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What volume does the v in pv denotes?

  1. Feb 12, 2014 #1
    what volume does the "v" in pv denotes?

    say that in a system where pressure is constant Mg reacts with O2.when dealing with above reaction thermodynamically,
    where H is the initial enthalpy of the system
    UI is the initial internal energy of the system.

    And my question is that what does the VI denotes in the equation.Is it the volume of magnesium and oxygen?
    Likewise when we write the equation after the reaction, what does the VF denotes?Is it the volume of magnesium oxide formed ?
    can anyone explain?
  2. jcsd
  3. Feb 12, 2014 #2


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    Staff: Mentor

    Most often, you would consider only the volume of the gas, as the volume of the solid will be negligible in comparison. For the reaction you mention, I would take both VI and VF to be the volume of O2 (assuming only O2 in the reaction vessel, not e.g. air).
  4. Feb 12, 2014 #3
    Interesting --
    pressure is held constant -
    Mg + O2 is exothermic reaction -
    For the pressure in the vessel to remain constant -
    the reaction must proceed slowly or there could be ( would be) an increase temperature resulting in an increase pressure unless the volume increases
    On the slow reaction method - (think rusting) the presure would slowly decrease as oxygen combines with the magnesium - MgO2 being a solid is much denser than O2 gas

    Pressure Volume Temperature o
  5. Feb 12, 2014 #4
    The heat of reaction is defined such that, in the initial state you have pure reactants and in the final state, you have pure products. Also, the final temperature is equal to the initial temperature, and the final pressure is equal to the initial pressure (for both the reactants and the products). The change in enthalpy is equal to the amount of heat you have to add to go from the pure reactants to the pure products at the constant temperature and pressure.
  6. Feb 13, 2014 #5
    why do we neglect the volume of solids ?

    so when there is a particular volume of O2 in a vessel,does it doing a work against the atmospheric pressure? I think I 've some wrong idea about "pv".
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