Transcript energy - New York Science Teacher

ENERGY
*FORMS
*KINDS
*PHASE DIAGRAMS
*PE DIAGRAMS
Kenneth E. Schnobrich
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.
FORMS OF ENERGY
Mechanical
Chemical
Thermal (Heat)
Radiant (Light)
Interconvertibility
Electrical
Nuclear*
*Nuclear changes are not a normal part of the
chemical reactions we will study.
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
KINETIC ENERGY
On a macroscopic 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
POTENTIAL ENERGY
On an everyday 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
PHASE DIAGRAMS
Phase diagrams are used to show the energy changes as
we convert from one phase to another phase
Temperature (oC)
Gas
Liquid
Gas
Liquid
Solid
Liquid
Solid
Time
PHASE DIAGRAMS
Here we are showing changes in potential and kinetic energy
D KE
Temperature (oC)
Endothermic
D PE
D KE
D PE
Exothermic
D KE
Time
PHASE DIAGRAMS
Energy calculations taken from the phase diagram
Temperature (oC)
Q = m(C)DT
Q = m(Hv)
Q = m(C)DT
Q = m(Hf)
Q = m(C)DT
Time
Some Calculations:
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 = 1045.00 J
Some Calculations:
Calculate the energy in Joules to convert 50 grams of ice
at 0oC to water at 0oC?
- The heat of fusion of ice is 336.0 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
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) = 4.18 J/goC
Hf = 336 J/g
Hv = 2260 J/g
MP = 0oC
BP = 100oC
*Think about the phase diagram
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
PE
Diagram(Exothermic)
Potential Energy
Activated
Complex
Activation
Energy (forward)
Activation
Energy (reverse)
Heat of
Reactants
Heat of Rx
(DH) = Heat of
Products
Reaction Pathway
PE
Diagram(Exothermic)
Potential Energy
Activated
Complex
Activation
Energy (forward)
Activation
Energy (reverse)
Heat of
Products
Heat of Rx
(DH) = +
Heat of
Reactants
Reaction Pathway
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
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
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 -7000 J the value for ∆G will be +2000 J, this
means that the reaction (change) is not spontaneous
∆G = ∆H – T∆S = (-5000 J) – (-7000 J) = +2000 J
There are times when the driving forces work in
opposition to one another.