Hess’s Law

Hess’s law states that energy change in the overall chemical reaction is the sum of energy change or enthalpy change in the individual reactions that comprises it.


  • Hess’s Law states that the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps.
  • Enthalpy change is the heat absorbed or given off during a chemical reaction.
  • So, in other words, Hess’s law states that the enthalpy change in a chemical reaction is the same regardless of the path taken.
  • For example, converting reactants A into products B, the enthalpy change is the same whether you take one step or several steps for the reaction.


Hess's law

The above example illustrates that a block of ice takes the same amount of energy to turn to gas regardless of the path taken.

Path 1 sublimation 300 kJ
Path 2 melting + evaporation 44 kJ + 256 kJ = 300 kJ

Example of Hess’s Law Calculation – Methane Formation
Methane formation >>

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