DoctorSaab said:
I am having a tough time understanding thermochemistry. EK confused the hell out of me. Is there a better sourse to get this topic down? My textbook covers way too much, so I dont want to go into that. Anyone have useful sites or tips how they got through thermochemistry? I feel like I dont understand any of this.
Not sure if this will help. I'm a little shaky on thermodynamics myself. Most of thermodynamics is just knowing terms and definition. Only the first part of thermodynamics may be confusing. It helped me to understand a few simple concepts. Here they goes:
Everything has internal energy (movement, vibrations, etc... of molecules). For many processes including chemical reactions, compression of gas, or melting of ice will result in a CHANGE in internal energy (dE), which is defined as the internal energy of the "products" minus the internal energy of the "reactants."
dE = Ef - Ei
-dE = "product" has less energy = energy is released in the process
+dE = "product" has more energy = energy was absorbed
However, as we all know, energy is conserved. So the energy released or absorbed must come to or from somewhere, which are work (W) and heat (Q). This leads us to another expression
dE = Q - W
Of course, positive dE and Q means the SYSTEM gain energy and heat, respectively. Negative means the opposite.
The tricky part is W. I'm using W as work done BY SYSTEM or ON SURROUNDING. (Some text may use W as work done ON SYSTEM or BY SURROUNDING, in this case, dE = Q
+ W)
Either way you look at it, it'll come to
dE = Q - P*dV where P = pressure and dV = change in volume
What helped me was to understand and remember that:
* Work done BY SYSTEM = Work done ON SURROUNDING (1)
* Work done ON SYSTEM = Work done BY SURROUNDING (2)
* (1) and (2) have equal magnitude but opposite sign
You'll have to be able to convert between these quantities. For example, if
Work done BY SYSTEM = 10kJ, then
Work done ON SURROUNDING = 10kJ (system does work on surrounding)
Work done ON SYSTEM = -10kJ (not only no one does work on system, system has to do work)
Work done BY SURROUNDING = -10kJ (surrounding didn't do any work, system did work on surrounding)
Use the quantity that make sense to you in your reasoning. For example, in this case, I'll say by work done by system was 10kJ instead of saying work done on system was -10kJ.
Good luck and sorry if I had confused you. If you post a sample problem regarding this matter, I'll tell you my reasoning process through it.
