# What volume does the v in pv denotes?

• abi.ayan
In summary, The "v" in pv denotes the volume of the gas in the system, as the volume of the solid is usually negligible. In the equation HI = UI + PVI, VI and VF both represent the volume of O2, assuming only O2 is present in the reaction vessel. This is because the heat of reaction is defined for the change from pure reactants to pure products at constant temperature and pressure. The volume of solids is not considered in this equation.
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,
HI=UI+PVI
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?

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).

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

abi.ayan said:
say that in a system where pressure is constant Mg reacts with O2.when dealing with above reaction thermodynamically,
HI=UI+PVI
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?
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.

DrClaude said:
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).
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".

## 1. What is the meaning of "v" in the equation pv?

The "v" in the equation pv stands for volume. It is a variable that represents the amount of space occupied by a substance.

## 2. How is the volume calculated in the pv equation?

The volume in the pv equation is calculated by dividing the pressure (p) by the temperature (T) and the number of moles (n) of the substance, or V = (p x n) / T.

## 3. Can the volume in the pv equation change?

Yes, the volume in the pv equation can change depending on the pressure, temperature, and number of moles of the substance. It is a variable that is affected by these factors.

## 4. What are the units for volume in the pv equation?

The units for volume in the pv equation depend on the units used for pressure, temperature, and number of moles. Common units for volume include liters (L), cubic meters (m³), and cubic centimeters (cm³).

## 5. How is the pv equation used in scientific research?

The pv equation is commonly used in research to study the behavior of gases. By manipulating the variables, scientists can determine the relationship between pressure, volume, temperature, and number of moles of a gas sample.

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