CHMA11H3 Lecture Notes - Lecture 23: Standard Molar Entropy, Zero Point Railway Station, Thermodynamics
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28 Feb 2019
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CHMA11 Lecture 20: Thermodynamics Part 2
- Criteria for spontaneity
o Suniverse = Ssystem + Ssurroundings
▪ Remember the 2nd law of thermodynamics
• All spontaneous processes will produce increases in the entropy
of the universe
• Therefore Suniverse will always be greater than 0
o Quantifying the entropy of systems
▪ The absolute entropy of a substance can be measured
• It is done in reference to a pure crystal of the substance in
question at a temperature of 0 K for which S will equal 0
o Zero entropy: the entropy of a perfect crystal at a 0 K is 0
▪ This is because all motion will stop at a
temperature of 0 K
▪ Zero Point Energy: tiny vibrations that molecules
exert on each other
• Proof that there is still a little motion at 0 K
• This energy is evenly distributed in a
perfectly crystal, hence the entropy value of
0
▪ Standard Molar Entropy: the absolute entropy of 1 mole of a pure
substance in its standard state at 25 degrees Celsius and 1 atm
• Represented by So
• Is a state function
o Therefore the change in standard molar entropy of a
system is equal to the sum of molar entropies of the
products in the system minus the sum of the molar
entropies of the reactants in the system
• As the complexity of a molecule increases, its absolute entropy
value will also increase
• A positive S value will indicate that a process is spontaneous, a
negative S value will indicate that a process in non-spontaneous
- Relationship between Entropy and Temperature
o Entropy increases as temperature increases
▪ This is due to the fact that more kinetic energy is available at higher
temperatures
• This results in more microstates being created, enabling a greater
entropy value to be calculated
o Therefore we can quantify the entropy of surroundings