Hess’s Law
Chapter 7
Reaction Mechanism



Complex reactions are actually a
series of simple reactions
Complex reactions can therefore
be broken down into a succession
of simple reactions to calculate
total enthalpy change
To do this the balanced from of
each simple reaction is required
2NO + O2 → 2NO2

1) 2NO → N2O2

2) N2O2 + O2 → 2NO2
Overall:

These intermediary steps are not
directly observed in a reaction as
the product of one intermediary
becomes the reactant for the next

Each intermediary also has a
certain activated complex
associated with it, which must be
considered when determining the
overall enthalpy change
Summation of Enthalpies

Hess’s law (or the law of constant
heat summations) states that if a
reaction can be broken down into
several simple steps, its enthalpy
change is equal to the algebraic
sum of the enthalpies of the simple
reactions
Hess’s Law
ΔH = ΔH1 + ΔH2 + ΔH3 …
Where:
ΔH = enthalpy change of the
overall reaction (kJ/mol)
ΔH1 + ΔH2 + ΔH3 = Enthalpy
change of each simple reaction
(kJ/mol)
Steps for calculation
1)
2)
3)
For each compound involved,
write the balanced chemical
equation
Refer to the chart on pg 418 to
see the appropriate enthalpy for
each simple reaction
Reverse any reactions (and the
value of it’s associated enthalpy)
so that it will be properly aligned
for adding
4) Multiply or divide across any
equations that require so based on
the molar value of the overall
equation
5) Add the simple thermochemical
equations by
a) combining like terms on
each side
b) subtracting equal terms
located on opposite sides
c) adding the enthalpy values
to get your final answer