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Energy
Energy: Ability to do Work
Potential Energy = Energy of position.– Also called STORED ENERGY.
Kinetic Energy = Energy of motion.
Radiant = Electromagnetic energy.– Ex: Sunlight
Kinetic Energy
K.E. = ½ X Mass X Velocity2 = ½ mV2
Potential Energy
Kleenex Box Spring Rubberband Popper
All these can have P.E. = energy of position = stored energy
Potential Energy can be converted to Kinetic Energy
Electromagnetic Radiation
Sunlight – Visible radiation Ultraviolet radiation Infrared radiation Gamma rays X-rays Microwaves Radiowaves
Applet spectrum
(The speed of light)
Magnets
The potential energy in the system of two magnets depends on their…
Types of Energy
K in e tic P o ten tia l
M ech an ica l
C h em ica l E lec trica l M ag n etic R ad ian t
N on -m ech an ica l
E n erg y
(Not a complete list!)
Units of Energy
SI system: unit of energy is the JOULE. (J) 1 Joule = amount of energy required to lift
a golf ball 1 meter.
Other Units: calorie, Calorie, BTU’s 1 calorie = 4.18 Joules 1 Calorie = 1000 calories = 1 kilocalorie
Energy in Chemistry
Chemical energy– energy stored in bonds
Heat – a form of ENERGY that flows from a warmer
object to a cooler object. (Macroscopic)– ENERGY associated with the motion of
atoms & molecules in matter. (Microscopic)
Bonding PE Curve
Separated atoms / High PE
o o
o—oStable Bondmin PE
oo nuclei approach each other high PE
Energy stored in chemical bonds
Stability and Energy
If PE is high, stability is low.– You standing on the edge of a cliff– Two highly separated atoms
If PE is low, stability is high.– You standing at the bottom of the cliff– Two atoms joined together in a bond
Heat Energy
Symbol for heat energy = Q or q.
Heat depends on the amount of substance present.
We measure changes in heat.
Temperature
A measure of the average kinetic energy of the particles of a substance.
Swimming Pool vs. Teacup
Temperature is NOT energy.– Temperature does not depend on amount of
substance; energy does.
Changing KE with Temperature
At higher temperatures molecules are moving at higher velocities
273°
373°
473°Faster molecules
At a given temperature, molecules have a distribution of velocity.
Slower molecules
Law of Conservation of Energy
Energy is neither created nor destroyed in an ordinary chemical or physical change.
Energy before = Energy afterEnergy before = Energy after
Energy Energy cancan be be convertedconverted from one from one form to another.form to another.
- potential to kinetic- potential to kinetic - radiant to electric- radiant to electric- electric to heat- electric to heat - chemical to - chemical to kinetickinetic- chemical to electrical- chemical to electrical
All physical and chemical changes are accompanied by…
So chemists are interested in energy changes. Thermochemistry!
Energy ChangesEnergy Changes
Energy Transfer
Measure changes in heat. – That is, the amount of energy transferred from one
substance to another.
You can measure the energy lost somewhere …or the energy gained somewhere else.
However, you cannot measure the absolute heat content of a system.
Energy of Universe is Conserved!
Environment
System
Energy
Energy can move
between the systemsystem and
the environmentenvironment
.
Perspective
When we talk about energy changes, we need a convention because direction is important.
Labels are from the system’s perspective!
Exothermic Change
System releases heat to the environment– What happens to the temperature of the
environment?
EXO - energy leaves system (exits).
What happens to the energy level of the system?– What happens to temperature of system?
Environment
System
Energy
Exothermic Change
TemperatureTemperatureof of
environmentenvironment
TemperatureTemperature of system of system
Exothermic Change
System has a net loss in energy!
Environment has a net gain in energy!
Energy lost = Energy gainedEnergy lost = Energy gained
Endothermic Change
System absorbs heat from environment– What happens to temperature of environment?
Endo - Energy enters system
What happens to the energy level of the system?– What happens to temperature of system?
Endothermic Change
Environment
SystemEnergy
Temperature Temperature of of
environment environment
Temperature Temperature of system of system
Endothermic Change
System has a net gain in energy!
Environment has a net loss in energy!
Energy lost = Energy gainedEnergy lost = Energy gained
Heat Flow
Heat flows from hotter object to cooler object.
Cold pack on leg: Heat flows from the leg to the cold pack!– Leg cools down; cold pack warms up.
Heat Transferred
Quantity of Heat Transferred depends on:
– Temperature change– Mass of substance– Specific Heat of substance
Specific to a particular substance
Specific Heat
The amount of heat required to raise the temperature of one gram of a substance by one degree C.
Symbol Cp
Specific heat is a physical constant. – Different for each pure substance.
Specific Heat Constants
Material Cp Joules/gram deg C
Silver 0.230
Gold 0.129
Graphite 0.720
Diamond 0.502
Copper 0.385
Brass 0.377
Mercury 0.139
Water 4.184
Ice 2.01
Ethanol 2.46
Calculating Heat Transferred
Q = mCT
Simple system: Pure substance in a single phase. To calculate heat gained or lost, use:
Q = amount of heat transferredQ = amount of heat transferredm = mass of substancem = mass of substanceC = C = specific heat capacityspecific heat capacity of the of the substance.substance.T = temperature change = TT = temperature change = Tfinalfinal – T – Tinitialinitial
