Energy

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Energy

• Chapter 10

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10.1 The Nature of Energy

• Energy the ability to do work or produce heat
• Potential Energy due to position or composition
• Kinetic Energy due to motion
• Depends on the mass of the object (m) and its
  velocity (v)
• KE ½ mv2

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The Law of Conservation of Energy

• Energy can be converted from one form to another
  but can be neither created nor destroyed
• The energy in the universe is constant

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Work

• Work Force x distance
• W Fd
• Frictional Heating 2 surfaces in contact with
  each other
• Depends on surface and force pushing the surfaces
  together

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State Function

• The property of the system that changes
  independently of its pathway
• The pathway is how you get there
• Example
• If you travel from Chicago to Denver what are
  state functions?
• The route you take to get there is your pathway,
  so it is not a state function
• Change in elevation doesnt depend on how you get
  there so it is a state function

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10.2 Temperature and Heat

• Temperature Measure of the random motion of the
  components of a substance

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• Heat The flow of energy due to a difference in
  temperature

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10.3 Exothermic and Endothermic Processes

• System part of universe we are looking at
• Surroundings everything else
• Exothermic energy flows out of a system
• Endothermic energy flows into a system

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• Where does energy as heat come from in exothermic
  reactions?
• It depends on the potential energy between the
  products and reactants

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10.4 Thermodynamics

• Law of Conservation of Energy (a.k.a. The First
  Law of Thermodynamics)
• Energy can neither be created nor destroyed under
  normal conditions
• The energy of the universe is constant

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E internal energy

• E is the sum of the kinetic energy and the
  potential energy
• Can be changed by the flow of work, heat, or both
• ? change in called delta
• w work
• q heat
• ?E q w
• Change in internal energy equals heat plus work

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• Thermodynamic quantities are made up of a number
  that shows magnitude and a sign that shows
  whether energy is flowing into the system
  (endothermic ) or out of the system
  (exothermic - )

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10.5 Measuring Energy Changes

• calorie amount of energy required to raise the
  temperature of 1 gram of water by one degree
  Celsius
• 1000 calories (1 kilocalorie) is what we refer to
  as a Calorie with a capital C
• 1 calorie 4.184 joules
• 1 cal 4.184 J
• To go from calories to joules multiply by 4.184
• To go from joules to calories divide by 4.184

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And now for a problem!

• How much heat, in joules, is required to raise
  the temperature of 7.40 g water from 29.0 C to
  46.0 C?
• We know we need 4.184 J of energy raise 1 g of
  water 1 C
• We have 7.40 g of water so it will take 7.4 g x
  4.184 J to raise it 1 C
• We also need to raise the temperature 17 C so
  17.0 C x 7.4 g x 4.184 J/ g x C
• So we need 526 J of energy
• Now try this
• Calculate the joules of energy required to heat
  454 g of water from 5.4 C to 98.6 C?

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• So we know that the amount of energy we need to
  raise the temperature of a substance depends on
  the amount of substance and the change in
  temperature
• But the substance also plays a big part
• Specific Heat Capacity the amount of energy
  needed to raise the temperature of 1 g of a
  substance 1 C

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Specific Heats

• Liquid water 4.184 J
• Aluminum 0.89 J
• Gold 0.13 J
• This explains why certain things heat up faster
  than others
• The pot heats up faster than the water in it
• The water in the pool is colder that the cement
  around it

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Now for another equation

• The amount of energy required the specific heat
  x mass x change in temperature
• Q m x Cp x ?T
• Try this sample
• A 1.6 g sample of metal that looks like gold
  requires 5.8 J of energy to change its
  temperature from 23 C to 41 C. Is the metal
  gold? (Hint you are finding what s is and
  comparing to what you know about golds specific
  heat)
• Answer No Golds s 0.13 J/ g C but this
  substance has an s 0.20 J / g C

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10.6 Thermochemistry (Enthalpy)

• Enthalpy (symbol H) is the same as the flow of
  heat
• ?Hp heat
• P tells us it occurred under constant pressure
• ? means change in
• So the enthalpy for a reaction at constant
  pressure is the same as heat

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Calorimetry

• Calorimeter device used to determine the heat
  associated with a chemical reaction
• Reaction is run in calorimeter and temperature
  change is observed
• We can use calorimeter to find ?H
• Once we know ?H for some reactions we can use
  those to calculate ?H for other reactions

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10.7 Hesss Law

• The change in enthalpy for a given process is
  independent of the pathway for the process (this
  means it is a state function)
• Hesss Law states that the change in enthalpy
  from reactants to products in a reaction is the
  same whether it takes place in one step or a
  series of steps
• N2 2O2 ? 2NO2 ?H 68 kJ
• or
• N2 O2 ? 2NO ?H 180 kJ
• 2NO O2 ? 2NO2 ?H -112 kJ
• So 180 kJ (-112 kJ) ?H 68 kJ

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Characteristics of Enthalpy Changes

• If a reaction is reversed, ?H is reversed
• Xe 2F2 ? XeF4 ?H -251 kJ
• XeF4 ? Xe 2F2 ?H 251 kJ
• Magnitude of ?H is proportional to quantities of
  reactants and products
• Xe 2F2 ? XeF4 ?H -251 kJ
• 2(Xe 2F2 ? XeF4) ?H -502 kJ

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10.8 Quality versus Quantity of Energy

• One of the most important characteristics is that
  it is conserved
• Eventually all energy will take the form of heat
  and spread evenly throughout the universe and
  everything will be the same temperature
• This means work wont be able to be done and
  universe will be dead called heat death
• We care more about what kind of energy (quality)
  than the amount of energy (quantity)

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10.9 Energy and Our World

• Fossil Fuels formed by decaying products of
  plants
• Petroleum
• Natural Gas
• Coal
• Greenhouse Effect Visible light travels through
  atmosphere, converted to infrared radiation
  (heat) which is absorbed by certain molecules,
  H20 and CO2 mainly, which radiate it back to earth

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10.10 Energy as a Driving Force

• Energy Spread in any given process,
  concentrated energy is dispersed widely
• Happens with every exothermic reaction
• When gas is burned, energy stored is dispersed
  into surrounding air
• Matter Spread molecules of a substance are
  spread out and occupy a larger volume
• Salt dissolves in water due to matter spread
• These 2 processes are important driving forces
  that cause events to occur

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Entropy

• Invented function that keeps track of disorder
• Entropy (S) is a measure of disorder or
  randomness
• So a cube of ice has a a lower S value than steam
• Energy spread and Matter spread lead to greater
  entropy
• The entropy in the universe is always increasing