Download Energy - Chemistry

Energy
Chapter 16
Energy: Ability to do Work
Potential Energy (PE) = Energy of position

aka STORED energy
Kinetic Energy (KE) = Energy of motion
Radiant Energy = Electromagnetic radiation

Ex: Sunlight
Types of Energy
Energy
Mechanical
Kinetic
Potential
(Not a complete list!)
Non-mechanical
Chemical Electrical
Magnetic
Radiant
Units of Energy
 SI
1
system - unit of energy is JOULE (J)
Joule ≅ amount of energy required to
lift 1 golf ball about 1 meter

other energy units:
calorie, Calorie, BTU

1 calorie = 4.18 Joules

1 Calorie = 1000 calories = 1 kilocalorie
Kinetic Energy
 KE
 So
= ½ x mass x velocity2 = ½ mv2
KE of matter depends on:
how heavy and how fast
Potential Energy
 Stapler
 Rubberband
 popper
 anything
can have PE
= energy of position
= stored energy
 PE
can be
converted to KE
Magnets
PE
in the system of 2
magnets depends on their
relative position


when magnets close together they will
pull together due to attraction
when magnets far apart they can’t
attract each other
Electromagnetic Radiation
 Sunlight
– Visible radiation
 Ultraviolet radiation
 Infrared radiation
 Gamma rays
 X-rays
 Microwaves
 Radiowaves
Applet spectrum
Energy in Chemistry
chemical
energy is energy stored
within chemical bonds
heat
is form of energy that flows
from warmer object to cooler
object
(Macroscopic)
Heat Energy
heat:
energy associated with motion of
atoms/molecules in matter
(Microscopic)
symbol
for heat energy = Q or q
Heat Energy
heat
depends on amount of
substance present
we
can only measure heat
changes
Temperature
= measure of average KE of particles
in substance
 Swimming
pool of water vs. mug of water
 temperature

is NOT energy
temperature does NOT depend on amount of
substance; energy does
Law of Conservation of Energy
 energy
is neither created nor destroyed
in ordinary chemical or physical change
energy before = energy after
Energy can be converted
from one form to another
- potential to kinetic
- radiant to electric
- electric to heat
- chemical to kinetic
- chemical to electrical
All physical & chemical
changes are accompanied by
change in energy
The chemistry of energy changes
is known as Thermochemistry!
Energy Transfer
 measure
changes in heat
 amount
energy transferred from
one substance to another
 can
measure energy lost somewhere or
energy gained somewhere else
 cannot measure absolute heat content of
system
Energy of Universe is conserved
Universe
EnvironmentEnvironment
System
Energy
energy can
move between
system and
environment
Exothermic Change
 system

releases heat to environment
what happens to temperature of environment?
 EXO
- energy leaves system (exits)
 what
happens to energy level of system?

what happens to temperature of system?
EXO - energy leaves system
(exits)
temperature of
environment 
Environment
temperature of
system 
System
Energy
Exothermic Change
system:
net energy loss!
environment: net energy gain!
energy
lost = energy gained
Endothermic Change
 system

absorbs heat from environment
what happens to temperature of environment?
 Endo
 what
- Energy enters system
happens to energy level of system
 what happens to temperature of system?
Endo - Energy enters system
(entrance)
temperature of
environment 
Environment
System
Energy
temperature of
system 
Endothermic Change
system
- net energy gain!
environment - net energy loss!
energy
lost = energy gained
Heat Flow
heat
flows from hotter object
to cooler object
cold
pack on leg: heat flows
from leg to cold pack!

leg cools down; cold pack warms up
Quantity of heat transferred
quantity
(amount) of heat
transferred depends on
temperature change
 mass of substance
 specific Heat of substance

Calculating Heat Transferred
simple system:
•pure substance in single phase
•calculate heat gained or lost using:
Q = mCT
Q = amount of heat transferred
m = mass of substance
C = specific heat capacity of the substance.
T = temperature change = Tfinal – Tinitial
Specific Heat
 amount
heat energy required to
raise temp of 1 gram of substance by 1oC
 symbol
=c
 specific
heat = a physical constant
 unique
for each pure substance
Calorimeter:
used to measure
heat changes
other examples
source
Calorimetry
 changes
in heat energy are measured by
calorimetry
 “universe”
contained in styrofoam cup
 “enviroment”
 “system”
is water****
is whatever put in water
Calorimetry
 energy
lost = energy gained
 difficult
to monitor “system”
 easy to monitor “environment” (water)
 energy
lost/gained by environment =
energy gained/lost by system
Calorimetry
10 grams of NaOH is dissolved in 100 g of
water & the temperature of the water
increases from 22C to 30C.
 was
dissolving process endothermic or
exothermic

how do you know?
Exothermic – temperature of environment ↑
Dissolving
 What’s
happening when NaOH dissolves?
Add H2O
molecules close together,
not interacting
molecules pulled apart &
interacting with H2O
Calorimetry
calculate energy released by NaOH as it
dissolves in water
energy lost by NaOH = energy gained by water

• easier to calculate from H2O perspective
Q = mCT
Q = energy (joules)
m = mass (grams)
c = specific heat capacity (Table B)
T = temperature change = Tf - Ti
Calorimetry & Q = mCT
 temperature
22C to 30C
 30C
 what
of water increased from
-22C = 8C = T
mass to use? temp change was for
water, so want mass of water
m = 100 g
 same goes for specific heat capacity;
calculate heat absorbed by water
cH 0 = 4.18J/g
2
Q = mCT
Q
= (100 g)(4.18 J/g)(8C)
Q
= 3344 Joules
Stability and Energy
 if
energy is high, stability is low
 if
energy is low, stability is high