State and explain Hess's law of constant heat summation.

Explanation

Statement of law of constant heat summation : 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 (i.e, in single step or in series of steps).
Explanation :
Consider the conversion of the reatant A into the final product C. Let `Delta_(r)H^(@)` be the enthalpy change for the direct conversion.
`A rarr CDelta_(r)H^(0)`
The same conversion can be carried out in two steps as,
Enthalpy change of Reaction
`{:(ArarrB,Delta_(r)H_(1)^(0)),(BrarrC,Delta_(r)H_(2)^(0)):}`
They by Hess's law, `Delta_(r)H^(0) = Delta_(r)H_(1)^(0) + Delta_(r)H_(2)^(theta)`.
Illustration : Consider the formation of `CO_(2^((g)))`.
In one step : `C_((g)) + O_(2(g)) rarr CO_(2(g)) , DeltaH = - 394 kJ`
In two steps : `C_((s)) + 1/2 O_(2(g)) rarr CO_((g)) , DeltaH_(1) = -83 kJ`
`CO_((g)) + 1/2O_(2(g)) rarr CO_(2(g)) , DeltaH_(2) = - 311 kJ`
`:. DeltaH =DeltaH_(1) + DeltaH_(2)`
`- 394 kJ = - 83 kJ + (-311) kJ`
Application : (1) To calculate heat of formation , combustion , neutralisation , etc.
(2) To calculate heats of extremely slow reactions.