Hess's Law Calculator
Build a target reaction from thermochemical steps, reverse or multiply reactions, and sum the adjusted Delta H values.
What to enter
Use one line per thermochemical equation. The calculator reads each line as reaction | factor | DeltaH. A negative factor means the reaction is reversed, so the enthalpy sign changes automatically.
| Entry | Meaning | Example |
|---|---|---|
| Reaction | Label or chemical equation for the step | C(s) + O2(g) -> CO2(g) |
| Factor | How the step is multiplied; negative reverses the step | -1 reverses, 2 doubles |
| DeltaH | Original enthalpy change for the written step | -393.5 kJ |
| Comma values | Already adjusted enthalpy values | -100, 50, -25 |
Formula used
Reverse reaction: DeltaH changes sign
Multiply reaction: DeltaH is multiplied by the same factor
Worked example
Target: C(s) + 1/2 O2(g) -> CO(g)
C(s) + O2(g) -> CO2(g), DeltaH = -393.5 kJ
CO(g) + 1/2 O2(g) -> CO2(g), DeltaH = -283.0 kJ. Reverse this step, so adjusted DeltaH = +283.0 kJ.
Net DeltaH = -393.5 + 283.0 = -110.5 kJ.
Why Hess's Law works
Enthalpy is a state function. That means the total Delta H depends on the initial substances and final substances, not on the route used to get there. If several known reactions can be added to form a target reaction, their adjusted enthalpy changes can also be added.
This is especially useful when a reaction is hard to measure directly but related combustion, formation, or decomposition data are available.
Strategy for solving Hess problems
- Write the target reaction clearly.
- Move each given equation so substances cancel to the target reaction.
- Reverse equations when needed and change the sign of Delta H.
- Multiply equations when coefficients need scaling and multiply Delta H too.
- Add the adjusted equations and check that unwanted substances cancel.
- Add the adjusted Delta H values.
Common mistakes
- Reversing a reaction but forgetting to change the sign of Delta H.
- Doubling coefficients but not doubling Delta H.
- Adding equations before checking that intermediates cancel.
- Mixing formation enthalpy signs for products and reactants.
- Rounding intermediate enthalpies too early.
Related Chemistry Tools
Hess's Law Calculator FAQs
What is Hess's Law in chemistry?
Hess's Law of constant heat summation, given by Germain Hess in 1840, states that the total enthalpy change of a chemical reaction depends only on the initial and final states of the system, and not on the path or number of steps taken. In other words, ΔH is a state function. So if a reaction occurs in stages, the sum of the individual ΔH values equals the ΔH of the overall reaction. This is a direct consequence of the First Law of Thermodynamics (energy conservation). ΔHoverall = ΔHstep 1 + ΔHstep 2 + … + ΔHstep n
How can an unknown ΔH reaction be determined using Hess's Law?
If the target reaction's ΔH cannot be measured directly (for example, the formation of CO from C and limited O2, which always also forms some CO2), we manipulate other reactions whose ΔH values are known. Reverse a reaction → reverse the sign of ΔH; multiply a reaction by k → multiply ΔH by k. Add the manipulated equations so that intermediate species cancel and the target equation remains. Then add the corresponding ΔH values — that gives the unknown ΔH.
How is Hess's Law applied in calculating enthalpy?
Hess's Law leads to two practical formulae chemists use every day: (a) ΔHrxn = ΣΔHf(products) − ΣΔHf(reactants), using standard enthalpies of formation; and (b) ΔHrxn = ΣΔHc(reactants) − ΣΔHc(products), using enthalpies of combustion (note the sign reversal!). Both are direct consequences of Hess's Law, treating the formation/combustion reactions as alternative paths.
How to calculate heat of formation using Hess's Law?
To find the heat of formation of a compound that cannot be directly synthesised from its elements, write reactions whose ΔH values you know (often combustion enthalpies) and combine them so the net reaction is the formation reaction. Example: ΔHf(CH4) can be found by combining the combustion of C, H2 and CH4: ΔHf(CH4) = ΔHc(C) + 2 ΔHc(H2) − ΔHc(CH4). Plug in numbers — that's the answer.
What is the importance of Hess's Law to thermodynamic calculations?
Hess's Law is invaluable because many ΔH values cannot be measured directly — the reaction may be too slow, too violent, or proceed by multiple parallel paths. Using Hess's Law we can compute these enthalpies indirectly from tabulated data. It also underlies the very idea of standard enthalpies of formation and reaction, on which all of thermochemistry rests. Without Hess's Law, the modern thermochemical database would not exist, and reaction-energy predictions in industry would be impossible.
How can you use Hess's Law to calculate ΔGrxn?
Gibbs free energy ΔG, like ΔH, is a state function — so Hess's Law applies to it too. You can write ΔGrxn = ΣΔGf(products) − ΣΔGf(reactants) using standard Gibbs energies of formation. Or you can compute ΔH and ΔS separately using Hess-type sums and combine them via ΔG = ΔH − TΔS. Both methods are widely used to predict spontaneity (ΔG < 0) and equilibrium constants (ΔG° = −RT ln K). ΔG° = ΔH° − T ΔS° ; ΔG° = − R T ln K