Thermodynamics Flashcards
First law of thermo
dU = dQ - dW
> dQ change in heat
> dW work done BY system
At constant pressure, first law becomes
dU = dH - PdV
> only mechanical work considered
> constant pressure change in heat is called enthalpy H
dH = (+) is heat \_\_\_ dH = (-) is heat \_\_\_
absorbed by system; released by system
Enthalpy of elements in standard state (25C and 1atm) equals
zero
Definition of closed system
Only energy (not mass) can be transferred, composition is fixed. Volume can change (expand and contract)
Definition of component
Smallest # of chemical constituents necessary and sufficient to express each phase present in the system
(NOT the smallest chemical component)
Definition of phase
A portion of the system with homogeneous [composition] AND [structure] with distinct boundaries from other parts of the system
Definition of equilibrium
A state in which the sytem’s parameters (composition and structure) no longer change, free energy is minimized
Thermal entropy
Uncertainty in LOCAL atomic positions due to distribution of thermal energy
> atomic vibrations
Configurational entropy
Disorder due to the variation in atomic arrangement across space, long length scale
> all materials: atomic arrangements
metals, ceramics & semicon: point defects/vacancies
polymers: chain configurations (amorphous, semicrystalline)
dS/dT =
Cp/T
dH/dT =
Cp
dStotal = dSsytsem + dSsurr
when reversible, dStotal =
zero
2nd law of thermo: for a reaction to be favored,
> dStotal > 0
dStotal > dH/T
dH - TdS < 0
dG < 0
For constant pressure: dG/dT =
V(dP/dT) - S(dT/dT) ==> dG/dT = -S (negative slope)
