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Hydrogen power maths

  1. Oct 11, 2008 #1
    hi.
    i think that hydrogen economy is possible. see this maths


    ther is gonna be a cylinder with a volume of exactly 1 litre. hydrogen and oxygen are introduced in the volume ratio of 2:1.

    volume of cylinder = 1000cm3
    volume of hydrogen = 666.6666......cm3
    volume of oxygen = 333.3333...cm3
    mass of hydrogen = 0.08988(density in g/L) * 0.666(mass in grams)
    = 0.0598g
    mass of oxygen = 1.429(density in g/L) * 0.333(mass in grams)
    = 0.475g
    energy liberated = 0.0598(mass) * 142800(calorific value converted into joules)
    = 8359.44J = 8.35944kJ
    mass of water vapour generated after the combustion will be equal to the mass of oxygen + mass of oxygen.
    mass of water vapour = 0.5348g = 0.0005348Kg
    specific heat of water vapour = 1.864kJ/KgK
    heat attained(in K) = heat evolved/(mass * specific heat capacity)
    = 8.35944/(0.0005348 * 1.864)
    = 29136 kelvin
    that is an incredible amount of heat. are my calculations right? if no please tell where the mistake is. anyone can ask clarifications. iam dead serious about this. please help me and help yourself. help mother earth.
     
  2. jcsd
  3. Oct 11, 2008 #2
    What process is this undergoing?
     
  4. Oct 11, 2008 #3

    uart

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    The 8.3kJ provided by that one litre of H2 O2 mixture has about the same energy as 0.2ml (about 3 drops) of gasoline so what exactly is your point?
     
  5. Oct 11, 2008 #4
    I suppose the point is that its not gasoline
     
  6. Oct 11, 2008 #5

    russ_watters

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    Welcome to PF, harnith.

    That little exercise about how much energy is generated by burning a liter of hydrogen doesn't really tell us anything useful "hydrogen power", (much less what you mean by "hydrogen power") or if a "hydrogen economy" is viable.

    I agree with uart: what is the point of that exercise? What, exactly does it tell us that is useful to know regarding hydrogen power or the hydrogen economy?
    It's not a tuna sandwich either, but so what? That's not a point.
     
  7. Oct 11, 2008 #6

    Borek

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    Harnit: there is no doubts that hydrogen can be used to store energy. But this single fact is not enough to make hydrogen economy possible.
     
  8. Oct 11, 2008 #7
    I suppose you're right...
     
  9. Oct 11, 2008 #8
    harnith

    hi
    i am asking u if my calculations are right. 666cm3 of uncompressed hydrogen can give us so much heat. so if we are using compressed hydrogen, we will be able to use an excellent and clean energy carrier. that too, the hydrogen can be electrolysisically obtained from water with the electricity from solar cells. this is a better way than carrying solar panels on our cars and fearing about no sun coz of clouds. we can have hydrogen bunkers instead of gasoline and diesel.
     
  10. Oct 11, 2008 #9
    i am not concerned about the 8.3kJ, but please take a look at the 26000 kelvin of heat. as we know the resulting water vapour will icrease the pressure in a closed container upto

    PV=nRT
    P * 1000(total volume of container) = 1/22.4(that is the number of moles) * 8.3144(universal gas constant) * 26000(temperature)
    therefore, 1000P = 9650
    P = 9650/1000
    pressure = 9.65 what is the unit for this? is the pressure enough to squeeze my hand so that i cry out of pain?
     
  11. Oct 11, 2008 #10

    russ_watters

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    Re: harnith

    You mean temperature, not heat. but no, it couldn't possibly be that high of a temperature. The adiabatic flame temperature of hydrogen in oxygen is 3100 K. But again, that doesn't tell us anything useful about the hydrogen economy.
    Those calculations do not support that conclusion unless you compare the numbers to other energy carriers.
    Perhaps, but solar panels are very expensive and don't work at night or when it is couldy. And since half or our electricity currently comes from coal, we should probably replace that before we even think about going after spare energy for making hydrogen.
    Solar panels couldn't power a real car anyway, but regardless - the hydrogen economy question is not a simple one. And what you started off with doesn't tell us anything at all about it's viability.
     
  12. Oct 11, 2008 #11

    russ_watters

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    Well since the temperature is wrong, the answer to that question is wrong too, but fyi, if you use metric, the volume needs to be in cubic meters and the pressure in n/m^3.
     
    Last edited: Oct 11, 2008
  13. Oct 11, 2008 #12

    russ_watters

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    FYI, the error in the calculation in the first post is in the specific heat of steam. Specific heat decreases with temperature and at those temperatures, you're dealing with ionized gas anyway and the thermodynamics become vastly different. But it doesn't really matter: flame temperature isn't really a useful thing to calculate here anyway.
     
  14. Oct 12, 2008 #13
    thank u everyone especially russ watters for pointing out the error in my calculations. but please tell me what to do to find the pressure in such a 1 litre container where hydrogen and oxygen are burnt in the ratio of 2:1. tell me the calculations if possible
     
  15. Oct 12, 2008 #14

    Borek

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    You have already assumed pressure - using density of the gas.

    PV=nRT - this is so called ideal gas equation, that combines number of moles of gas with its teperature, pressure and volume. R is a universal gas constant (very important one, that you will find in many places).

    Assuming there is 1 mole of the gas, it occupies volume of

    V = nRT/P

    Let's put n=1, that means you use just 1 mole of gas. Gas has molar mass M, if there is just 1 mole of it you have M grams of the gas, so its density is

    d = M/V = MRT/P

    You can use this formula to calculate pressure knowing molar mass, density and temperature. You asked about hydrogen - molar mass is 2 g/mol, density is 0.08988 g/L, let's assume temperature of 300K (warm summer day). You may need to properly select gas constant for these units, but once you will do you will find the pressure to be 1.097 atm, not much higher than atmospheric.

    In fact 0.08988 g/L is just a hydrogen density at 0 °C and 1 atm.

    Now, your pressure will be higher, as you have added oxygen, but we are still in the realm of low pressures.

    Sure, once the gases reacted and got hot the pressure will be much higher.
     
  16. Oct 12, 2008 #15
    i am asking only the pressure after the gases after they get hot. i am not concerned about them being cool. please someone tell me. this is breaking my mind
     
  17. Oct 12, 2008 #16
    one more thing. i am not a college student or someone like that. i am just a 9th standard school student from india
     
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