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## Energy (Ch. 10)

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Title: Energy (Ch. 10)

1
Energy(Ch. 10)
2
What is Energy?
10.1 Energy, Temperature and Heat
• Energy is the ability of something to do work or
make heat.

3
Forms of Energy
• Heat
• Light
• Sound
• Motion
• Electrical

4
Energy Change
• The Law of Conservation of Energy Energy cannot
be made or destroyed, but it can change from one
type to another type (of energy).
• If you ride a bicycle down a hill, you move
faster and faster. When the hill ends, you slow
down and stop. Why?
• You slow down because friction of the tires on
• The energy change Motion ? Heat

5
Work
• When you ride your bicycle down the hill, gravity
does work on you.
• Work is a force pushing or pulling on something
over distance.

6
Work Heat Energy
• Case A You ride down a hill on your bicycle and
go very fast.
• Case BYour friend uses breaks, goes slow and
stops at the bottom.
• For both people, the energy change is the same!
• Energy is a state function because it does not
depend on the pathway (how the energy change
happened).
• Speed is NOT a state function, because it depends
on

7
Temperature and Heat
• Temperature is the measure of random motions of
the things.
• Heat is the movement of energy.
• Thermal energy is the random movement of
molecules and atoms.

8
Temperature and Heat
• If you have a hot cup of coffee, after about an
hour it will cool to be the same temperature as
the room.
• What happened to the molecules in the coffee and
the air?
• At first, the molecules in the coffee are moving
faster than the molecules of air.
• The coffee molecules move slower.
• The air molecules move faster.
• The energy moves from the coffee
• to the air.

9
Exothermic Vs. Endothermic
• The system is the thing we are interested in.
• The surroundings are everything else in the
world.
• Example I want to measure the temperature change
of my cup of coffee.
• The system the coffee cup
• The surroundings everything else (the air,
chairs in the room, the students)

10
Exothermic Vs. Endothermic
• Exothermic energy being released
• Endothermic energy being absorbed

11
Exothermic Vs Endothermic REACTIONS
• To determine if a chemical reaction is exothermic
or endothermic you must look at the energy of the
whole reaction
• Exothermic Reaction OVERALL, the reaction
releases more energy than it takes in
• Endothermic Reaction OVERALL, the reaction
absorbs more energy than it releases

12
How this looks on a graph
Energy released to the surroundings
13
Does this Graph Represent an Exothermic or
Endothermic Reaction?
• Exo or Endo?
• Exothermic, because molecules have less energy
then when they started

14
Does this Graph Represent an Exothermic or
Endothermic Reaction?
• Exo or Endo?
• Endothermic, because molecules have more energy
then when they started

15
10.2 The Flow of Energy
16
Thermodynamics
• Thermodynamics is the study of energy
• The first law of thermodynamics the law of
conservation of energy the energy of the
universe is constant.
• Internal energy (E) energy of the system
• ? E q w
• ? E means the change in energy
• q heat
• w work

17
Measuring Energy Changes
• You can measure heat in joules (J) or calories
(cal)
• One calorie energy needed to raise the
temperature of one gram of water one degree
Celsius.
• 1 cal 4.184 J
• Practice Convert 44.2 cal to J

18
Thermodynamic quantities
• Thermodynamic quantities (q, w and ? E) have two
parts
• Number the size of the change
• Sign ( or )shows the direction of the flow.

19
Calculating Heat from lab data
• Energy Specific heat x Mass x Temp change
• Q sm?T
• This equation tells us how much energy was gained
by a substance (if it was heated) or lost by a
substance (if it was cooled)
• What part of this equation will tell you if
something is heated or cooled?
• Temp change (also written as ?T)

20
What is a Specific Heat?
• Specific Heat Capacity the amount of energy
required to heat 1 gram of something by 1C.
• Every substance has a different specific heat
• What is the specific heat of water
• 4.18 J/(gC) ? memorize!
• This number tells us that if we want to raise the
temperature of a container of water by 1C, then
each gram of water in the container must gain
4.18 J of energy.

21
Lets do an example
• Lets say your making some delicious Ramen
noodles for dinner mmm..mmmm good!
• The directions say to boil 2 cups of water (500
grams of water)
• If the water starts at room temp ( 20C), how
much energy must it gain to boil?

22
10.3 Thermochemistry
• Enthalpy ?H the heat for a reaction
• The units of Enthalpy are kJ
• Example If you burn one mole of CH4 the enthalpy
(?H) is 890 kJ.
• In other words
• CH4 O2 ? CO2 H2O ?H -890 kJ
• How much heat is released if you burn 5.8 grams
methane?

23
• An exothermic reaction has a negative (-)
enthalpy.
• Is this reaction exothermic or endothermic?
• CH4 O2 ? CO2 H2O ?H -890 kJ
• Yes, exothermic
• The reverse reaction would be endothermic and so
it has a positive () sign
• CO2 H2O ? CH4 O2 ?H 890 kJ

24
Calorimeter
• A calorimeter is a device used to measure the
heat in a chemical reaction.

25
Hesss Law
• Hesss Law The change in enthalpy for a reaction
is the same whether a reaction takes place in one
step or many steps.
• Example What is the change in enthalpy for
• N2 O2 ? NO2 ?H ???

26
• N2 O2 ? NO2 ?H ???
• You know
• N2 O2 ? 2NO ?H 180 kJ
• 2NO2 ? 2NO O2 ?H 112 kJ
• Can we make NO2 from N2 and O2 in two steps?
• Yes, butyou have to do the second reaction
backwards
• 2NO O2 ? 2NO2 ?H -112 kJ

27
• If we add these two reactions, we can find the ?H
of N2 O2 ? NO2
• N2 O2 ? 2NO ?H 180 kJ
• 2NO O2 ? 2NO2 ?H -112 kJ
• N2 O2 ? NO2 ?H 180 (-112) 68 kJ

28
10.4 Using Energy in the Real World