Spontaneous processes occur naturally without an external stimulus, while nonspontaneous processes require something to be done to occur. Whether a reaction is spontaneous can be determined using thermodynamics by calculating the enthalpy and entropy. Energy exists in various forms including thermal, electrical, chemical, and kinetic, and it can be transferred or changed between objects and forms. During chemical reactions, energy is either absorbed or released as bonds break and form.

1. Why changes take place
Spontaneous process
Takes place ‘naturally’ with no apparent
cause or stimulus.
Nonspontaneous process
Requires that something be done in order
for it to occur.
Spontaneous Nonspontaneous
6-1

2. When will a reaction
be spontaneous?
Spontaneity of a reaction can be determined
by a study of thermodynamics.
Thermodynamics can be used to calculate
the amount of useful work that is produced
by some chemical reactions.
The two factors that determine spontaneity
are enthalpy and entropy.
entropy
6-2

3. Energy
Energy - the ability to do work.
Work - when a force is applied to an object.
There are several types of energy:
• Thermal - heat
• Electrical
• Radiant - including light
• Chemical
• Mechanical - like sound
• Nuclear
6-3

4. Energy
Energy can be classified as:
Potential energy
Stored energy - ability to do work.
Kinetic energy
Energy of motion - actually doing work.
Energy can be transferred from one object
to another. It can also change form.
6-4

5. Kinetic vs. potential energy
Potential
Potential
Energy
Energy
6-5

6. Converting potential
to kinetic energy

7. Kinetic vs. potential energy
Kinetic
Energy
6-7

8. Energy and
chemical bonds
During a chemical reaction
• Old bonds break.
• New bonds are formed.
• Energy is either absorbed or released.
Exothermic Energy is released.
New bonds are
more stable.
Endothermic Energy is required.
New bonds are less
stable. 6-8

9. Exothermic
Reactants
Energy
Products
Since excess energy is released,
the products are more stable.
6-9

10. Endothermic
Products
Energy
Reactants
Additional energy is required
because the products are less stable.
6 - 10

11. Entropy
Entropy - a measure of the ‘disorder’ or
randomness of a system.
Disorder is favored over order and may
account for reaction occurring
spontaneously even if it is endothermic.
Increased
entropy
solid gas
6 - 11

12. Rate of change
Not all spontaneous changes take place in a
useful time period.
Some may require some initial energy to get
them started.
spark
2H2 (g) + O2 (g) 2H2O (l)
Others can be made faster by adding a
catalyst.
I-
2H2O2 (l) 2H2O (l) + O2 (g)
Kinetics - the study of the rate of a reaction.
6 - 12

13. Temperature, energy and heat
Temperature. An intensive property of a
material.
Thermal energy. Energy of motion of
molecules, atoms or ions. All materials
have this energy if at a temperature above
0 K.
Heat. Thermal energy transfer that results
from a difference in temperature. Thermal
energy flows from warm objects to cool
ones.
6 - 13

14. Law of conservation of energy
“Energy cannot be created or destroyed in a
chemical reaction.”
During a reaction, energy can change from
one form to another.
Example. Combustion of natural gas.
Chemical bonds can be viewed as potential
energy. So during the reaction:
2CH4 (g) + 3O2 (g) 2CO2 (g) + 2H2O (l) + thermal energy + light
some potential energy is converted to
thermal energy and light.
6 - 14