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Energy
Energy
• Energy – the ability to do work or
produce heat
• Energy exists in two different forms –
kinetic energy & potential energy
Potential Energy
• Potential energy – energy due to
composition or position of an object
• Potential energy is stored energy that
results from the attractions or repulsions
of other objects
Kinetic Energy
• Kinetic energy – the energy of motion
• Kinetic energy depends on as objects
mass & its velocity
• Atoms has mass & they are in motion;
therefore they will have kinetic energy
Energy
• A roller coaster at the top of a hill has a
great amount of potential energy.
• As the rollercoaster begins to speed
down the hill, the potential energy is
turned into kinetic energy
Energy
• The SI unit for energy is the joule (J)
• 1 J = 1 Kgm2 / s2
• Another unit of energy that you may be
more familiar with is the calorie
• calorie – amount of energy required to
raise 1 g of water 1°C
• 1 cal = 4.18 J
Energy
• The calories that you eat are actually
kilocalories or Calories (with a big C)
• 1000 calories = 1 Kilocalorie = 1 Calorie
Energy Conversions
• Convert 15,500 joules into Calories
• 15500 J x 1 cal x 1 Cal
=
•
4.18 J 1000 cal
• 3.71 Cal
Formulas – Kinetic Energy
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Kinetic energy
KE = ½ mv2
KE = kinetic energy (joules)
m = mass (must be in Kg)
V = velocity (must be in m/s)
Formulas – Potential Energy
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Potential Energy
PE = mgh
PE = Potential Energy (J)
m = mass (Kg)
g = gravitational constant = 9.8 m/s2
h = height (m)
Formulas - Work
• Work (w) – the energy used to move an
object against a force
• Force (f) – a push or pull on an object
• W = mgd = fd = PE
• Work and potential energy can be
looked at in the same light
Work
• It is important to understand that if there
is no movement, there is no work done
• If I push and push on the demonstration
table with all of my might. I may get hot
and sweaty and feel like I have done a
TON of work, but in reality I have done
NO work because the table has not
moved
Examples
• A bowler lifts a 5.4 kg bowling ball 1.6m
and then drops it to the ground.
• How much work was required to raise
the ball?
• W = mgd
• W = (5.4 kg)(9.8 m/s2)(1.6m)
• 85 Kgm2/s2 = 85 J
Examples
• How much potential energy does that
ball have at this height?
• 85 J
Examples
• If the bass is dropped and we assume
that all of the potential energy is turned
into kinetic energy, at what velocity will
the bowling ball hit the ground?
• KE = PE = 85J
• m = 5.4 Kg
• V=?
Examples
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KE = ½ mv2
85 J = ½ (5.4 Kg) v2
v2 = 31.5
v = 5.6 m/s
More examples
• What is the kinetic energy of 1 atom of
Ar moving at 650 m/s?
• KE = ½ mv2
• 1atom Ar x
1mol Ar
x 39.95 g Ar x 1 Kg Ar
=
•
6.02 x 1023 atoms
1 mol Ar
1 x 103 g Ar
• 6.64 x 10-26 kg Ar
More Examples
• KE = ½ mv2
• KE = ½ (6.64 x 10-26)(6502)
• KE = 1.4 x 10 -20 J
1st Law of Thermodynamics
• 1st Law of Thermodynamics – energy is
conserved
• The law of conservation of energy
states that in any chemical reaction or
physical process, energy can be
converted from one form to another, but
it is neither created nor destroyed.
1st Law of Thermodynamics
• Since energy can neither be gained nor
lost, the change in E can be calculated
using:
• E = Ef – Ei
• In a chemical reaction i indicates
reactants and f indicated products
E
•
E has 3 parts:
1. A # indicating the magnitude
2. A sign (+/-) indicating the direction
3. A unit
Thermochemistry
• Thermochemistry is the study of heat
changes that accompany chemical
reactions and phase changes.
• In thermochemistry, the system is the
specific part of the universe that
contains the reaction or process you
wish to study.
Thermochemistry
• Everything in the universe other than
the system is considered the
surroundings.
• Therefore, the universe is defined as
the system plus the surroundings.
universe = system + surroundings
Relating E to heat & work
• The system can exchange energy with
its surroundings in 2 ways: as heat or
work
• E = q + w
• E = change in energy
• q = heat
• w = work
q&w
• Don’t forget q & w must have signs
• In order to get the sign you must look at
the system as a box and the
surroundings as everything else
System
Surroundings
q&w
• Anything going INTO the box will be +
• Anything going OUT of the box will be –
+
-
q&w
• If heat is transferred from the
surroundings to the system and work is
done on the system what are the signs
for q & w?
q=+
w=+
q&w
• If heat is lost to the surroundings and
work is done on the system what are
the signs for q & w?
q=-
w=+
Summary for q & w
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q + = heat into system
q - = heat into surroundings
w + = work done on the system
w - = work done on the surroundings
Examples
• A system loses 1150 J of heat to the
surroundings and does 480 J of work on
the surroundings. Calculate E.
• E = q + w
• E = (-1150J) + (-480J)
• E = -1630 J
Examples
• A system absorbs 140 J of heat from
the surroundings and does 85 J of work
on the surroundings. Calculate E.
• E = q + w
• E = (+ 140J) + (-85J)
• E = + 55 J
Endothermic & Exothermic
• Endothermic
– system absorbs heat
– Heat flows into the system
– Temperature goes down
• Exothermic
– Heat flows out of the system and into the
surroundings
– Temperature goes up
• Only look at heat (q) to determine if the
system is endo or exo