Internal Energy: U
-the total energy of a system
-4 components: vibration, rotations, translation of atoms/molecules, entropy
-depends on T and P (higher T = higher U)
-extensive property- depends on mass
-Um (molar internal energy) = U/n
-not measured absolutely
-ΔU=ΔProducts - ΔReactants
      =W+q (W=energy transferred by work, if P is constant, W=0), (q=energy transferred by heat)
For a perfect gas, isothermal expansion results in the energy of the system being constant so, Δ=0 and q=-W.

Internal energy is a "state function", it depends only on the final and initial states, not on the pathway
Conclusion: ΔU, ΔG (Gibb's free energy), ΔH (enthalpy), and S (entropy) are all independent of the pathway, "state functions"
ΔU, ΔG, ΔH, and S are thermodynamic properties

1st Law of Thermodynamics
Isolated systems:
1.) cannot do work
2.)U is constant (1st law)

How do you measure change in U (ΔU)?
 -conduct experiments at fixed V, thus V=0, and W=0, so ΔU=q at constant V (use bomb calorimeter)

Bomb Calorimeter
-used to measure C, C=q/ΔT, at constant V, Cv=ΔU/ΔT

enthalpy
-in the real world, most reactions take place in ways that are not at constant V
-very hard to work with ΔU in many circumstances, thus we use enthalpy (H)
H=U+PV
-extensive property (depends on mass);
Hm=H/n= Um*PVm -> ΔH=Qp @constant P
ΔH=ΔU+Δ(PV) *can measure ΔH but not H*