hess
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MODULE 2AS: Energetics ITo understand the principles behind Hess’ LawTo be able to define the enthalpy of formation and enthalpy of combustionTo use the definitions to calculate a standard enthalpy change
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Exothermic energy level diagram ∆H (‘delta H’) is the symbol
for the ‘change in energy’. In an exothermic reaction
the products have less energy than the reactants.
∆H is negative for an exothermic reaction.
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Endothermic energy level diagram
In an endothermic reaction the products have more energy than the reactants.
∆H is positive for an endothermic reaction.
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Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.
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Definitions Standard enthalpy change of formation is the enthalpy
change for the formation of one mole of a compound from its elements under standard conditions
Standard enthalpy change of combustion is the enthalpy change for the complete combustion of one mole of a substance under standard conditions.
Standard enthalpy change of neutralisation is the enthalpy change when 1 mole of H+ ions is just neutralised by 1M alkali under standard conditions
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Hess’ Law
The enthalpy of combustion of pentane is -3520 kJ/mol. The enthalpy of formation of
carbon dioxide is -395kJ/mol and that of water is -286kJ/mol.
What is the enthalpy of formation of pentane?