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The science of alcohol
The science of alcohol

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2.2.1 Burning of ethanol

When you burn – or combust – ethanol (C2H5OH) in air (which contains oxygen, O2), the final products obtained are carbon dioxide gas (CO2) and water (H2O). Looking at this reaction is important as it produces the same products as does the metabolism of ethanol in the liver, which you will look at in Week 5 of this course.

Now try to produce a balanced equation for the combustion of ethanol:

  • Start by writing out the reactants and the products.

  • The reactants and products are:

    cap c sub two times cap h sub five times cap o times cap h times prefix plus of cap o sub two right arrow cap c times cap o sub two plus cap h sub two times cap o

  • Now compare the number of carbon atoms in the reactants and products, and start to balance the equation.

  • The two carbons of ethanol must make two molecules of carbon dioxide:

    cap c sub two times cap h sub five times cap o times cap h times prefix plus of cap o sub two right arrow two times cap c times cap o sub two plus cap h sub two times cap o

  • Now balance the hydrogens in the equation.

  • The six hydrogens of ethanol must produce three molecules of water (as every molecule of water contains two hydrogens):

    cap c sub two times cap h sub five times cap o times cap h times prefix plus of cap o sub two right arrow two times cap c times cap o sub two plus three times cap h sub two times cap o

  • Finally, consider the oxygens. What do you have to do to balance them in the equation?

  • There are now seven oxygens on the right-hand side and so three molecules of oxygen are needed on the left – you now have a fully balanced equation:

    equation left hand side cap c sub two times cap h sub five times cap o times cap h times prefix plus of three times cap o sub two equals right hand side two times cap c times cap o sub two plus three times cap h sub two times cap o

Notice that if you chose to start with oxygen then the oxygens already balance; but it is important that you still go on to balance each of the other elements until all are balanced. There are no fixed rules for which atom to balance first, and sometimes a trial and error approach is needed. It is important to remember that, since atoms cannot be created or destroyed, an equation, if it shows a feasible process, can always be made to balance. An arrow or an equals (=) sign can be used to denote a completely balanced reaction. You will see either used, although the most common convention is to use an equals sign when the equation is completely balanced, and arrows in the previous steps of ‘working out’. In the following video, Louise MacBrayne demonstrates the combustion of ethanol. Despite burning ‘slow’ a great deal of heat is generated or, put another way, the reaction releases a lot of energy. In fact there is so much energy released that an ethanol rocket can be manufactured using an empty plastic soda bottle as illustrated in this video.

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Transcript

LOUISE MACBRAYNE
When you drink an alcoholic drink, your body metabolises the ethanol in that drink via some enzymes. We can recreate that metabolism of ethanol in the lab essentially by burning the ethanol into carbon dioxide and water. But in contrast to what happens inside your body, when we burn ethanol in the lab, we can produce a huge amount of energy just in one step. In fact, we can produce so much energy that we can use it to fire an ethanol rocket. So to demonstrate to you an ethanol rocket, we can take an empty carbonated drinks bottle. But into the bottom of the bottle we've drilled a hole of about five millimetres to act as an exhaust. We can then add our rocket fuel, which is just going to be liquid ethanol, which we add into our bottle. And then ensuring the hole is covered and the lid is on, we can shake the bottle to get an even coating of ethanol inside the bottle. And then, very importantly, the fuel for a rocket is actually ethanol vapour. So we're going to remove the liquid. And we're going to give the bottle a good shake to get as much of the liquid out as possible. And now we have our rocket loaded with our fuel, which is ethanol vapour mixed with oxygen from the air. So we can now go and ignite our rocket. We're now ready to fire our rocket from our ignition ramp here that we've made a piece of guttering. And I'm going to stand well back to light it.

[CRASHING SOUNDS]

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Burning ethanol in air happens in one fast high-temperature reaction as seen here. In contrast the liver does the same overall process in three low-temperature stages. This will be explored in Week 5, discussing how much energy is generated and comparing the result with other foods and fuels.