# 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