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Probably a simple question ........

  • 28-05-2006 2:44pm
    #1
    Panserborn
    Registered Users, Registered Users 2 Posts: 807 ✭✭✭


    Howye, was pondering a question today but couldn't totally satisfy myself with an answer.

    If you enclose a lighted candle in a container that was sealed in open countryside (78% N2, 21% O2, 0.0002% CO2 etc etc etc) and waited until the O2 was used up and the candle went out. What would the gas composition be then made up of?

    All the O2 would presumably be converted to something, would the pressure also remain the same in the tube or would it change?


Comments

  • #2
    Civilian_Target
    Registered Users, Registered Users 2 Posts: 5,618 ✭✭✭Civilian_Target


    Well, yeah, but the pressure inside the tube would get higher because warm air is more excited and therefore tries to expand, increasing the pressure. Thats the same reason the top of a pot of boiling spuds bounces, the air pressure has increased because of the heat in the system.

    I'd imagine the candle will only use all the accesible O2, so you'll probably find it's something along the lines of 78% N, 8% O2, 13% CO2 and other waste, I'm not a chemist so I can't help you there.


  • #3
    Thomas_S_Hunterson
    Closed Accounts Posts: 6,151 ✭✭✭Thomas_S_Hunterson


    The curning candle gives out CO2 and H2O because it's a hydrocarbon (I think the chemical formula for candle wax is generally C18H38)


  • #4
    ApeXaviour
    Registered Users, Registered Users 2 Posts: 5,523 ✭✭✭ApeXaviour


    Probably a bit of carbon monoxide too. As the oxygen runs out the carbon doesn't combust entirely eh?

    The pressure wouldn't change due to the CO2 (single gas molecule to single gas molecule) but it would increase slightly as a result of the gaseous H2O products. Though then it would decrease again to slightly below its initial pressure as a result of the H2O condensing (assuming an ambient temperature < 100C)


  • #5
    Zaphod
    Registered Users, Registered Users 2 Posts: 1,733 ✭✭✭Zaphod


    In an effort to confuse things even further, here's my reckoning:

    The candle will initially burn to give carbon dioxide and water. However, as ApeXaviour mentions, when the amount of oxygen decreases, you will get incomplete combustion with carbon/water or carbon monoxide/water being formed.
    http://www.wpbschoolhouse.btinternet.co.uk/page04/OilProducts.htm

    The pressure will depend on a number of things:
    The number of moles of gas in the system before and after combustion.
    Is the water formed a vapour or liquid?
    The heat of the system.

    P = nRT/V

    For ease of calculation, assume a perfect system where full combustion takes place:
    C18H38 (s) + 27.5O2 (g) -> 18CO2 (g) + 19H2O (l)
    The heat from a candle probably won't be enough for maintain the water as a vapour i.e. it will quickly condense.

    27.5 moles vs. 18 moles => pressure falls. The pressure from the increase in heat will be negligible in comparison to decrease in number of moles.


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