Hess’ Law
Chapter 11.4
Page 502-605
Hess’ Law
Not all chemical energy changes can be
studied conveniently using simple
calorimetry.
Methods used to study these
reactions are based on the
principle that net changes in all
properties of a system are
independent of the way the
system changes from the initial
state to the final state.
Predicting ΔrH: Hess’ Law
Hess’ law states that the addition of chemical equations
yields a net chemical equation whose enthalpy change is the
sum of the individual enthalpy changes.
ΔrH° = Δ1H° + Δ2H° + Δ3H° + … = ΣΔrH°
Two things to remember:
• If a chemical equation is reversed, then the sign of ΔrH
changes.
• If the coefficients of a chemical equation are altered by
multiplying or dividing by a constant factor, then the ΔrH is
altered by the same factor.
Sample Problem
Use Hess’ law to determine the standard enthalpy change for
the formation of carbon monoxide.
C(s) + ½ O2(g)  CO(g)
∆fH°= ?
Use the following given equations and their standard enthalpy
changes.
Rearrange the equations so that you can obtain
the chemical equation for the formation of
carbon monoxide:
C(s) + O2(g)  CO2(g)
∆1H°= -393.5 kJ
CO2(g)  CO (g) + ½ O2(g)
∆2H°= +283.0 kJ
Now add the reactants, products and standard
enthalpy changes to get a net reaction
reaction:
C(s) + ½ O2(g)  CO(g)
∆fH°= -110.5 kJ
Another Example...
In an experiment to find the standard
molar enthalpy of formation of butane
(from it’s elements), the following values
were determined by calorimetry:
Hint: Make sure to find the net equation first!
Answer...
∆fHm°= -125.6 kJ/mol
Note: You should have only flipped the first
equation and remember, it was asking for molar
enthalpy. Since you were not given any mass,
you use the number of moles in the equation (1
mole)
 Read pgs. 502 – 504
 pgs. 504, 505 Practice #’s 1 – 4
 pgs. 508 – 509 Section 11.4 Questions #’s 1 –
10