9.4 Hess' law and enthalpy cycles
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What does Hess’ law allow
Allows enthalpy changes to be determined indirectly
What is Hess’ law
States that if a reaction can take place by two routes and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
Where does Hess’ law come from
The idea of the conservation of energy Can easily be visualised with a diagram
In what situations can the principle of Hess’ law be used
Any number of enthalpy changes provided that all enthalpy changes are known except for one, the unknown enthalpy change can always be determined
What is the ΔfHΦ value for elements
ZERO
Rules to use to calculate using enthalpy changes of formation and combustion: • Using enthalpy changes of formation ΔfH • Using enthalpy changes of combustion ΔcH
• ΔrH = ΣΔfH products - ΣΔfH reactants 2. ΔrH = ΣΔcH reactants - ΣΔcH products