Natural Limits on Chemical Technologies

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Natural Limits on Chemical Technologies

• Laws of Thermodynamics, Conservation of Mass,
  Rates of Reactions, Chemical Equilibrium

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(No Transcript)
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Restrictions on Matter Law of Conservation of
Mass

• Law of Conservation of Mass - during an ordinary
  chemical reaction, matter is neither created nor
  destroyed
• Means that raw materials must be continuously
  expended and that waste products must be
  continuously recycled back into the environment

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Restrictions on Energy First Law of
Thermodynamics

• First Law of Thermodynamics - during an ordinary
  chemical reaction, energy is neither created nor
  destroyed, but can be changed from one form to
  another
• Means that energy resources must be continuously
  expended and degraded energy must be continuously
  recycled back into the environment

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Restrictions on Energy 2nd Law of Thermodynamics

• Second Law of Thermodynamics - all spontaneous
  changes result in a net increase in the disorder
  (entropy) of the universe
• A spontaneous change is one that can occur
  without outside intervention
• A non-spontaneous change requires outside
  intervention in order to occur

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Continued...

• A decrease in disorder in one part of the
  universe results in a greater increase in
  disorder somewhere else in the universe
• In any process in which energy changes from one
  form to another, part of the energy is converted
  to heat so as to increase disorder (Nature's heat
  tax)

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Continued...

• The energy that is converted to heat is degraded
  energy since it readily disperses into the
  surroundings and ultimately becomes unavailable
  energy (escapes from planet Earth into space)
• All technologies involve spontaneous changes and
  therefore degrade energy to heat

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Efficiencies of Technologies

• 1st Law states that energy is conserved but may
  be converted from one form to another
• 2nd Law implies that some energy is degraded to
  heat during a conversion
• Due to 2nd Law "heat tax", the output of useful
  work will always be less than the energy input -
  indicated as -efficiency

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Hinrichs, Energy, 2nd, Saunders, NY,1996, 78-79
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Hinrichs, Energy, 2nd, Saunders, NY,1996, 85
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Miller, Living in Environment, 12th,
Brooks/Cole,CA, 2002, 66
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Rates of Reactions

• Rate of reaction is the amount of product formed
  per unit time
• Collision Theory - for a reaction to occur,
  reactant molecules having Ea must collide with
  each other
• Factors that affect rates alter the collision
  frequency and/or the fraction of collisions
  having Ea

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Surface area - increasing surface area of a solid
increases collision frequency with gases or
liquids
Concentration - increasing concentration
increases collision frequency and hence increases
rate
http//www.wpbschoolhouse.btinternet.co.uk/page03/
3_31rates.htm
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Temperature - increasing temperature increases
collision frequency and fraction of molecules
having Ea thus increases rate of reaction
Increasing temperature by 10 degrees C doubles to
triples rate of reaction
http//www.wpbschoolhouse.btinternet.co.uk/page03/
3_31rates.htm
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Increasing temperature increases fraction of
molecules having the energy of activation
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Catalyst - presence of catalyst increases rate by
lowering the Ea
http//www.wpbschoolhouse.btinternet.co.uk/page03/
3_31rates.htm
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Restrictions on Technologies Rates of Reactions

• Limits the choice of chemical reactions that can
  be utilized in a technology to those that are
  fast enough to fulfill the demand
• Range of possible reactions can be expanded by
  manipulating the reaction conditions of
  temperature, concen-tration, surface area, and
  catalyst(best)

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Chemical Equilibrium

• Chemical equilibrium is the condition which
  exists when the concentrations of reactants and
  products no longer change with time

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Attaining Chemical Equilibrium
McMurray/Fay, Chemistry,Prentice Hall, NJ, 1995,
497
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Equilibrium

• Most chemical reactions involve two reactions
  one is the reverse of the other (called
  reversible reactions)

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Rates of Reactions and Equilibrium

• As the forward reaction occurs, the rate of
  forward reaction decreases and the rate of the
  reverse reaction increases due to changes in
  concentrations
• Equilibrium is attained when the rate of forward
  reaction rate of reverse reaction, resulting in
  no further net changes in concentrations

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McMurray/Fay, Chemistry,Prentice Hall, NJ, 1995,
498
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Yields of Equilibrium Reactions

• When equilibrium is established, there will be
  constant amounts of both reactants and products
  left in the system
• The relative amounts of reactants and products at
  equilibrium depend on their relative stabilities
  making equilibrium yields of less stable products
  low

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Equilibrium as a Restriction on Chemical
Technology

• Equilibrium limits the percent yields of
  reactions, yield
• (100)(actual yield/calculated yield)
• The percent yield of the previous reaction
  (100)(0.0125/0.08) 16 which would not be cost
  effective in an industrial process

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Best Practices for Chemical Technologies

• Minimize by-products requiring disposal
• Carry out reactions as near to ambient
  temperature and pressure as possible
• Use a catalyst to speed up a slow reaction
• Use preparative reactions that have favorable
  yields at equilibrium

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Current Technologies to be Discussed in This
Course

• Food
• Materials - Clothing and Shelter
• Energy and Transportation
• Water and Air Quality