Energy (Ch. 10)

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Energy(Ch. 10)
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What is Energy?
10.1 Energy, Temperature and Heat

• Energy is the ability of something to do work or
  make heat.

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Forms of Energy

• Heat
• Light
• Sound
• Motion
• Electrical

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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 road changed your energy to heat.
• The energy change Motion ? Heat

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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.

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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

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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.

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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.

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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)

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Exothermic Vs. Endothermic

• Exothermic energy being released
• Endothermic energy being absorbed

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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

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How this looks on a graph
Energy released to the surroundings
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Does this Graph Represent an Exothermic or
Endothermic Reaction?

• Exo or Endo?
• Exothermic, because molecules have less energy
  then when they started

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Does this Graph Represent an Exothermic or
Endothermic Reaction?

• Exo or Endo?
• Endothermic, because molecules have more energy
  then when they started

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10.2 The Flow of Energy
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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

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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

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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.

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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)

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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.

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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?

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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?

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• 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

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Calorimeter

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

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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 ???

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• 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

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• 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

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10.4 Using Energy in the Real World

• Read p339-349 and answer questions 1-7 on p350
• Go to this websites
• http//www.adventuresinenergy.org/ - What are
  Oil and Natural Gas? and Refining
  Distillation
• http//science.howstuffworks.com/oil-refining4.htm
  
• And answer the questions on the worksheet,
  Petroleum Assignment

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