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Bond enthalpies

Energy required to break 1 mole of a specified type of bond in a gaseous molecule
Endothermic, +ve value


Limitations of bond enthalpies

Actual bond enthlapy depends on chemical environment of bond
Average bond enthalpy calculated from actual bond enthalpies, no 1 true value
Not all enthalpy changes occur under standard conditions


Calculating enthalpy change from average bond enthalpy

Total bond enthalpy of reactants-total bond enthalpy of products=enthalpy change


Hess’ law

If a reaction can take place by 2 routes and the starting and finishing conditions are the same, total enthalpy change is the same


Hess cycle for the enthalpy change for delta f H

Reactants =>Products
^. ^


Hess cycle for the enthalpy change of delta c H

Reactant => Products
Combustion products