hess

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

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.

Endothermic energy level diagram

In an endothermic reaction the products have more energy than the reactants.

∆H is positive for an endothermic reaction.

Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.

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

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?