Hess’s Law and Hess Cycles
Hess' Law is a law named after Germain Hess, a Switzerland-born Russian chemist and physician who published it in 1840.
It states that if a reaction can take place by more than one route from the same initial and final conditions then the total enthalpy change is the same for each route.so regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes.
Take care you need to write a definition close to your specification request
see the list of definitions provided for each syllabus
A Hess Cycle is a cycle that can be constructed to find out the Enthalpy change of one reaction based on other reactions of known enthalpies
so 1 = 2 + 3
if you can represent 2 reactions with any 2 of the 3 pathways then the third one can be easily found
algebraically .
This means that ΔH1 = ΔH2 + ΔH3
Application
Hydrogen gas can be generated by the reaction of carbon (coal) and water:
this reaction requires the input of 90.1 kJ of heat for every mole of consumed.
The cycle above shows how the Hess law applies
each pathway has the amount of energy absorbed or released on it
and we can see three reactions in the cycle
No 1 the conversion of carbon and water to carbon dioxide and hydrogen
No 2 the formation of carbon and water from their constituents (hydrogen and oxygen and carbon)
No 3 the formation of carbon dioxide and hydrogen from their constituents(hydrogen and oxygen and carbon)
As per the Hess law and cycle moving in the pathway in reaction 1 is the same as moving through the other
if change the arrows directions so we can construct the cycle ( remember that changing direction requires changing the sign of the enthalpy change)
is the same as going through pathway 2
going from carbon and water to carbon dioxide and hydrogen through pathway 1
is the same as going through multiple steps 2 then 3
this means 1 = 2+3
so ΔH1 = ΔH2 + ΔH3 ( just note that ΔH2 sign has to be reversed )
this means ΔH1 = ΔH3 - ΔH2
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