Alcohol and Flammable Water
Country: United Kingdom
Date: Winter 2009-2010
My question is about flammability: alcohols are flammable
ethanol, butanol, etc. and as the carbon chain increases so to does
the energy (heat) required to cause ignition so this would lead me
to think that its ability to combust lies in its OH functional
group. Assuming that my assumption is correct... why is water NOT
flammable? I would be extremely grateful if your answer could be
detailed and as complex as needed. Thank you so much!
In order to have a fire you need fuel (various alcohols in your case, you
need (in air/oxygen) and you need an ignition source. Let us ignore that
there are flammability minima and maxima of the fuel / oxygen ratios
necessary. Be aware that measuring flammability is a tricky business (to use
the technical jargon).
When faced with a proposition that does not seem to work, look at the
assumptions. Maybe there is a hidden variable. Here, the hidden variable
could be the vapor pressure (volatility) of the fuel. In your words:
"Assuming that my assumption is correct."... begs the question, that is,
"Suppose your assumption isn't correct? Maybe the flammability is NOT just
its ability to combust lies in its OH functional group."
Couldn't an alternative explanation be the volatility of the particular
fuel? Have and how would you eliminate that possibility?
A very good question, but you open a box of options that would be a major
project to disentangle.
I am glad that you are trying to create theories of your own. The
next step is to test your hypothesis with the knowledge that you
have or try to perform (safe) experiments. You can also try to
research flammability and see what you can find. You know that
water is not flammable, so the basis of your theory must have some
error or incomplete explanation.
The process of burning is called combustion. Combustion is a
chemical process that takes hydrocarbons and converts them to carbon
dioxide, water, heat and light. This occurs through a process
called oxidation. Believe it or not, pure oxygen is not
flammable! While oxygen is required for combustion, there has to be
some other source of fuel (gasoline, propane, acetylene, wood etc)
for the oxygen to oxidize. When this occurs much heat and light are
emitted, which you can see and feel. The carbon dioxide and water
you cannot see because they are given off as gases.
What I have said above accounts for cases where there is pure
combustion. The longer the carbon chain, the harder it is to
achieve 100% combustion. When this occurs, other by-products form
such as carbon monoxide and soot. If the fuel contains higher
levels of sulfur then sulfuric acid is generated and this can result
in acid rain.
While I will not go into mechanistic detail in this account, the
actual combustion process occurs when oxygen reacts with a
carbon-carbon bond. The hydroxy (OH) group is not involved in the
combustion process. If this were the only way that combustion could
happen, then pure hydrocarbons like gasoline, propane, kerosine etc
would not be able to burn. None of those fuels have hydroxy groups.
There are two main things that control ignition: the ease in which
some bonds in the compound react with the oxidant (in this case
oxygen gas), and how much of the liquid is in vapor phase in order
to quickly react with the oxidant.
In this case, what is being reacted are the C-C, C-H, C-O and O-H
bonds. While there are some variations to the energy required to
initiate the reaction with O2 of these bonds depending on the length
of the molecule, this is not significant in in this case since the
only effect is the length of the molecules and the number of C-C and
C-H bonds present.
What is significant is the difference in vapor pressure of these
compounds at a particular temperature. The longer molecules which
have a larger degree of London Forces will necessarily require more
energy to get them into vapor phase. Since ignition occurs in the
vapor phase, then the temperature at which there are sufficient
molecules in the vapor phase will control when exactly the system
will ignite. This should explain why, as the molecules get larger,
the ignition temperature gets higher.
Water is not directly related to this series. Water does not have
C-C or C-H bonds. You can imagine then that ignition of hydrocarbons
involves primarily the reaction of C-C, C-H with O2, and this can
occur at lower initiation energies than the reaction of O-H bonds with O2.
Greg (Roberto Gregorius)
Interesting question! There are lots of bits entwined in this problem.
When one of these alcohols is burned in oxygen the products of
complete combustion are H2O and CO2. The energy that is released
comes from the bonds forming in products (minus the bonds that must
be broken in reactant alcohol and the O2 molecules). There is also
some energy change from the intermolecular forces that must be
broken or made in the reactants or products. The reason they burn at
all is due to this release in energy (the reaction is exothermic)
and there will also be a role from the entropy change during the
combustion but it is the heat energy (enthalpy) that drives this process.
Longer alcohols do, indeed, need more heat to cause ignition but
this is primarily due to all the C-C bonds that need to be broken
(this is a strong bond) and the stronger intermolecular forces that
must be overcome in order for the molecules to react. These two
factors imply a larger activation energy which is the barrier that
must be overcome for the molecules to react. Interestingly the
strong C-C bonds also make the flames of the longer alcohols
"dirtier" at a certain temperature and reactant concentrations, i.e.
they contain more uncombusted carbon (soot) and turn objects they
are heating black.
Water itself does not burn in oxygen. This is mainly due to the
possible products (eg H2O2) being higher in enthalpy so energy would
have to be put it in for them to form. The only way more O can be
incorporated into the molecule is to string oxygens together and the
O-O bond is very weak so this is unlikely to happen, especially as
the O=O in O2 must be broken: the numbers don't add up and it always
takes more energy to add more oxygens to the water molecule from O2.
The OH group is actually the only part of the alcohol molecule that
is unchanged during the reaction as it ends up in one of the water
molecules after the reaction.
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Update: June 2012