Internal energy vs. Enthelpy vs. Entropy

In summary, the conversation discusses the first law of thermodynamics and how it can be used to calculate heat transfer in different processes. The use of enthalpy and internal energy is also discussed, with the conclusion that enthalpy is generally used for constant pressure processes and internal energy for constant volume processes. It is also mentioned that for open systems, the first law for open systems must be used.
  • #1
JSBeckton
228
0
Ok, I must admit that I am becoming a bit confused about these concepts. I understand that enthalpy is u + Pv, and entropy has something to do with molecular randomness. I was fine until we started to solve for heat transfer when dealing with entropy and now I am confused, sometimes we use:

Q=m(u2-u1) to describe heat transfer

and other times we use:

Q=m(h2-h1)

Can anyone please explain to me how to tell which should be used where? I know that its a very important concept that i must have missed.

Thanks
 
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  • #2
Let's start with the first law. It says the change in the internal energy [itex]\Delta U[/itex] is

[tex] \Delta U = Q + W [/tex]

Similarly the change in enthalpy is

[tex] \Delta H = \Delta U + \Delta (PV) [/tex]

From this, you can calculate the heat exchange for a constant volume process and a constant pressure process.

So for a
1) Constant volume process
The first law reduces to
[tex] \Delta U = Q[/tex]
So, the heat transfer will be [itex] Q =m(u_2-u_1)[/itex]

2) Constant pressure process
In this case, the first law is
[tex] \Delta U + P\Delta V = Q [/tex]
But, from the definition of enthalpy, you also have
[tex]\Delta H = \Delta U + P\Delta V [/tex]

So, can you complete this and figure out how you calculate Q in each case?

If you need to calculate Q for a general process, try calculating the change in internal energy and the work, then use the first law.
Finally, in some cases you may be able to calculate the heat transferred if you know the change in entropy. (For example, a reversible isothermal process)
 
Last edited:
  • #3
Thanks a lot siddharth

[text]
\begin{array}{l}
Q = \Delta U + P\Delta V \\
{\rm where }\Delta H = U + P\Delta V \\
{\rm therefore }Q = \Delta H \\
\end{array}
[/text]

So, is it safe to say that I should use U for constant volume and H for constant pressure?
 
  • #4
Sorry for the double post, can't figure out this text editor

Thanks a lot siddharth

Q=delta H

So would it be safe to say that i should use U for constant volume and H for constant pressure?
 
  • #5
JSBeckton said:
Sorry for the double post, can't figure out this text editor

Thanks a lot siddharth

Q=delta H

So would it be safe to say that i should use U for constant volume and H for constant pressure?

For an ideal gas, and ignoring changes in Kinetic energy and such, yes.
 
  • #6
When I have a steam turbine I use enthalpy not internal energy even though its not a constant pressure process. I know that steam is not an ideal gas but how do I choose which to use?

Is this true?

Q-W=U2-U1
W=(U1-U2)+Q
W=(H1-H2)

But if its adaibatic Q=0 so,
W=(U1-U2)
 
  • #7
JSBeckton said:
When I have a steam turbine I use enthalpy not internal energy even though its not a constant pressure process. I know that steam is not an ideal gas but how do I choose which to use?

Is this true?

Q-W=U2-U1
W=(U1-U2)+Q
W=(H1-H2)

But if its adaibatic Q=0 so,
W=(U1-U2)

Whoops, my error. You can use it for a non-ideal gas, but the system should be closed.

For a steam turbine, you have a flow process in an open system. In that case, you'll have to use the first law for open systems.

[tex] \frac{dE}{dt} = \dot{Q} - \dot{W} + \sum_i \dot{m_i} h_i - \sum_j \dot{m_j} h_j[/tex]

You'll find how you get this in any thermodynamics book. It's different from the first case, which is for closed systems.
 

1. What is internal energy?

Internal energy is the total energy contained within a system, including the kinetic and potential energy of all its particles. It is a measure of the microscopic energy of a system and can change through processes such as heating or cooling.

2. What is enthalpy?

Enthalpy is the measure of the heat content of a system at constant pressure. It takes into account the internal energy of a system as well as the work done by or on the system. It is often used to calculate the heat transferred in a chemical reaction or physical process.

3. How are internal energy and enthalpy related?

Internal energy and enthalpy are closely related as they both measure the energy of a system. The difference between them is that enthalpy includes the energy associated with the system's pressure and volume, while internal energy does not. Enthalpy is often described as the "heat content" of a system, while internal energy is the total energy of the system.

4. What is entropy?

Entropy is a measure of the disorder or randomness of a system. It is a thermodynamic property that increases with the amount of energy distributed among the particles of a system. In simple terms, it is a measure of the system's tendency towards equilibrium and the amount of energy that is unavailable to do work.

5. How are enthalpy and entropy related?

Enthalpy and entropy are related through the Gibbs free energy equation, which states that the change in Gibbs free energy (ΔG) is equal to the change in enthalpy (ΔH) minus the temperature (T) multiplied by the change in entropy (ΔS). This relationship shows that an increase in entropy can result in a decrease in enthalpy, and vice versa. It also highlights the importance of both enthalpy and entropy in determining the spontaneity of a chemical reaction.

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