How can I burn without alcohol
Prof. Blume's tip of the month June 2007 (tip no.120)
The following experiment is one of the classics of the fairground games and is nowadays above all a welcome chemical showpiece. This was also the aim of a student's request. This is material for a tip of the month!
First of all, we need a 1: 1 mixture of ethanol with water
We remember the well-known introductory experiment in which 50 ml + 50 ml = 96 ml comes out. Please note: the mixture gets really warm! Either we combine the preparation of the miracle experiment with the initial attempt, or with the miracle experiment we dispose of the alcohol-water mixtures that arise during the initial attempt in a funny and at the same time spectacular way.
Now the experiment, which is about today:
Experiment 1: The burning handkerchief
The very brave even try this with one Bank note in front. However, this smooth paper does not absorb enough liquid; therefore the experiment is not particularly spectacular.
|(Photo: Daggi) |
There is a film about this (1.5 MB)
Here is the reaction equation for burning the alcohol:
C.2H5OH + 3 O2 ———> 2 CO2 + 3 H.2O
We could leave it at that. But many people want to know why and why the handkerchief or the banknote were not burned with them.
1: 1 mixtures of alcohol and water can be ignited without further heating
This is because at these concentrations some of the alcohol evaporates.
Experiment 2: Burning water-alcohol mixtures
Result: We find that the mixture starts to burn from around 45% by volume of alcohol. Every cook who has to use a higher percentage to flambé dishes knows this too.
Sometimes the paper burns, sometimes it doesn't. What's happening?
Paper is usually made of cellulose. And it burns when you light it with the help of an alcohol flame. But why doesn't this work in our miracle experiment 1?
If you put a 1: 1 alcohol-water mixture on the paper or on the handkerchief, this mainly absorbs the water. The alcohol evaporates particularly well due to the large surface. When lighting, only the alcohol burns and evaporates faster and faster due to the heat of combustion, which accelerates its burning off.
The cellulose does not start to burn because its ignition temperature is not reached under the conditions of the experiment. The most important reason for this is the moisture in the cellulose. The evaporation of the adsorbed water requires a surprising amount of energy, so that the paper stays moist longer than the alcohol needs to burn.
The exact value of the spontaneous ignition temperature of dry paper can be found in the title of a well-known book by Ray Bradbury (or the film from Fran ois Truffaut): Fahrenheit 451. This temperature specification, which comes from the American novel and is inconvenient for us, has to be converted to degrees Celsius that are more common in us using the following relationship:
T (Celsius) = (T (Fahrenheit) - 32) 5/9
T (Celsius) = (451 - 32) 5/9 = 233 C
That should be the spontaneous ignition temperature of paper. After all, the novel or the film is about book burns.
Bradbury is very wrong: Namely, he confuses degrees Fahrenheit with degrees Celsius. The usual paper (viscose paper) ignites spontaneously at 451 ° Celsius. That would be 843 Fahrenheit.
What is the reason for the high ignition temperature of dry paper?
To ignite, all substances must first be converted into a flammable gas that can mix with the oxygen in the air. This not only applies to gasoline or candle wax, but also to cellulose. But it doesn't evaporate; it only decomposes, producing flammable gases. To this Pyrolysis a lot of energy is required. This can also be demonstrated using wood distillation.
Far more energy is required than the brief flare-up of alcohol in our experiment can provide. In addition, the energy has to act longer than in this experiment. You notice this when you want to ignite dry paper - with a match, for example: you have to hold the flame up to the paper a little longer before it even catches fire.
R diger flower
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