HESS’s LAW or HEAT SUMMATION:
This law is given by G.N Hess in 1864 in which he explained the characteristics of heat energy absorbed or released during any chemical process.
According to Hess’s law:
“The amount of heat energy absorbed or released or enthalpy during any chemical process is independent of number of steps involved in it.”
“If a chemical process is completed in a single step or in several steps then the total energy absorbed or released is found equal”.
Consider the formation of “D” by using s “A” using following two methods,
Reactant A is directly converted into D in a single state with the change in enthalpy of ∆H
A —————–> D ∆H
In another method the same process is completed in three steps with following enthalpies.
A ————> B ∆H1
B ————> C ∆H2
C ————> D ∆H3
In method 1 reaction is completed in one step while in method no.2 the process involving 3 steps. Now according to Hess‘s law of heat summation.
∆H = H1 + H2 + H3
∆H = H1 + H2 + ……. Hn
APPLICATION OF HESS‘S LAW
HEAT OF FORMATION
The amount of heat energy absorbed or released when one mole of a compound is prepared by its own elements is called “Heat of formation” of that compound. E.g.
C + O2 —–> CO2 ∆H = -394 KJ/mol
H2 + 1/2O2 —–> H2O ∆H = -286 KJ/mol
HEAT OF CONISBUSTION
The amount of heat energy released when one mole of any substance is combusted in the presence of oxygen to form an oxide is called “Heat of Combustion”, e.g.
CH4 + 2O2 —–>CO2 ∆H = -890.24 KJ/mol
CHCOOH + 202 —–> 2CO2 + 2H2O ∆H = -870.8 KJ/mol