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Homework Help: Hydrogen oxygen ignited volume expansion

  1. Apr 19, 2015 #1
    1. The problem statement, all variables and given/known data
    there is an expandable container with 1m3 of hydrogen and 1/2 m3of oxygen at stp. the gas is ignited.what is th final volume if there is no heàt lost and the pressure outside the container is 1atm? assume complete combustion

    2. Relevant equations
    other than the ideal gas equation not sure what other equations to- use

    3. The attempt at a solution
  2. jcsd
  3. Apr 19, 2015 #2


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    Staff: Mentor

    And how could you apply the ideal gas law?
  4. Apr 19, 2015 #3
    thinking along the lines that when the hydrogen is ignited it releases energy which would increase temperature. increased temperature, increased volume because pressure remains the same. knowing that the end product of hydrogen combustion is water would this container then be filled with water vapor?
  5. Apr 19, 2015 #4
    Yes. This is all correct.

    Here are some hints:

    If there is no heat lost and the expansion occurs against constant pressure of 1 atm., from the first law, what is the change in enthalpy of the system?

    If there had been no change in temperature, what would the change in enthalpy have been, and how much heat would have been lost?

  6. Apr 20, 2015 #5
    thanks chet will get back to you-m
  7. Apr 21, 2015 #6


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    Gold Member

    1.Thats right and one value you need is the molar heat capacity of water vapor in calories/mole K
    2. How many moles of H2O produced ? ( 22.4moles/liter STP) How many liters in m3
    3. What is enthalpy in calories released in total reaction ?
    4. From above you should get K° for T
    5.R should be ..08206 L atm/mole K
    6. V = nRT/P

    I got xx.x m3
  8. Apr 21, 2015 #7
    hate to'sound ignorant (but i am) 1) i am not finding any molar heat capacity of water VAPOR. would not the heat capacity of vapor depend on temperature which would change once the hydrogen is combusted? 2)there are aproximately 67 moles in the 1 1/2 m3. 44.6 of hydrogen and 22.3 for oxygen. not understanding "number of moles H2O" moles being gaseous does that mean water vapor? Am i making this more complicated than it has to be? I really appreciate the help ,I am trying to get my brain in the right perspective and not just do the math.
  9. Apr 21, 2015 #8
    So how many moles of water vapor are produced when 44.6 moles of hydrogen react with 22.3 moles of oxygen. What is the heat of this reaction (with the product being water vapor)? This is the same as the heat of formation of water vapor at standard conditions. Are you saying that you can't find any data on Cp for water vapor? (Did you consider getting the value per unit mass, and then multiply by the molecular weight?).

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