Hess Law



Hess 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’ Law can be difficult to understand because of all the big words like enthalpy change, heat summation, state function, etc. Here’s a short video clip to help understand the basics of Hess’ Law before diving into the deep!

antEnergy change in a system is also called enthalpy change. It is the heat absorbed or given off during a chemical reaction.

Find out more about enthalpy change enthalpy change


Hess’ 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.

So, in other words, Hess law states that the enthalpy change in a chemical reaction is the same overall whichever route you take. If you want to convert reactants A into products B, the enthalpy change is the same whether you take one step or several steps to achieve that.

Example of Hess Law Calculation – Methane Formation

methane formation

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