1. Unit 10 Lesson 4 Spontaneity In Chemical Reactions Videodisk Unit 4
Demos
The Purple Cow
Hot and Cold Packs
In Chemical Reactions

2. Spontaneous Processes
Spontaneous processes (physical or chemical) are those that can proceed without any outside intervention.
The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously return to vessel B.
© 2009, Prentice-Hall, Inc.

3. Spontaneous Reactions
A Spontaneous Reaction proceeds without needing a constant input of energy (they usually have an output of energy)
Think about iron rusting, it happens “automatically” and releases heat
A non spontaneous reaction will not proceed without a constant input of energy
Think about water decomposing, it needs a constant supply of electricity

4. Spontaneous Processes
Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.
© 2009, Prentice-Hall, Inc.

5. Entropy (DS) A measure of the disorder or randomness in a system.
The law of disorder states that the natural tendency of systems is to move towards a state of maximum disorder or randomness.
It is related to the various modes of motion in molecules.
Increasing disorder = + DS
Decreasing disorder = - DS

6. Examples Increasing entropy examples (positive DS)
Solid  Liquid  Gas
Gas particles are less “orderly” than solids
Dissolving of a solid
Orderly crystals dissolve into ions in solutions
When temperature increases
Particles begin to move faster, have more collisions, and more disorder

7. Spontaneous Reactions
If a reaction is spontaneous it will occur without the addition of energy.
The spontaneity of a reaction is measured by free energy.
Negative free energy (DG) is spontaneous.
It means there is a “surplus” of energy so the reaction has more than enough to proceed

8. Gibb’s Law of Free Energy
DG = DH – TDS
+ DG = Non-spontaneous
- DG = Spontaneous
Entropy
Free Energy
Enthalpy
Temp (in K)

9. DG = (-411.6 kJ/mol) – (298 K*.0318 kJ/molK)
Example Problem
Calculate the free energy for a reaction at 25°C
if DHrxn = kJ/mol & DSrxn = 31.8 J/molK.
Is the reaction spontaneous?
DG = DH - TDS
DG = ( kJ/mol) – (298 K*.0318 kJ/molK)
DG = kJ/mol
Yes, it is spontaneous

10. DG = DH - TDS Enthalpy Entropy Spontaneous Rxn? (neg.) Negative (exo)
Positive
Yes
Positive (endo)
Only at high temperatures
Negative
Only at low temperatures No