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Name: Kristian
Status: student
Grade: other
Country: United Kingdom
Date: Winter 2009-2010


Question:
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!



Replies:
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.

Vince Calder


Kristian,

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.

Matt Voss


Kristian,

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)
Canisius College


Kristian,

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.

Best wishes,

Tom Collins



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