What volume does the v in pv denotes?

[abi.ayan]

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,
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?

 

[DrClaude]

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 O₂ (assuming only O₂ in the reaction vessel, not e.g. air).

 

[maltman]

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

 

[Chestermiller]

say that in a system where pressure is constant Mg reacts with O₂.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?

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.

 

[abi.ayan]

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 O₂ (assuming only O₂ in the reaction vessel, not e.g. air).

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".