Hess's law — Chemistry, 14–17
Calculate an enthalpy change indirectly by combining reactions whose values are known. Chemistry, 14–17 years.
The total energy change
Hess's law says that the enthalpy change depends only on the starting substances and the final substances. It does not matter whether the reaction happens in one step or through several imagined steps. You can therefore add, reverse or multiply equations to find a value that is difficult to measure directly.
Why use an indirect route?
Some reactions are too slow, dangerous or incomplete to measure cleanly in a calorimeter. Burning methane is possible, but measuring how methane forms from its elements is much harder. Hess's law solves this problem by letting measured reactions act like energy building blocks.
Finding methane's formation enthalpy
Use: C + O₂ → CO₂, −393.5 kJ mol⁻¹; H₂ + ½O₂ → H₂O(l), −285.8; and CH₄ + 2O₂ → CO₂ + 2H₂O, −890.3. For the last equation, −890.3 = (−393.5 − 571.6) − ΔHf(CH₄). Rearranging gives ΔHf(CH₄) = −74.8 kJ mol⁻¹.
Signs change with equations
A common mistake is to reverse an equation but keep its enthalpy sign. That feels reasonable because the same chemicals are still present, but reversing the process reverses the energy transfer too. If you multiply an equation by two, multiply its enthalpy by two as well.
Where it is used
Chemists use Hess's law to estimate energy changes for fuels, minerals and industrial reactions that are hard to test directly. It also helps compare fuels: a more negative combustion enthalpy means more heat is released per mole, although mass and safety matter too.
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