Chemistry 0983 Learning Centre
Hess’s Law of Heat Summation
Finding ΔH usually requires either experimentation or using the values for ΔH for related
reactions. Your chemistry textbook has a list of standard heats of formation (ΔH°f) for
many substances. These values of ΔH describe the enthalpy change for the reaction in
which a particular compound is formed from its component elements under standard
conditions. The elements are always in their most stable natural form, and “standard
conditions” means:
• a standard temperature (usually 25° C)
• atmospheric pressure of 1 atm (or 1 bar)
• 1.000 M concentration of any solutions
A ΔH measured under standard conditions is designated ΔH° (“delta H naught”). The “°”
symbol tells you that the value was found under standard conditions. The symbol ΔH° is
used with any reaction, while ΔH°f is used specifically for one mole of compound formed
from the reaction of elements in their standard states.
We can find ΔH° for any reaction using a table of standard heats of formation and
Hess’s Law of Heat Summation, which says that the heat absorbed or evolved in a
reaction is fixed, and independent of the number of steps of a reaction; the reaction can
take place in one step or multiple steps. This means we can “build” a reaction from
simpler steps.
ΔH overall reaction (net) = ΔH step 1 + ΔH step 2 + ΔH step 3 …
Example 1: Find ΔH° for the combustion of propane gas.
Solution: First we need the balanced equation:
C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (g)
We will need to calculate the ΔH° for the reaction from the standard heats of formation
for all the species involved (both products and reactants), which we look up in the table
in the textbook:
3 C (graphite) + 4 H2 (g) → C3H8 (g) ΔH°f = −105 kJ∕mol
O2 (g) → O2 (g) ΔH°f = 0
C (graphite) + O2 (g) → CO2 (g) ΔH°f = −393.5 kJ∕mol
H2 (g) + ½ O2 (g) → H2O (g) ΔH°f = −241.8 kJ∕mol
[Notice that for CO2 there was also a value for the aqueous form, and for H2O there was
a value for liquid form. The state of matter counts! The ΔH°f for oxygen gas is zero
because that is the natural elemental form of oxygen.]
We can think of using Hess’s Law like this: if we wanted to, we could take propane and
dissociate it into its constituent elements (which would mean reversing the formation
reaction for propane listed above). Then we could react the elemental hydrogen and
carbon with oxygen to create water and carbon dioxide. It would be a grossly inefficient
Authored by Gordon Wong & Darren Rigby
This work is licensed under a Creative Commons Attribution 4.0 International License
Hess’s Law of Heat Summation
Finding ΔH usually requires either experimentation or using the values for ΔH for related
reactions. Your chemistry textbook has a list of standard heats of formation (ΔH°f) for
many substances. These values of ΔH describe the enthalpy change for the reaction in
which a particular compound is formed from its component elements under standard
conditions. The elements are always in their most stable natural form, and “standard
conditions” means:
• a standard temperature (usually 25° C)
• atmospheric pressure of 1 atm (or 1 bar)
• 1.000 M concentration of any solutions
A ΔH measured under standard conditions is designated ΔH° (“delta H naught”). The “°”
symbol tells you that the value was found under standard conditions. The symbol ΔH° is
used with any reaction, while ΔH°f is used specifically for one mole of compound formed
from the reaction of elements in their standard states.
We can find ΔH° for any reaction using a table of standard heats of formation and
Hess’s Law of Heat Summation, which says that the heat absorbed or evolved in a
reaction is fixed, and independent of the number of steps of a reaction; the reaction can
take place in one step or multiple steps. This means we can “build” a reaction from
simpler steps.
ΔH overall reaction (net) = ΔH step 1 + ΔH step 2 + ΔH step 3 …
Example 1: Find ΔH° for the combustion of propane gas.
Solution: First we need the balanced equation:
C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (g)
We will need to calculate the ΔH° for the reaction from the standard heats of formation
for all the species involved (both products and reactants), which we look up in the table
in the textbook:
3 C (graphite) + 4 H2 (g) → C3H8 (g) ΔH°f = −105 kJ∕mol
O2 (g) → O2 (g) ΔH°f = 0
C (graphite) + O2 (g) → CO2 (g) ΔH°f = −393.5 kJ∕mol
H2 (g) + ½ O2 (g) → H2O (g) ΔH°f = −241.8 kJ∕mol
[Notice that for CO2 there was also a value for the aqueous form, and for H2O there was
a value for liquid form. The state of matter counts! The ΔH°f for oxygen gas is zero
because that is the natural elemental form of oxygen.]
We can think of using Hess’s Law like this: if we wanted to, we could take propane and
dissociate it into its constituent elements (which would mean reversing the formation
reaction for propane listed above). Then we could react the elemental hydrogen and
carbon with oxygen to create water and carbon dioxide. It would be a grossly inefficient
Authored by Gordon Wong & Darren Rigby
This work is licensed under a Creative Commons Attribution 4.0 International License
