jasoncurious Posted December 24, 2012 Share Posted December 24, 2012 Hi guys, happy holidays. I need some help in choosing the usage of H (enthalphy) and U (internal energy) when dealing with thermodynamics. Any guidelines? Link to comment Share on other sites More sharing options...
juanrga Posted December 24, 2012 Share Posted December 24, 2012 (edited) U is useful when working with variables (S,V,N,...). H is useful when working with variables (T,V,N,...). Precisely enthalpy is obtained from internal energy when the variable entropy S is substituted by the variable temperature T. Edited December 24, 2012 by juanrga Link to comment Share on other sites More sharing options...
jasoncurious Posted December 25, 2012 Author Share Posted December 25, 2012 Sorry, but I can't help but wonder, isn't that every thermodynamic situation involves entropy? Link to comment Share on other sites More sharing options...
juanrga Posted December 25, 2012 Share Posted December 25, 2012 U is useful when working with variables (S,V,N,...). H is useful when working with variables (T,V,N,...). Precisely enthalpy is obtained from internal energy when the variable entropy S is substituted by the variable temperature T. Sorry, but what I wrote above is plain wrong. Enthalpy is obtained from internal energy when the variable volume V is substituted by the variable pressure p. U=U(S,V,N,...) H=H(S,p,N,...) Enthalpy is useful in situations when volume is variable or cannot be easily controlled, but pressure can. Link to comment Share on other sites More sharing options...
Zeppos10 Posted March 5, 2013 Share Posted March 5, 2013 In general use enthalpy H: since H=U+pV, enthalpy includes internal energy (never mind the conditions) consider the following, symbols: U=internal energy, p=atmospheric pressure, V=system volume, H=enthalpy, Q=heat-transfer, W=work, W'=work done by atm, W"=technical/usefull work (eg work pistonrod transfers to the system. First law: dU=Q+W=Q+W'+W" W'=-pdV, dU+pdV=dH=Q+"W", hence if W"=0 then dH=Q. (this is a conditional equality, not an identity) enthalpy is best understood as a form of energy, ie the sum of internal energy and energy of displacement pV. Heat Q is NOT a form of energy although this is what they tell us in highschool. reference: Mannaerts SHWM, Energy-balance of the Joule-Thomson experiment: Enthalpy change at decompression. npt-procestechnologie. 2010; 17(4)18-22. (can be retrieved at Reseachgate). Link to comment Share on other sites More sharing options...
CaptainPanic Posted March 5, 2013 Share Posted March 5, 2013 (The following is slightly simplified) When engineers make a mass and energy balance of a process, the energy balance is expressed in Enthalpy. Enthalpy is used to describe how something heats up or cools down, and what the temperature becomes if you put a certain amount of energy in, or if you remove some. Adding or removing energy can be dome in multiple ways (external heating, reaction, stirring), and will result in a change in enthalpy, which is (often) expressed and measured as a change in temperature. Sometimes "enthalpy balance" is used as a synonym for "energy balance", but I have to admit that this may be because some people are a little flexible with definitions. Link to comment Share on other sites More sharing options...
Zeppos10 Posted March 5, 2013 Share Posted March 5, 2013 If there are no changes in kinetic and gravitational energy, energy balance=enthalpy balance. Hess'Law is a good example of its application to reactive (isothermal) systems: dH=Q. Link to comment Share on other sites More sharing options...
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