π₯ Hess's Law Calculator
Combine step enthalpies to get ΞHrxn.
How to Use This Calculator
Enter each step's ΞH and how you use it: as written, reversed, or scaled. The calculator applies the sign and multiplier rules and sums everything.
Hess's Law Rules
Scale by n: multiply ΞH by n
ΞHrxn = sum of adjusted step enthalpies
Because enthalpy depends only on start and end states, any path between them sums to the same total. Arrange the given steps so intermediates cancel, leaving only your target equation.
Worked Example
Find ΞH for C + O2 to CO2 using: C + O2 to CO2 is given directly as -393.5 kJ in one step, so ΞHrxn = -393.5 kJ. With two steps (C to CO at -110.5, CO to CO2 at -283.0), sum = -393.5 kJ. Same answer either path.
Why Hess's Law Works
Enthalpy is a state function: it depends only on where you start and end, not the path between. This is like elevation gain on a hike; whether you take the direct trail or the scenic route, the total climb from trailhead to summit is identical. Hess realized chemists could exploit this by combining known reaction steps like trail segments to find the enthalpy of reactions too dangerous or slow to measure directly.
Standard Enthalpies of Formation
The most powerful application uses formation reactions, where each compound forms from its elements. Tabulated ΞHfΒ° values let you compute any reaction's enthalpy without doing the experiment: products minus reactants. This is how engineers estimate the heat of combustion for new fuels before ever lighting them.
Born-Haber Cycles
The Born-Haber cycle is Hess's law applied to ionic compounds: it breaks salt formation into atomization, ionization, electron affinity, and lattice energy steps. Since lattice energy cannot be measured directly, the cycle calculates it from the other measurable steps. This is how chemists know NaCl's lattice energy is 787 kJ/mol.
Frequently Asked Questions
What is Hess's law?
The total enthalpy change of a reaction is the same regardless of the path taken. If you can add reaction steps to get your target equation, you add their ΞH values the same way. Enthalpy is a state function.
What happens to ΞH when I reverse a reaction?
The sign flips. If A to B is -100 kJ, then B to A is +100 kJ. Reversing means going uphill instead of downhill by the same amount.
What happens when I multiply an equation by 2?
Multiply ΞH by 2 as well. Enthalpy is extensive: twice the material, twice the heat. This is the most common Hess's law mistake.
Can I mix formation enthalpies with Hess's law?
Yes, that is exactly what the standard formula does: ΞHrxn = sum of formation enthalpies of products minus reactants. It is Hess's law applied to formation reactions.
Why do some Hess's law problems need three steps?
Because the target equation's species appear across multiple given reactions. You scale and flip each step until every species cancels except the target ones, tracking ΞH through each operation.