Adiabatic Process, Internal Energy vs. Enthelpy.

In summary, the conversation discusses the relationships between enthalpy, internal energy, and work in an ideal gas system. The speaker is struggling to understand how both ΔU and ΔH can equal work (W) in different scenarios. They come to the realization that ΔU=Qv and ΔH=Qp in a closed adiabatic system and an open steady-state adiabatic process, respectively. This leads to the conclusion that nCvΔT will equal to W because at constant volume, no work is done and heat (Q) is equal to ΔU. This is also the proof for the equation Cp-Cv=R.
  • #1
Jeremy1789
2
0
I'm struggling to understand a concept which I assume is basic, but I can't seem to fit the pieces together. When speaking about an ideal gas, I understand that

ΔH = ΔU + Δ(PV) = ΔU + RΔT

So far so good. I also understand the relationship:

ΔU = Q + W... (W here is work being done on the system)

In an adiabatic reversible process, Q = 0, which also makes sense. So,

W = ΔU = nCvΔT

Now, where my confusion lies is in the next part. My book works out a problem, and says:

ΔH = W = nCpΔT

How can both ΔU and ΔH equal W? This doesn't make sense to me, unless Δ(PV) from the first equation was 0. I don't see how this could be 0 unless we were talking about an isothermal case. It also doesn't make sense because W can't equal both nCvΔT and nCpΔT simultaneously, since Cp = Cv + R.

Am I missing something?
 
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  • #2
Never mind. Just answered my own question. The first instance refers to a closed adiabatic system, and the second refers to an open steady-state adiabatic process.
 
  • #3
How nCvΔT will equal to W.It is the heat absorbed/released at constant volume only(no work is done at constant volume,the expression
Cv).i think instead of work it is heat,ie ΔU=ΔH=qv at constant volume and ΔH=qp at constant pressuer
actually it is the proof for Cp-Cv=R
 

1. What is an adiabatic process?

An adiabatic process is a thermodynamic process in which there is no transfer of heat or matter between a system and its surroundings. This means that the change in internal energy of the system is equal to the work done on or by the system.

2. What is the difference between internal energy and enthalpy?

Internal energy is the total energy of a system, including the kinetic and potential energy of its particles. Enthalpy is the sum of the internal energy and the product of pressure and volume of the system. Enthalpy takes into account the work done by the system while internal energy does not.

3. How does an adiabatic process affect the internal energy of a system?

In an adiabatic process, the internal energy of a system remains constant. This is because there is no heat transfer, so the change in internal energy is equal to the work done.

4. What factors affect the enthalpy of a system?

The enthalpy of a system is affected by the internal energy of the system, as well as the pressure and volume of the system. Additionally, changes in temperature and the number of moles of a substance can also affect enthalpy.

5. Can the enthalpy of a system change without changing its internal energy?

Yes, the enthalpy of a system can change without changing its internal energy. This can happen if there is a change in pressure or volume, as enthalpy takes into account these factors while internal energy does not.

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