Thermodynamic def vs. mathematical def.

[Dominic]

In physical chemistry, when the thermodynamic definition of a value is discussed vs. its mathematical definition what difference are we talking about

[fafalone]

I believe this may be referring to enthalpy... the net energy change of a reaction can be negative, which means energy added to the system.

[Dominic]

I was assuming that it meant to express a value a collection of thermodynamic values (such as H, enthalpy, as you mentioned or H or U) which would give you a "thermodynamic definition" vs. if you defined the same value with "measurables" such as Pressure, Volume, and temp.

 

But I'm not sure about that and I can't seem to find a definition in my textbook or online.

[fafalone]

/\H can be measured... calorimeter, table of bond energies

[Dominic]

I was under the impression that dH is calculated. You measure the temp change.

 

such as in dH=CpdT

In the lab you don't directly measure the H, you measure the dT and calc the dH.

[fafalone]

Normally, but volume could also be considered indirect because you either measure it by l w h or water displacement. If you've got some really expensive equipment you might be able to measure H directly

[Dominic]

Items that are measured directly:

Temp measured by thermometer

Pressure by barometer

Volume by ruler or water displacement...

Enthalpy by ?????

 

Enthalpy can be measured, it is how we have our Cp and Cv values (probably) but if someone wants a H or U or S value they won't use an instrument to do it they will measure other simpler things to measure and then calculate the others.

[blike]

Dominic, you in college or highschool?

[Dominic]

Just starting college. Why? Are my posts a little simple minded?

[blike]

No, not at all.

 

I was just wandering if you were in highschool chem or in college. Sounds like my gen chem class last semester.

[Dominic]

Yeah, this is for physical chemistry. I took gen chem last semester and that was easy enough but for some reason p-chem isn't sinking in fast enough.

[blike]

I'm in for gen chem II this semester, and organic next spring.

 

Is physical chem part of your major requirements?

[Dominic]

taking organic right now. had to take p-chem or biochem to get into upperdivision and I can't take bio til next spring. So here I am in pchem.

[blike]

Hows organic? It has a reputation of being rough, but I've heard its not so bad from some people..

[Dominic]

I don't have much of a prob with ochem. Concepts are easy enough to deal with, some memorization, and some spacial orientation issues for some people.

 

The pchem is what is getting me. It is multivariable calculus based and I have to admit I didn't enjoy multi-variable calculus either.