1. ENERGY *FORMS *KINDS *PHASE DIAGRAMS *PE DIAGRAMS
Kenneth E. Schnobrich

2. Energy: Physical & Chemical Changes
In our discussion of matter and its changes
we discussed both physical and chemical
change. As we convert a solid to a liquid or
a liquid to a gas there is an energy change
that takes place.
In our discussion of chemical changes it is
important to remember that bonds are being
made and broken as we form new substances.
This making and breaking of bonds involves
energy.

3. FORMS OF ENERGY Mechanical Chemical Thermal (Heat) Radiant (Light)
Electrical
Nuclear*
Interconvertibility
*Nuclear changes are not a normal part of the
chemical reactions we will study.

4. FORMS OF ENERGY
Kinetic Energy – energy of motion (temperature is a measure of average kinetic energy)
Potential Energy – stored energy or energy of position. The energy a system has as a result of forces interacting

5. KINETIC ENERGY On a macroscopic level: On a molecular or atomic level
A car rolling down a hill
A skier skiing downhill
A book falling off a desktop
On a molecular or atomic level
Translational motion
Rotational motion
Vibrational motion

6. POTENTIAL ENERGY On an everyday level On atomic level
Stretching a rubber band
Falling off a ladder
Jumping down a series of stairs
On atomic level
The interaction of the protons & electrons
The energy stored in a chemical bond
Changing phase solid to liquid to gas

7. PHASE DIAGRAMS Phase diagrams are used to show the energy changes as
we convert from one phase to another phase
Gas
Liquid
Gas
Temperature (oC)
Liquid
Solid
Liquid
Solid
Time

8. PHASE DIAGRAMS
Here we are showing changes in potential and kinetic energy
D KE
Endothermic
D PE
Temperature (oC)
D KE
D PE
Exothermic
D KE
Time

9. PHASE DIAGRAMS Energy calculations taken from the phase diagram
Q = m(C)DT
Q = m(Hv)
Temperature (oC)
Q = m(C)DT
Q = m(Hf)
Q = m(C)DT
Time

10. Some Calculations: Q = m(C)DT Q = (50g)(2.09 J/goC)(10oC)
Calculate the energy in Joules to convert 50 grams of ice
at -10oC to 0oC?
The specific heat capacity (C) of ice is 2.09 J/goC
The mass (m) is 50 grams
The change in temperature (DT) is 10oC
Q = m(C)DT
Q = (50g)(2.09 J/goC)(10oC)
Q = J

11. Some Calculations: Q = m(Hf) Q = (50g)(336.0J/g) Q = 16,800 J
Calculate the energy in Joules to convert 50 grams of ice
at 0oC to water at 0oC?
The heat of fusion of ice is J/g
The mass (m) is 50 grams
There is no change in temperature (DT) – phase change
Q = m(Hf)
Q = (50g)(336.0J/g)
Q = 16,800 J

12. Here is one for you:
What is the total amount of energy needed to convert 10g of water from -20oC to 120oC?
Cp of H2O(s) = 2.09 J/goC
C pof H2O(g) = 2.09 J/goC
Cp of H2O(l) = J/goC
Hf = 336 J/g
Hv = 2260 J/g
MP = 0oC
BP = 100oC
*Think about the phase diagram

13. POTENTIAL ENERGY DIAGRAMS
In cases where there is an actual chemical reaction, PE diagrams are useful tools to help understand what is happening energywise as we convert reactants to products.
Heat Content (Enthalpy) of Reactants
Heat Content (Enthalpy) of Products
Activation Energy (forward & reverse rx)
Activated complex (Intermediate)
Heat of Reaction (DH)
* Reactions generally do not occur in a single step

14. PE Diagram(Exothermic)
Activated
Complex
Activation
Energy (forward)
Potential Energy
Activation
Energy (reverse)
Heat of
Reactants
Heat of Rx
(DH) = -
Heat of
Products
Reaction Pathway

15. PE Diagram(Exothermic)
Activated
Complex
Activation
Energy (reverse)
Activation
Energy (forward)
Heat of
Products
Potential Energy
Heat of Rx
(DH) = + -
Heat of
Reactants
Reaction Pathway

16. Driving Forces
Remember, there are two fundamental driving forces for changes that occur –
ENERGY
ENTROPY
Energy – if it is at all possible, systems like to change from a higher energy state to a lower energy state. From an energy standpoint exothermic processes are favored.
Entropy – entropy describes the relative disorder of a system. It is natural for systems to seek a state of higher entropy (greater disorder). Entropy is temperature dependent, so we frequently speak of T∆S
If T∆S is + it means that the entropy of the system (relative disorder) is increasing – this is a favored change
If T∆S is – it means that the entropy of the system (relative disorder) is decreasing – this is not a favored change

17. Driving Forces For energy changes –
if ∆H = - it means that the change is exothermic and that represents a favored change
if ∆H = + it means the change is endothermic and that does not represent a favored energy change
Remember that ∆H represents the change in energy and is also called the Enthalpy change
If both the change in Entropy (T∆S) and the change in Enthalpy (∆H) are favorable the change will be spontaneous (T∆S = + and ∆H = -)
For other cases it is helpful to calculate what we call the change in Free Energy (∆G). ∆G = ∆H – T∆S
If ∆G = - it means the reaction will still be spontaneous
If ∆G = + it means that energy will be required to bring about the change

18. Driving Forces
if ∆G = 0 it means the system has reached a point where the two driving forces are equal and the system has reached equilibrium.
Entropy like Enthalpy has units associated with it and they are generally J/K or J/mol•K (they are sometimes called eu’s – entropy units)
If the ∆H for a reaction is – 5000 J and the value for T∆S is J the value for ∆G will be J, this means that the reaction (change) is not spontaneous
∆G = ∆H – T∆S = (-5000 J) – (-7000 J) = J
There are times when the driving forces work in opposition to one another.