State Functions for Internal Energy and Enthelphy

AI Thread Summary
The discussion centers on the state functions for internal energy (u) and enthalpy (h), specifically their dependence on volume and pressure, respectively. Internal energy is often expressed as a function of temperature and volume, while enthalpy is related to temperature and pressure, reflecting their applications in different thermodynamic processes. Enthalpy is primarily used in chemical processes at constant pressure, where volume is harder to measure, whereas internal energy is more relevant in physical processes with measurable volume. The choice of these state functions is seen as a matter of convenience rather than a fundamental necessity, as internal energy could also be defined in terms of temperature and pressure. Overall, the conversation seeks a more intuitive understanding of these relationships without relying heavily on mathematical proofs.
Kushwoho44
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Hi,

As is commonly known,

u = u(T,v)
h = u(T,p)

I've worked with some maths proofs of this a while ago, but do you guys have an intuitive way of understanding this without the maths, that is, why the state function for internal energy is defined by intensive volume and enthalpy with pressure?

I seem to recall, I used the model for thought, that if a system had a moving boundary, as in the case of a changing enthalpy, then the volume would not stay constant, so we could not use the constant volume specific heat capacity, so we used specific pressure, but I feel like a new way of thinking about it is required for me..
 
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Hi Kushwoho44!

Enthalpy is typically used for chemical processes in an open environment, meaning that pressure is constant at standard atmospheric pressure, and volume is not easily measurable.
Internal energy is more used for physical processes where volume is measurable.
 
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Kushwoho44 said:
Hi,

As is commonly known,

u = u(T,v)
h = u(T,p)

I've worked with some maths proofs of this a while ago, but do you guys have an intuitive way of understanding this without the maths, that is, why the state function for internal energy is defined by intensive volume and enthalpy with pressure?

I seem to recall, I used the model for thought, that if a system had a moving boundary, as in the case of a changing enthalpy, then the volume would not stay constant, so we could not use the constant volume specific heat capacity, so we used specific pressure, but I feel like a new way of thinking about it is required for me..
All of thermodynamics could be derived without ever introducing the enthalpy function. Also, an equation of state requires that f(P,v,T)=0, so that specifying any two of these determines the third. So, internal energy could also be specified as u = u (T,p). Given these facts, we must conclude that expressing u as a function of T and v, and h as a function of T and P must merely be a matter of convenience in solving various types of problems. There is nothing fundamental about it.
 
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Thanks guys! That makes so much sense.
 
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