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
∆ST1T2 = 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.
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
∆ST1T2 = 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.
