For example, if the solver outputs −1.0, flip the reaction and negate its ΔH.
You need at least as many known reactions as unique chemical species in your target. Add more known reactions to constrain the system.
Example: Formation of Liquid Water
Known reactions:
1. 2H₂ + O₂ → 2H₂O(g), ΔH = −483.6 kJ
2. 2H₂O(g) → 2H₂O(l), ΔH = −88.0 kJ
Target: 2H₂ + O₂ → 2H₂O(l)
The solver finds coefficients 1.0 and 1.0, giving ΔH = (1.0 × −483.6) + (1.0 × −88.0) = −571.6 kJ.