Download Energy, Heat, and Work* Oh My*

Energy, Heat, and Work… Oh My…
 Energy
– The capacity to do work
 Kinetic
Energy – Energy of Motion
 Thermal
A
Energy – Associated with Temperature
type of kinetic energy – motion of atoms and molecules
 Potential
Energy – Stored Energy
 Chemical
A
Energy – Position of electrons and nuclei
type of potential energy – changes in position change stored energy
Energy, Heat, and Work… Oh My…

joule (J) is the amount of energy needed to move a 1-kg
mass a distance of 1 meter


1 J = 1 N∙m = 1 kg∙m2/s2
calorie (cal) is the amount of energy needed to raise the
temperature of one gram of water 1°C
kcal = energy needed to raise 1000 g of water 1°C
 food Calories = kcals

Energy Conversion Factors
1 calorie (cal) = 4.184 joules (J)
1 Calorie (Cal) = 1000 cal = 1 kcal = 4184 J
1 kilowatt-hour (kWh) = 3.60 x 106 J
Energy, Heat, and Work… Oh My…

nanojoule (nJ)


160 nJ is about the kinetic energy of a mosquito
megajoule (MJ)
Kinetic energy of a 1 ton car moving at 100 mi/hr
 3.6 MJ = 1 kilowatt-hour


gigajoule (GJ)


6 GJ = chemical energy in 1 barrel of oil when burned
exajoule (EJ = 1x1018 J)
1.41 EJ = 2011 earthquake in Japan
 52 EJ = Energy release per day of the average hurricane
 94 EJ = annual energy consumption in the US

Energy, Heat, and Work… Oh My…

Heat – The transfer of thermal energy as a result of
a temperature difference
Energy, Heat, and Work… Oh My…

Heat – The transfer of thermal energy as a result of
a temperature difference

Work – Force acting through a distance
Lifting a book
 Pushing a desk
 Inflating a balloon
 Breathing

Energy, Heat, and Work… Oh My…
 Law
of Conservation of Energy
 “Energy
can be neither created nor destroyed”
Etotal @ start = Etotal @ end
 Energy
CAN be transformed from one type to another
 Energy CAN be transferred between system
 Transfer
can take place through WORK or HEAT
Energy, Heat, and Work… Oh My…
 Energy
can be transferred between systems
 System
 Stuff
– The “stuff” in which changes in energy are
being studied
= material or process
 Surroundings
– Everything with which the system can
exchange energy
System + Surroundings = UNIVERSE!
(energy of the universe must remain constant)
Energy, Heat, and Work… Oh My…
 Energy


can be transferred between systems
System – The “stuff” in which changes in energy are being studied
Surroundings – Everything with which the system can exchange energy
system
UNIVERSE
surroundings
Energy, Heat, and Work… Oh My…

Law of Conservation of Energy

“Energy can be neither created nor destroyed”
Etotal @ start = Etotal @ end

Energy can be transferred between systems
a system and its surroundings
If energy is lost by the system, what must happen?
Its surrounding must gain the same amount of energy!
If energy is gained by the system, what must happen?
Its surrounding must lose the same amount of energy!
Energy, Heat, and Work… Oh My…

When energy flows out of a system, it
must all flow into the surroundings

When energy flows out of a system,
DEsystem is ─

When energy flows into the surroundings,
DEsurroundings is +

Therefore:
─ DEsystem= DEsurroundings
Surroundings
DE +
System
DE ─
Energy, Heat, and Work… Oh My…

When energy flows into a system, it
must all come from the surroundings

When energy flows into a system,
DEsystem is +

When energy flows out of the
surroundings, DEsurroundings is ─

Therefore:
DEsystem= ─ DEsurroundings
Surroundings
DE ─
System
DE +
Energy, Heat, and Work… Oh My…
A student drops some magnesium turnings into a styrofoam coffee cup containing HCl(aq)
and uses a thermometer to monitor the reaction. Assume that the cup is a perfect insulator.
1) What is the system?
the reaction (Mg(s) + 2 H+  Mg2+ + H2(g))
gives off energy
2) What are the surroundings?
water
thermometer
air in the cup
absorbs energy
Energy, Heat, and Work… Oh My…

Energy is a “State Function”
Path independent – does not matter HOW it got there
 Matters only on the state of the system

Change in elevation is
Elv final – Elv initial
OR
D Elv = Elv final – Elv initial
Change in ENERGY is
E final – E initial
OR
D E = E final – E initial
Energy, Heat, and Work… Oh My…

Changes in Energy
Change in ENERGY is
E final – E initial
OR
DE = E final – E initial
OR
DEreaction = E products – E reactants
Conditions
DErxn sign
E prod > E react
+ (DErxn > 0)
E prod < E react
– (DErxn < 0)
Energy, Heat, and Work… Oh My…
Changes in Energy
DEreaction = E products – E reactants
Internal Energy

C(s), O2(g)
CO2(g)
energy
energy
released
absorbed
DE
DErxn
+
rxn==─
Conditions
DErxn sign
E prod > E react
+ (DErxn > 0)
Surroundings
E prod < E react
– (DErxn < 0)
System
CO
O22
C
+ 2O→C
2 →+CO
Energy, Heat, and Work… Oh My…

System and surrounding exchange energy through:

heat (thermal) energy: q

work: w
 work
and heat are NOT state functions!!
DE = q + w
q (heat)
system gains heat (+)
w (work) system gains energy from work done to it (+)
DE
system gains energy (+)
system loses heat (–)
system loses energy doing work (–)
system loses energy (–)
If the burning of the fuel in a potato cannon performs 855 J of work on the potato and
produces 1422 J of heat, what is DE for the burning of the fuel?
What is the system being investigated? Fuel
What are the surroundings? Everything else (potato, air, birds, etc.)
wfuel = – 855 J
qfuel = – 1422 J
DE = q + w
DE = –1422 J + –855 J
DE = –2277 J
If the burning of the fuel in a potato cannon performs 855 J of work on the potato and
produces 1422 J of heat, what is DE for the burning of the fuel?
What is the system being investigated? Fuel
What are the surroundings? Everything else (potato, air, birds, etc.)
wpotato = + 855 J
wpotato = - wfuel
wfuel = – 855 J
Reacting 50 mL of H2(g) with 50 mL of C2H4(g) produces 50 mL of C2H6(g) at 1.5
atm. If the reaction produces 3.1 x 102 J of heat and the decrease in volume
requires 7.6 J of work, what is the change in internal energy of the gases?
What is the system being investigated? Reaction
What are the surroundings? Everything outside the container
qrxn = – 310 J
wsur = – 7.6 J
wsur = – wrxn
wrxn = + 7.6 J
DE = q + w
DE = –310 J + (+7.6 J)
DE = –3.0x102 J