Combustion Reaction
Heat of Reaction (q)
C6H14(l) + O2 (g) 
Energy
required to
break bonds
CO2 (g) + H2O (g)
Energy
released as
bonds form
Heat of Reaction (q)
• Reactants use energy to break their bonds
– C-H (413 kJ per bond in hexane)
– O=O (498 kJ per bond in oxygen gas)
• Products release energy as bonds reform
–
–
C=O (803 kJ per bond in carbon dioxide)
O-H (463 kJ per bond in water)
• Once the accounting is done, there will be a net
difference in energy (q).
• Combustion reactions always produce MORE
energy than they use up  EXOTHERMIC
Measuring Heat of Combustion (q)
• Calorimetric experiments are performed to
indirectly measure the amount of heat
released by a fuel
• A known quantity of water will be heated by
the combustion of a fuel
• It is assumed:
Heat absorbed = heat released by
by the water
combustion of fuel
q = mCΔT
• q is the amount of heat energy (in Joules)
absorbed by the water.
• Therefore, all values (m, C, T) are related to water
m=mass of water (grams)
C=heat capacity of water (constant: 4.18 J/Kg)
ΔT=temperature rise of water (Tf–TI)
*q is always be (+) for combustion reactions
Calorimetry
• A known quantity of water must be
measured and weighed
• The exact amount of fuel used to
heat the water must be determined.
This is done by accurately weighing
the mass of the spirit burner before
and after the water is heated. Keep
the cover on the burner to prevent
any loss in mass by evaporation.
• Measure the starting and final
temperature of the water.
• Calculate q (heat of reaction).
Calculate q
Eg.
m = mass of water = 150.0 g
C = heat capacity of water = 4.18 JK-1g-1
ΔT = temperature rise = 18.0o
q = 150.0 x 4.18 x 18.0
= 11,286 Joules
= 11.3 KJ (heat absorbed by water)
ΔH (enthalpy) and q (heat)
q = heat energy absorbed by water
ΔH = heat energy released by the reaction
So, ΔH = -q (opposite signs)
If q = 11.3kJ, then ΔH = -11.3 kJ
*ΔH will always be (-) for an exothermic reaction
Calculate ΔH per gram
q = 11.3 kJ
Recall that the
spirit burner
used 1.5 grams
of fuel
Recall that ΔH= -q
Therefore, ΔH = -11.3 kJ
ΔH= -11.3 kJ = - 7.6 kJ of energy released per
1.5 grams
gram of hexane burned
Calculate ΔH per mole
• Molar heat of combustion tells us how much
the water was heated by for every mole of
hexane used.
• The spirit burner needs to be weighed
before/after to determine the amount used
(grams)
• mass will need to be converted into moles
ΔH=q/mol
Calculate molar heat of
combustion
ΔH = -11.34 kJ
mol = 1.5 g x
Recall that
hexane has a
molar mass of
86 g/mol
1 mol = 0.0174 mol
86.0 g
ΔH = q = -11.34 kJ = - 650 kJ/mol
mol
0.0174 mol
Comparing ΔH values
• q values will vastly differ as it depends on the
amount of water being heated and the
temperature rise of the water; q values cannot
be compared or published
• Molar heat of combustion values (ΔH) are
widely known and published for every type of
fuel; ΔH can be compared