# Lecture 15

```Lon-Capa
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4th HW assignment due Friday, 10/10, 5 pm.
It is open now.
3rd Quiz due Sunday, 10/12 by 10 pm. It
will open Friday, 10/10 at 5 pm.
5th HW assignment is also open!
1
Chapter 9: Problem #99
When water is supercooled, it freezes at a
temperature below 0.0°C. If 10.9 kJ of energy
as heat is released when 2.00 mol of
supercooled water at –15.0°C freezes, calculate
the molar enthalpy of fusion for ice at 0.0°C and
1 atm. Assume the molar heat capacities are
temperature independent.
[for H2O(l): 75.3J/Kmol, for H2O(s): 37.5J/Kmol]
2
We know it should. But why?
Determine q, w
ΔE, ΔH for this
process.
Which helps us
to explain why
this happens?
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Questions To Consider
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How can we predict which reactions are
spontaneous (and what do we mean by
spontaneous)?
How can we provide support (mathematical and
qualitative) for observations?
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Ice cream melts
Coffee cools
Why do systems reach equilibrium? That is, why
doesn’t the ball go all the way down the hill?
4
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Entropy is related to heat/temperature.
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ΔS = qrev/T
Entropy as “disorder”.
S = k●logW
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Points To Consider
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We want to see why entropy is related to the
logarithm of the number of configurations or
microstates.
We want to show that thermal equilibrium is
preferred when two bodies of different
temperatures are put in contact with one another.
We want to see how probability plays into all of
this.
What does “reversible” mean when we say the
change is entropy equals qrev/T?
6
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