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Summary Spontaneity, entropy and free energy

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Summary of 1st chapter from Chemical Principles: Zumdahl and Decoste. Notes containing key concepts from the chapter and thorough explanations of the terminology. Also includes formulas and relevant course-related information.

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  • Chapter 1
  • July 19, 2019
  • 5
  • 2018/2019
  • Summary

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Spontaneity, Entropy and Free
Energy
10.1 Spontaneous Processes

 A process is said to be spontaneous if it takes places without outside intervention.
 The change in the entropy of the universe for a given process is a measure of the
driving force behind that process.
 Nature spontaneously proceeds towards the states that has the highest probabilities
of existing.
 Microstates are the possible arrangements of molecules that can take place within a
system.
 The probability of occurrence of a particular state depends on the number of
microstates in which the state can be achieved.
 Positional probability depends on the possible positional microstates that yield a
particular state.
 Positional probability is higher for gasses as they have more possible microstates
available.

10.5 Entropy and second law of thermodynamics

 Second law of thermodynamics: In any process, there is always an increase in the
entropy of the universe.
 ∆Suniverse = ∆Ssurroundings + ∆Ssystem
- If ∆Suniverse equals 0, then the process has no tendency to occur and the universe if
at equilibrium.
- If ∆Suniverse is positive, then the entropy increases, and the process is spontaneous
in the direction written
- If ∆Suniverse is negative, then the process is spontaneous in the opposite direction.

10.6 The Effect on Temperature on Spontaneity

 Entropy changes in the surrounding are primarily determined by the heat flow.
 The sign of ∆Ssurroundings depends on the direction of the heat flow.
 The magnitude of ∆Ssurrroundings depends on the temperature
 ∆Ssurr = - ∆H/T

, 10.7 Free Energy

 For a process done at constant temperature, free Energy (G) is defined as:
- ∆G = ∆H – T∆S
 ∆Sunniverse = - ∆G / T at constant T and P
 This indicates that process carried out at constant temperature and pressure will be
spontaneous only if ∆G is negative. A process is spontaneous in the direction in
which the free energy decreases.
 The point at which ∆Hº and ∆Sº equate, indicates the boiling or melting point of a
substance as there is a coexistent state of matter; where the reaction is equally
favored to be spontaneous in both directions.




10.8 Entropy changes in chemical reactions

 The change in positional possibility in reactions involving gaseous molecules Is
determined by the difference in moles of products and reactants.
 In case of the increase in positional probability, ∆S is positive.
 The entropy of a perfect crystal at 0K is zero.  Third law of thermodynamics
∆ST1T2 = nC x ln(T2/T1)
 As entropy change is not pathway dependent, the change in entropy in a reaction
can be determined by finding the difference between the total entropy of the
products and the reactants.

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