PChem312 f20 w3

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Monday, Aug 31, 2020

Continue reading chapter 2

Sec 2.1: Internal Energy and !st Law of Thermodynamics

U <-- internal energy (1st thermodynamic state function)

U_f - U_i = ΔU, the change in the internal energy

• we cannot measure U_f or U_i, only ΔU.

First Law of Thermodynamics: the change in the system's internal energy is equal to, but opposite in sign, to the change in the surround's internal energy.

ΔU_sys = - ΔU_surr

How does one measure ΔU?

- changes in the system variables (ΔP, ΔV, ΔT, Δn) are an indication of ΔU; changes in system variables result in either heat (q) transfer or work (w) being done on or by the system; calculation of q or w leads directly to ΔU.

ΔP, ΔV, ΔT, Δn --> q (heat), w (work) --> ΔU

Sec 2.2: Heat (q)

Heat is transitory; heat is not a thing. We DO NOT use the term Δq (for change in heat), we instead say that heat flows from one place to another.

Heat naturally flows from hotter/higher temperature to cooler/lower temperature objects (Zeroth Law of Thermodynamics).

Heat sign convention

- we speak of heat from the perspective of the system (unless otherwise stated).

- when the system loses heat, we identify this quantity of heat as negative, -q; loss of heat is generally associated with a decrease in temperature, ↓T.

- when the system gains heat, we identify this quantity of heat as positive, +q or q; ; gain of heat is generally associated with a increase in temperature, ↑T.

Thursday, Sept 3, 2020

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