Chapter 11.5 MOlar Enthalpies of formation and Hess’ Law Web view05.11.2016 ·...
Transcript of Chapter 11.5 MOlar Enthalpies of formation and Hess’ Law Web view05.11.2016 ·...
THERMO CHEMISTRYCHAPTER 11.5 MOLAR ENTHALPIES OF FORMATION AND HESS’ LAW
When chemists measure energy changes, they must use a starting point. When looking at energy states of molecules, the elemental state is considered to be the point with zero potential energy:the reference energy state. This means all elements (O2, Mg, Ag, S8, etc…) in their elemental forms have an enthalpy of formation of zero kJ/mol.
When a compound is built from elements, the reaction either requires or releases energy which is called the standard enthalpy of formation. ΔfHo. The middle of your data booklet consists of a large table of these values. Combined with Hess’ Law, we can use these to determine the energy produced or absorbed in a reaction. (enthalpy of reaction).
Combustion of Methanol:
Combustion of Propane
C3H8(g) +5O2(g) 4H2O(g) + 3CO2(g)
C3H8(g) = -103.8 kJ/mol
O2(g) = 0 kJ/mol (element)
H2O(g) = -241.8 kJ/mol
CO2(g) = -393.5 kJ/mol
ΔrHo = [products – reactants] = [(4mol x -241.8kJ/mol) – (3mol x -393.5kJ/mol)] - [1mol x -103.8kJ/mol]
ΔrHo = (-2147.7kJ) – (-103.8kJ) = -2043.9kJ (if one mole of propane is combusted)
The molar enthalpies of formation can also be used to describe the thermal stability of a compound (which is its resistance to decomposition when heated).
Rule: the lower the enthalpy of formation (including negatives!!!!), the more stable it is
Ex: What is more stable: water or carbon dioxide?
Water: -285.8kJ/mol
Carbon dioxide: --393.5kJ/mol
Since CO2 is more negative (lower) it is more thermally stable.
Practice: page 514 #1-6