The Hess's law allows us to determine the enthalpy change of a reaction because enthalpy is a state function. It does not depend on the individual path take in going from reactants to products in the reaction.
Enthalpy changes are the heat changes accompanying physical and chemical changes.
It is the difference between the heat content of product in the final state and the reactants.
Enthalpy changes for some reactions are not easily measurable experimentally.
To calculate such heat changes, we apply the Hess's law of heat summation.
The law states that "the heat change of a reaction is the same whether it occurs in a step or several steps".
The Hess's law is simply based on the first law of thermodynamics by which we know that energy is conserved in every system.
<span> You can do it on the icing of roads, reverse osmosis for desalination of water, dissolved CO2 in soda cans, osmotic pressure involving blood vessels and IV solutions, etc.</span>
