Hess’s Law:
The algebraic addition of chemical
equations yields a net equation
whose enthalpy of reaction is the
algebraic sum of the individual
enthalpies of reaction.
Hnet =  Hr
1.
2.
If the reaction equation is
reversed, the sign of the Hr also
reverses.
If the coefficients of a reaction
equation are changed by
multiplying or dividing, then the
value of Hr is changed in the
same way.
1.
2.
3.
4.
5.
Write the balanced chemical equation.
Manipulate the given equations so
they will add to the net equation.
Cancel and Add the remaining
reactants and products.
Add the component enthalpy changes
to obtain the net enthalpy.
Determine the molar enthalpy of
reaction if required.
Practice
a) Using the reactions given below, determine
the heat of reaction for the equation:
2U(s) + 3O2(g)  2UO3(s)
1) 2UO3(s)  2UO2(s) + O2(g) Hr = 260 kJ
2) U(s) + O2(g)  UO2(g)
Hr = -1130 kJ
b) Determine the molar heat of reaction



elements are set as the reference energy
state where the potential energy is 0.
formation reactions can be endothermic
(+H) or exothermic (-H) (p 4-5 data book)
simple decomposition is the reverse of a
formation reaction so reverse the sign of
H


thermal stability: ability to resist
decomposition
the more exothermic a formation reaction,
the more endothermic the decomposition=
greater thermal stability
Compound
Molar Enthalpy of
Formation kJ/mol
aluminum oxide
-1675.7
copper (I) oxide
-168.6
nitrogen monoxide
+91.3
Most stable: aluminum oxide
Least stable: nitrogen monoxide


To determine the standard enthalpy of
a reaction, the sum of the enthalpies
of formation of the reactants is
subtracted from the sum of the
enthalpies of formation of the
products.
Mathematically, this can be written as:
Hnet = nHf(prod) - nHf(react))

watch state of the compound as this
affects the molar enthalpy of formation
◦ Ex) H20(l) Ho = -285.8 kJ/mol
H20(g) Ho = -241.8 kJ/mol
-combustion in bomb calorimeter
produces H20(l)
Examples:
1. Show the decomposition and formation
reactions, with their respective enthalpies for
the combustion of propane.
2. Determine the heat of reaction for the
reaction of sulfuric acid with magnesium
chloride.
3. Determine the enthalpy of reaction for the
combustion of 100 g of sucrose.
4. What mass of nitrogen monoxide must react to
release 1.00 MJ of energy for the following
reaction:
2NO(g) + O2(g)  2NO2(g)
5. The molar enthalpy of combustion of octane is
-5047.1kJ/mol. Determine the molar enthalpy
of formation of octane.