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General Chemistry A: Thermodynamics Lecture Notes #2
- Institution
- Orange Coast College
Lecture notes of 8 pages for the course General Chemistry A185 at Orange Coast College (Professor Maughan)
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Spontaneity, entropy, and free energy 2Entropy and the second law of thermodynamics
Review of 1st and 3rd law of thermodynamics
first law of thermodynamics: energy cannot be created nor
destroyed
third law of thermodynamics: the entropy of a perfectly
ordered crystal at absolute zero (0 kelvin) is zero.
Second law of thermodynamics
- we know that change in enthalpy ( h) and change in
entropy ( s) determine spontaneity of a process
1
- g is what we use to define the combined contribution
from h and s DISPERSION of Energy.
-
TS Molecular Randomness
G = H -
enthalpy: heat flow at constant pressobe
- all quantities in above equation pertain to system
- these changes lead to change in entropy of the
universe
- 2nd law of thermodynamics: for any spontaneous process,
the entropy of the universe increases
- remember: entropy is s. Basically, the universes
randomness increases
qualitative understanding of 2nd law of thermodynamics
- the 2nd law tells us that all processes occur
spontaneously in the direction that increases the total
entropy of the universe
- ie: Suniverse O
- since universe consists of system and surroundings, to
predict spontaneity of a process, consider 2 enthalpy
changes — the system and the surroundings.
- the criterion for spontaneous process is based on
sum of 2, called entropy of universe:
, S =
0S - S
suppounding
system
- 2nd law provides clear cut criterion for spontaneity.
Direction of spontaneous change always determine by sign
of total entropy change:
T
sunirease:
5 universe =
-
suniverse =
0
quantitative aspect of 2nd law of thermodynamics
- general equation for entropy change when a process/
reaction occur under constant temp:
sig, heat flow at constant temp: H
system T
- this is general equation for rxns w/ heat flow at
constant t. Applies to processes such as phase
change and heat flow from system to surrounding
- subscript t is reminder that equation is restricted to
processes that occur at constant temp. B/c q is an
energy term, units of s are energy/temperature, or j/k
entropy changes for phases changes
- phase changes take place at constant temp, absorbing
or releasing heat in process
- change in entropy for this type of processes can be
calculated using equation above
- when more ordered phase changes to less ordered one,
entropy change is positive. For given substance,
entropy increases as substance changes from solid to
liquid to gas
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