Change in enthelpy at constant VOLUME

In summary: So, in summary, enthalpy is a mathematical potential used in isobaric processes and is equal to the heat transferred. In isochoric processes, the change in enthalpy is equal to the change in internal energy plus the change in pressure-volume work, but this is just a mathematical representation and the actual heat flow is still equal to the change in internal energy. Enthalpy is useful in engineering and chemistry because most processes occur at constant pressure.
  • #1
Tabeia
9
0
Hi, I know enthalpy is used a lot to calculate isobaric processes, but what about isochoric ones?
For example, if I have gas in a piston and I add heat at constant VOLUME.
I know that [itex]Q=\Delta U[/itex]
But since [itex]H=U+PV[/itex] or [itex]\Delta H=\Delta U + \Delta PV[/itex]
I see that although V is constant P is not, it increases and so my entropy increase is bigger than my internal energy increase.
That is ok, I've searched this forum and found the same results, but my question is. Since enthalpy is energy, and energy is conserved from where does this energy come from?
Since the heat supplied to the system is smaller than the change in enthalpy.
Let's say I give 1000 Joules to the system in form of heat, the internal energy increases by 1000J, but the Enthalpy increases more...
Any help appreciated.
 
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  • #2
Tabeia said:
I see that although V is constant P is not, it increases and so my entropy increase is bigger than my internal energy increase.

by how much? do you have any idea?
 
  • #3
Since enthalpy is energy, and energy is conserved from where does this energy come from?
Enthalpy is not the energy, U is the energy. Enthalpy only has the dimensions of energy. Enthalpy is a mathematical potential useful in processes that take place at constant pressure. The change in enthalpy during an isobaric process is equal to the heat that is transferred. In the fields of engineering and chemistry enthalpy is more useful than internal energy, since most processes occur isobarically.
 
  • #4
Bill_K said:
Enthalpy is not the energy, U is the energy. Enthalpy only has the dimensions of energy. Enthalpy is a mathematical potential useful in processes that take place at constant pressure. The change in enthalpy during an isobaric process is equal to the heat that is transferred. In the fields of engineering and chemistry enthalpy is more useful than internal energy, since most processes occur isobarically.
Yes. Exactly. I was in the middle of responding but this is a much more succinct and clearer answer than I was about to give.

AM
 
  • #5
@Bill K, but the question is under isochoric process.
 
  • #6
It feels weird that in an ISOCHORIC process the enthalpy(the potential to do work? Can I say that way?) increases more than the heat supplied...so I get more work(energy) than I gave in(in the form of heat)
Yes, I know that I'm wrong but I was having trouble seeing this.
But thanks for the clarification.

What I'm thinking is that this isn't a perpetual energy machine(duh) because to keep the constant volume I need to have something holding the piston, exerting some force on it to keep it from expanding. And also because I would need to compress it again and in the end the delta H would be zero.
Going to try some calculations later.
 
  • #7
121910marj said:
@Bill K, but the question is under isochoric process.
In an isochoric process, the change in enthalpy differs from the change in internal energy by the amount [itex]\Delta PV[/itex]. But it is just a mathematical book-keeping thing. The only actual heat flow/energy change is [itex]\Delta Q = \Delta U[/itex] in a constant volume process. In fact, the VdP term is already incorporated into [itex]\Delta U[/itex] so, in effect, it is being counted twice in the expression for [itex]\Delta H[/itex] in a constant volume process.

AM
 
  • #8
Andrew Mason said:
In fact, the VdP term is already incorporated into [itex]\Delta U[/itex] so, in effect, it is being counted twice in the expression for [itex]\Delta H[/itex] in a constant volume process.
AM

Aha, now everything is clear in my mind.
 

1. What is enthalpy?

Enthalpy is a thermodynamic property that represents the amount of energy in a system, including both its internal energy and the work required to change its volume or pressure.

2. What is the difference between change in enthalpy at constant volume and constant pressure?

Change in enthalpy at constant volume refers to the change in energy in a system that occurs when the volume is held constant, while change in enthalpy at constant pressure refers to the change in energy when the pressure is held constant.

3. How is change in enthalpy at constant volume calculated?

Change in enthalpy at constant volume can be calculated using the formula ΔH = Q, where Q represents the heat added or removed from the system.

4. Why is constant volume used for some thermodynamic processes?

Constant volume is used for some thermodynamic processes because it allows for easier and more accurate measurements of the change in energy, as the volume remains constant and there is no work being done by the system.

5. What are some real-life examples of processes that occur at constant volume?

Some examples of processes that occur at constant volume include the combustion of fuel in an engine, mixing of two liquids in a closed container, and the melting or freezing of a substance in a sealed container.

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