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What volume does the v in pv denotes? 
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#1
Feb1214, 11:00 AM

P: 37

say that in a system where pressure is constant Mg reacts with O_{2}.when dealing with above reaction thermodynamically,
H_{I}=U_{I}+PV_{I} where H is the initial enthalpy of the system U_{I} is the initial internal energy of the system. And my question is that what does the V_{I} denotes in the equation.Is it the volume of magnesium and oxygen? Likewise when we write the equation after the reaction, what does the V_{F} denotes?Is it the volume of magnesium oxide formed ? can anyone explain? 


#2
Feb1214, 01:16 PM

Sci Advisor
PF Gold
P: 1,356

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 V_{I} and V_{F} to be the volume of O_{2} (assuming only O_{2} in the reaction vessel, not e.g. air).



#3
Feb1214, 02:56 PM

P: 8

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 


#4
Feb1214, 03:35 PM

Sci Advisor
HW Helper
Thanks
PF Gold
P: 5,260

What volume does the v in pv denotes?



#5
Feb1314, 07:19 PM

P: 37

so when there is a particular volume of O_{2} 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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