Energy
What is energy?
• Not matter, because it has no mass and
does not occupy space
• Ability to do work (work = moving an
object a distance by a force)
What are the forms of energy?
• Potential or kinetic
• Potential – stored energy;
energy an object has by
virtue of its position
• Kinetic energy – energy of
motion
What are the forms of energy?
•
•
•
•
•
•
•
Electrical
Thermal
Light (electromagnetic)
Nuclear
Chemical
Mechanical
Gravitational
Energy is interconvertible
• Potential ↔ Kinetic
Energy is interconvertible
conversion
to coal
over time
Photosynthesis converts
light to chemical energy
Electrical energy is
converted to light energy to
promote photosynthesis
As coal burns, chemical
energy is converted to
thermal and light energy
The moving turbines allow a
generator to convert the
mechanical energy to electrical
energy.
Heat causes water to change to steam so
water particles move faster and turn the
blades of a turbine gnerating mechanical
energy.
Energy Forms
• In chemistry, the forms of energy
encountered the most are thermal and
light.
• Thermal energy is related to particle
motion
• Which has the highest thermal energy?
Energy Forms
• Are heat and temperature the same?
NO!
• Heat = transfer of thermal energy
based on temperature differences
• Temperature = intrinsic property
related to the hotness or coldness of an
object; measures the effect of heat on
an object; determines direction in which
heat will be transferred
Heat Units
• Calories – amount of heat required to
raise one gram of water by 1oC
nutritional calories – 1000 chemist
calories or 1 kcal
• Joule - equal to the work required to
move a 1 kg mass against an opposing
force of 1 newton 1 J = 1 kg m2 s-2
4.184 J = 1 cal
• BTU (British thermal unit) – energy to
raise 1 pound of water by 1oF
Temperature
• Measured with a
thermometer
• Laboratory thermometers
contains a liquid that expands
as the liquid particles speed
up.
• Thermometers may work on
the expansion of metals
(thermostats).
• Ear thermometers measure
infrared energy and convert
it to a temperature.
Temperature
Scales
What is a
comfortable
room
temperature in
oC?
What is body
temperature in
oC?
Heat In and Out
When we talk about energy transfer we need to
define the “system” we are talking about.
Surroundings
Energy out =
Exothermic
SYSTEM
Energy in =
Endothermic
Surroundings
Heat In and Out
• When a system absorbs energy the
process is endothermic.
• When a system releases energy, the
process is termed exothermic.
• Of course, if a process is exothermic for
the system, it is endothermic for the
surroundings!
• What do you think is preferred by a
system- an endothermic or exothermic
process?
Heat In and Out
What processes involve energy?
• Phase changes
• Chemical changes
• Dissolving
• Heating or cooling a substance
Energy Conservation
• Sometimes it appears that a process
“loses” energy. However, the Law of
Conservation of Energy says that energy
cannot be created nor destroyed in a
normal chemical process.
So where does the energy go?
• The system releases to or absorbs energy
from its surroundings so it isn’t really lost.
How much energy is transferred?
The amount of energy transferred to or
from a system depends on three factors:
• Amount of matter affected – the more
matter, the more energy will be involved
• Nature of the matter – different
materials will absorb or release energy at
different levels depending on their
properties like particle forces and particle
mass
• Temperature change – the energy will
depend on how much the particles change
their motion
Specific Heat
• Specific heat is the amount of energy
required to raise one gram of a substance by
1oC.
Substance
Helium gas
Water
Specific Heat Substance
cal/gram.oC
1.267
Air
1.000
Concrete
Ethyl alcohol 0.588
Ice
0.480
Vegetable oil 0.478
Glass
Copper
Gold
Specific Heat
cal/gram.oC
0.244
0.210
0.201
0.092
0.031
If you put an equal amount of heat into oil
and copper, which one reach a higher
temperature? Why?
Calculating Heat Exchange
• How much energy is needed to change
the temperature of 200 grams of water
from 25oC to 40oC?
Energy =
sp.ht.
x mass
x
ΔT
Energy = 1.00 cal/g oC x 200 g x 15 oC
Energy = 3000 cal or 3.0 kcal
Heat in Phase Changes
• When a substance is boiling or freezing,
does the temperature change?
NO!
• When phase changes occur, the energy lost
or gained comes from the formation or
breaking of forces between the particles
• The amount of heat exchanged in the
phase change depends on the heat of
fusion (melting) and/or the heat of
vaporization (boiling). The units are
kcal/gram for each so the energy
exchanged depends on the mass but not
the temperature.
Cooling a gas…
Heating Curve
Chemical Changes and Electricity
• You know that in some
reactions there is an
exchange of electrons
(redox). If we
design the system
correctly we can tap
the energy of these
electrons as they
move.
• To generate energy
the reactions have to
be spontaneous.
Redox
• CuSO4 + Fe -----> Cu + FeSO4
• Cu + AgNO3  Ag + Cu(NO3)2
Metal Plating
• You can deposit layers of metal on a
surface using an electrochemical
reaction. This is a common industrial
process.
Copper plating on paperclips
Deposition of silver (plating)
Batteries
• Batteries used contained redox
reactions to move electrons from a
negative terminal or electrode to a
positive terminal.
Batteries
• Lead Storage
battery used in
vehicles
Dry and Wet Cell Batteries
• When the
electrolyte used to
conduct the
electrons has little
moisture and/or is
pasty, the battery is
called a dry cell.
• An auto battery
which has liquid acid
solution is called a
wet cell.
Battery Life
• When the chemical reaction in a battery
stops, the battery “dies”.
• If the chemical reaction used in the
batteries is reversible, the battery can
be recharged. Generally reactions that
involve gases are not reversible.
Exploded batteries
Chemical Changes and Electricity
• We can also
use flowing
electrons to
make a nonspontaneous
reaction
occur.
Hydrolysis of water