Energy Around Us

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Energy Around Us
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Energy

• Energy ability to do work or produce heat
• Potential Energy energy due to position or
  chemical composition (bonds)
• Kinetic Energy energy due to motion
• KE ½ mv2
• Law of Conservation of Energy energy is neither
  created nor destroyed, merely converted from one
  form to another.

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Conservation of Energy

• Example Rolling a ball down a hill.
• a- What energy is there initially?
• b- What is the energy in the final positions?
• c- Where is the energy lost by A?
• Ball B moved from a lower to higher position and
  therefore gained potential energy. This means
  that work must have been done on B to give it
  energy.
• ? definition Work is force acting over a
  distance.

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• So where is the remaining energy?
• ? the answer is in the interaction between the
  hills surface and the ball!
• A will always lose the same amount of potential
  energy. Sometimes a lot of it will be
  transferred to ball B however, some energy will
  be lost to the frictional heating of the hill.
  If the hill is very rough, more work is done in
  the rolling, producing more heat.

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• ? The work and heat produced depend on the
  pathway.
• Energy change is independent of pathway,
  while heat and work are dependent on pathway.
• State function A property of a system that
  changes independently of its pathway.

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State Functions

• State function A property of a system that
  changes independently of its pathway.
• EX Take a trip from Chicago to Denver. Which is
  a state function?
• Distance traveled
• Change in elevation

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Energy

• Temperature measure of the random motions of
  the components of a substance (average velocity)
• Thermal energy the total energy that a
  substance contained due to the random motion.

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Figure 10.2 Equal masses of hot and cold water.
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Heat

• Heat the flow of energy due to a temperature
  difference.
• What will eventually happen? The two water
  samples will reach the same temperature.
• The final temp will be an average of the two
  original temps.

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Exothermic and Endothermic Reactions

• Exothermic energy flows out of the system
• Endothermic heat flows into a system
• So we say that a process that results in the
  evolution of heat is exothermic while a process
  that absorbs energy from the surroundings is said
  to be endothermic.

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• Total energy is conserved and energy flows from
  the system into the surroundings in an exothermic
  reaction. Energy gained by the surroundings must
  be equal to the energy lost by the system.
• Match burned match has lost potential energy
  (potential energy stored in the bonds of the
  reactants)
• In any exothermic reaction, some of the potential
  energy stored in the chemical bonds is converted
  to thermal energy (random kinetic energy) via
  heat.

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Figure 10.5 The energy changes accompanying the
burning of a match.Exothermic change
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Figure 10.3 H2O molecules in hot and cold water.
System Cold Water Surroundings Hot
water Endothermic process heat is absorbed
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Endothermic Reaction
Potential Energy
SYSTEM
SURROUNDINGS
Products
Energy is absorbed from surroundings
?PE
Reactants
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Thermodynamics

• When we heat a substance to a higher temperature,
  we increase the motions of the components of the
  substance- we increase the thermal energy of the
  substance.
• Different materials will respond differently to
  being heated.

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Measuring Energy

• Calorie the amount of energy (heat) required to
  raise the temperature of one gram of water by one
  Celsius degree.
• When have you heard of the term calorie?
• Joule the SI unit for measuring energy
• 1 calorie 4.184 joules abbreviated
• 1 cal 4.184 J
• Ex Express 60.1 cal of energy in units of
  joules.
• 251 J

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• Think about heating a substance from one
  temperature to another.
• How does the amount of the substance affect the
  energy required?
• In 2g of water, there are twice as many molecules
  as in 1g of water. Will it take the same amount
  of energy? No, it takes twice as much energy to
  change the temperature of 2g of water by 1 oC
  because we must change the motions of twice as
  many molecules.
• Similarly it takes twice as much energy to
  change a given sample of water by 2 degrees as it
  does to raise the temperature by 1 degree.

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Example

• Ex Determine the amount of energy (heat) in
  joules required to raise the temperature of 7.40
  g water from 29ºC to 46ºC.
• What we know about raising water temp
• It takes 1 calorie to move 1 g up 1ºC
• Account for the mass of our sample
• 7.40 calories to raise 7.40 g up 1ºC
• Account for the temp change for our sample
• 7.40 calories x 17ºC
• Summarized calculation 125.8 calories

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On what does heating depend?

• So far
• We have seen that the energy (heat) required to
  change the temperature of a substance depends on
• 1. The amount of substance being heated
  (number of grams)
• 2. The temperature change (number of degrees)

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• One more important thing the identity of the
  substance!
• The numbers we have are for water, but different
  substances react differently to being heated.

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Specific Heat

• Some substances require relatively large amounts
  of energy to change their temperatures, whereas
  others require relatively little. Chemists
  refer to this as different heat capacities.
• The amount of energy required to change the
  temperature of one gram of a substance by one
  Celsius degree is called the specific heat
  capacity, or the specific heat.
• The specific heat for water is very high compared
  to those of other substances.
• Where have you encountered this in your life?

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How to calculate

• Q s x m x DT
• Where
• Q energy (heat) required
• s specific heat capacity
• m mass of the sample in grams
• DT change in temperature in Celsius
  degrees
• This equation always applies when a substance is
  being heated (or cooled) and no change of state
  occurs.

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Calorimetry

• A calorimeter is a device used to determine the
  heat associated with a chemical reaction.

• The reaction is run in the calorimeter and the ?T
  of the calorimeter is observed. Knowing the ?T
  and and the s of the calorimeter enables us to
  calculate the heat energy released or absorbed by
  the reaction.

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Quality vs. Quantity of Energy

• Remember
• The total energy of the world is constant
• Energy crisis
• Not about quantity of energy
• About quality of energy

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Example Chicago to Denver

• When driving, you put gasoline in your car
• The energy stored in the bonds of the gasoline
  and the oxygen that reacts with it is changed to
  thermal energy.
• This gets spread along the highway.
• Total energy stays the same
• We take concentrated energy and spread it out.

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Fossil Fuels

• Fossil fuel A fuel that consists of
  carbon-based molecules derived from decomposition
  of once-living organisms
• Examples
• Coal
• Petroleum
• Natural gas

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Petroleum

• Petroleum is a thick, dark liquid that is made of
  hydrocarbons.
• Hydrocarbons are made of the elements carbon and
  hydrogen
• Petroleum is usually a liquid that contains
  hydrocarbon chains that range from 5 to 25
  carbons
• C1 C4 are gases at room temperature
• C5 C20 are liquids at room temperature
• C20 and above are solids at room temperature
• To use petroleum efficiently, we must boil it so
  that we can separate the different length chains
  from each other

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Fractional Distillation

• The lighter molecules are being boiled off,
  leaving the heavier molecules behind.

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Why do we use this for heat?

• The bonds store a lot of potential energy
• Write a combustion reaction for hexane (C6H14)
• O2, CO2, and H2O will be in this reaction as a
  reactant or a product!
• 2C6H14 19O2 ? 12CO2 14 H2O Energy
• Exothermic!

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