Statement of law of constant heat summatlon:
It states that, the heat of a reaction or the enthalpy change in a chemical reaction depends upon initial state of reactants and Final state of products and independent of the path by which the reaction is brought about (l.e. in single step or in series of steps).
\(\mathrm{OR}\)
Heat of reaction is same whether it is carried out in one step or in several steps.
Explanatlon:
Consider the formation of \(\mathrm{CO}_{2 / g)}\)
In one step: \(\mathrm{C}_{i v}+\mathrm{O}_{2 v} \rightarrow \mathrm{CO}_{2 v v} \Delta H=-394 \mathrm{~kJ}\)
In two sleps : Cin \(+\frac{1}{2} \mathrm{O}_{26 w} \rightarrow \mathrm{CO}_{4} \Delta H_{1}=-83 \mathrm{~kJ}\)
\(\mathrm{CO}_{21}+\frac{1}{2} \mathrm{O}_{2(e)} \rightarrow \mathrm{CO}_{2 x)} \Delta \mathrm{H}_{2}=-311 \mathrm{~kJ}\)
\(\therefore \Delta H=\Delta H_{1}+\Delta H_{2}\)
\(-394 \mathrm{~kJ}=-83 \mathrm{~kJ}+(-311) \mathrm{kJ}\)
Hess's law treats thermochemical equations mathematically Le., they can be added, subtracted or multiplied by numerical factors like algebraic equations.
Applicatlons:
Hess's law is used for:
1. To calculate heat of Formation, combustion, neutralisation, ionization, etc.
2. To calculate the heat of reactions which may not take place normally or directly.
3. To calculate heats of extremely slow or fast reactions.
4. To calculate enthalpies of reactants and products.
