Enthalpy Changes of Combustion of Different Alcohols
Aim:
The aim of this experiment is to find out how the enthalpy change
(total energy released when the alcohols are completely combusted in a
plentiful supply of air) for 5 different alcohols is affected by the
number of carbon atoms in the alcohol and other factors contributing
to the molecular structure.
Prediction:
I predict that as the amount of carbon atoms in the alcohol increases,
the higher the enthalpy of combustion will be. I have made this
prediction, using the values for the enthalpy change of combustion for
each alcohol, calculated using bond enthalpies and Hess' law.
Methanol's molecular formula is CH3OH. This is the basic structure for
all the alcohols, then to make the larger ones an extra carbon is
added to the existing carbon each time and the oxygen-hydrogen
molecule gets added to the atoms added to the new carbon atom
When methanol combusts in air, it reacts with oxygen molecules to from
water and carbon dioxide. The balanced equation fort this is:
[IMAGE] CH3OH (l) + 1.5O2 (g) CO2 (g) + 2H2O (l)
This means that the bonds broken are; 3 carbon- hydrogen, 1
carbon-oxygen, 1 oxygen-hydrogen and 1.5 oxygen- oxygen (double bond)
and the bonds broken are; 2 carbon- oxygen (double bond) and 4
oxygen-hydrogen.
Constructing a Hess' law cycle will show how these are linked
together:
[IMAGE] CH3OH (l) + 1.5O2 (g) CO2 (g) + 2H2O (l)
[IMAGE][IMAGE]
C (g) + 4H (g) + 4O (g)
N.B. bond enthalpies are for elements in their gaseous states
If a calculation for the amount of energy needed to break the bonds is
made and then the amount of energy given out from bond formation, the
resultant energy difference (negative because the reaction is
exothermic) is the enthalpy change of combustion.
Average bond enthalpies for elements in their gaseous states
(kJmol-1):
Carbon - Carbon (C-C) = +347
Carbon - Hydrogen (C-H) = +413
Oxygen - Hydrogen (O-H) = +464
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from both sides, leaving us with ½ V2 = GH. When the above equation is
Text Box: CaCO3 + HCl = CaCl2 + CO2 + H2O calcium carbonate + hydrochloric acid = calcium chloride + carbon dioxide + water
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CaCl2 + H2O + CO2
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