Having got some advertising mileage out of sticking the prefix "bio" onto ethanol, the glossy advertising brochure then adds some more enticing words like "eco", "friendly", "green" and/or "smart".
But the real selling point of the slick advertising is that you don't need a flue. No chimney, no pipes, just set your "bioethanol" fireplace up where ever you want it in the room.
Which is really very, very, interesting (for all you Chemists out there).
Chemistry of Combustion
Ethanol combusts (burns) by combining with oxygen gas in the atmosphere.
A balanced chemical equation for the complete combustion of ethanol is:
C2H5OH + 3O2 → 2CO2 + 3H2O
This tells us that for every 1 mole of ethanol (whether it has the "bio" prefix or not), 2 moles of carbon dioxide gas will be evolved (and yes, that's the same carbon dioxide gas that contributes to the greenhouse effect).
According to the brochure, 5 litres of "bioethanol" would last about 10 hours.
That is, 1 litre of bioethanol keeps your fire going for about 2 hours, just enough to watch a movie.
So how much carbon dioxide will be released into your room while you watch the movie?
First we can use the density of ethanol and the volume of ethanol burnt to calculate the mass of ethanol consumed:
- Density of ethanol is 0.79 g cm3 at 25oC
- If we let 1cm3 = 1 mL
- then the mass of 1 mL of ethanol is 0.79 g
- so the mass of 1 L of ethanol = 1000 x 0.79 g = 790 g
Now we can calculate the moles of ethanol in the 1 L of "bioethanol" we burnt:
- moles = mass/molar mass
- molar mass = 2 x 12 + 6 x 1 + 16 = 46 g/mol
- so moles of ethanol in 1 L = 790/46 = 17.2 mol
Use the balanced chemical equation to calculate the moles of carbon dioxide produced:
- From the balanced chemical equation, 1 mole of ethanol burns to produce 2 moles of carbon dioxide gas.
- Therefore, 17.2 moles of ethanol burns to produce 2 x 17.2 moles of carbon dioxide gas.
- moles of carbon dioxide produced = 34.4 moles
We can then calculate the volume of carbon dioxide released into your room while you watch the movie:
- At 25oC, 1 mole of gas occupies a volume of 24.79 L
- So, 34.4 moles of carbon dioxide occupies a volume of 34.3 x 24.79 = 853 L
853 L of carbon dioxide gas sounds like lot!
But is it really? What proportion of the "air" in your room will be carbon dioxide after 2 hours?
Let's calculate the volume of a room:
- A small room is about 3m x 3m x 3m
- or 300cm x 300cm x 300cm = 27,000,000 cm3 = 27,000 L
- So the volume of air in the room before you start burning your ethanol is 27,000 L(ignoring loss of volume due to you and the furniture )
And now we can see what proportion of the "air" in your room will be carbon dioxide after 2 hours:
- Assuming there is no ventilation in the room (all the doors and windows are closed), burning 1 L of ethanol adds about 853 L carbon dioxide gas to the room, so the total volume of gas is now 27,853 L.
- The percentage of that due to the carbon dioxide we have produced is 853/27853 x 100 = 3%
Or does it?
Did you know that at a concentration of about 1%, carbon dioxide will make you feel sleepy.
At a concentration of about 7%, you can suffocate.
Which makes you think you should probably be opening a window!
Except, wouldn't that defeat the purpose of lighting a fire to keep warm in the first place?
Suggested Study Questions:
- Draw a structural formula for ethanol.
- Explain the difference between complete and incomplete combustion of ethanol.
- How you could you tell by observation whether the ethanol in your "bioethanol" fireplace was undergoing complete combustion or incomplete combustion?
- The glossy brochure states that your "bioethanol" fireplace will not produce soot. What does this tell you about the type of combustion occurring in the fireplace?
- If the ethanol in the "bioethanol" fireplace was undergoing incomplete combustion, would the amount of carbon dioxide produced be more or less than that produced during complete combustion?
- Calculate the moles of carbon dioxide gas that would be produced if you burnt 1 L of "bioethanol" fuel in your fireplace in a room that measured 5m x 7m x 3m
- What volume of the room in question 6?
- At 25oC, how many moles of gas are present in this room from question 6?
- What percentage of gas in this room from question 6 would be carbon dioxide after you burn 1 L of "bioethanol"?
- One of your friends suggests that you should take a "bioethanol" fireplace with you on your camping trip because it will be a perfect way to heat your tent. Do you agree with your friend or not? Explain your answer.