Third Law of Thermodynamics

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Third Law of Thermodynamics

• If increasing temperature increases entropy, then
  the opposite should be true also.
• Is it possible to decrease the temperature to the
  point that the entropy is zero?
• At what T does S 0?
• If entropy is zero, what does that mean?

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Third Law of Thermodynamics

• 3rd Law of Thermodynamics
• Entropy of a perfect crystalline substance at 0 K
  is zero
• No entropy highest order possible
• Purpose of 3rd Law
• Allows S to be measured for substances
• S 0 at 0 K
• S? standard molar entropy
• The entropy of the pure substance is our zero
  (reference point)

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What is a Perfect Crystal?
Perfect crystal at 0 K
Crystal deforms at T gt 0 K
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Third Law of Thermodynamics

• The entropy of a pure crystalline substance at
  absolute zero is 0.

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Third Law of Thermodynamics

• The entropy of a pure crystalline substance at
  absolute zero is 0. This allows us to calculate
  an absolute entropy.

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Measuring Heat and Enthalpies
Constant-Volume Calorimetry
qsys qwater qbomb qrxn
qsys 0
qrxn - (qwater qbomb)
qwater msDT
qbomb CbombDT
Reaction at Constant V
DH qrxn
No heat enters or leaves!
Why only approximately the enthalpy?
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Constant-Pressure Calorimetry
qsys qwater qcal qrxn
qsys 0
qrxn - (qwater qcal)
qwater msDt
qcal CcalDt
Reaction at Constant P
DH qrxn
No heat enters or leaves!
6.5
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Thermochemistry
The study of heat transfer due to chemical
reactions and phase changes
qreaction qcalorimeter 0
qreaction - qcalorimeter - Ccalorimeter DT
For 18 gm C(gr) or equivalently nC 18/121.5
moles DHc 1.5(-393.5) kJ
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Energy is a State Function if and only if DUa
DUb

• Assume the contrary and take DUa gt DUb .
• Go forward on a (gaining Ua) and backward along b
  (lossing -DUb)
• During each cycle there is a net gain in energy
  (DUa - DUb ) gt 0, available to do work and no
  change in the system.
• We could repeat this and gain energy from doing
  nothing.
• Nothing is Free so its not possible.
• Not possible, therefore, DUa DUb. Energy is
  conserved.
• Energy is a state function
• So is Enthalpy (same arguments apply), (Entropy,
  etc)

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Thermochemistry Hesss Law
DHa DHb Hf - Hi
DH1 DH2 HCD - HAB HEF - HCD DH3
If 2 (or more reactions) are added (or
subtracted) to get a new reaction, then adding
(or subtracting) enthalpy changes for the two
reactions gives enthalpy change for the new
reaction. Hesss law takes advantage of the fact
that Enthalpy is a state function.
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Examples of Hesss Law
When carbon burns it creates mainly carbon
dioxide and water. What is the enthalpy of
combining Carbon with Oxygen to make Carbon
Monoxide?
DH3 DH1 DH2 -393.5 kJ/mol DH1 -283.0 kJ/mol
DH1 283.0 kJ/mol -393.5 kJ/mol -110.5
kJ/mol
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Another Example of Hesss Law
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Latent Heat in Phase Changes
H2O(liq) H2O(gas)
DHvap 40.66 kJ/mol
Phase changes are accompanied by latent heat. Can
add heat but temp doesnt rise until all one
phase.
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What happens to diamonds in a house fire?
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Enthalpy of Formation (defining a zero point)
DHo standard enthalpy change for reaction
when reactants and products are in their
standard states
Standard state of compound X is the most stable
form of X at P1 atm and T298.15 K.
DHof standard enthalpy of formation of a
compound.
DHof DH of reaction in which pure elements in
standard states combine to form products in std
states.
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Examples of Enthalpy of Formation
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Standard heats of reaction from DHof
DH298o c DHfo(C) dDHfo(D) - a DHfo(A) -
bDHfo(B)
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Examples using Heats of Formation
DH298o 3 DHfo(H2O) 2 DHfo(CO2) - DHfo(C2H2OH)
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Bond Enthalpies
In reactions bonds are broken and bonds are
formed, eg C--H
Define average bond strengths C-C
DHC-C 348 kJ
CC DHCC 615 kJ

C-H DHC-C 413 kJ
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Practical uses surroundings system
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Condition of Spontaneity
Make this equation nicer
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Third Law of ThemodynamicsNernst Heat Theorem

• In any thermodynamic process involving only pure
  phases in their equilibrium states, the entropy
  change

The entropy of any element in equil state is
defined to approach 0 as T approaches 0K. This is
the reference state (sea level) for Entropy.
?S---gt0 as T---gt 0K for chem reaction where pure
elements ---gt pure compounds.
Entropy of any pure substance (element or
compound) in its equil state ---gt0 as T--gt0
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Measuring Heat and Enthalpies
Hot metal (373K) put in cool water (283K). What
is Tf?
Heat absorbed by water
Hot Metal
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Mechanical Equivalence of Heat
1 calorie 4.184 Joules