Chap 16

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• Chap 16
• Equilibrium
• Chap 22
• Kinetics

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What is the focus of chemical kinetics

• To study what effects the rate of rxn

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What is a reaction rate?

• How fast reactants disappear (or products) form

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What equation is used to find average rate of
reaction?

• change con/change in time?con? T

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• What conditions are necessary for a rxn to occur

• particles must collide
• They must have a minimum energy
• they must have the proper orientation

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What is this called collectively

• Collision Theory

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What is minimum energy

• Activation energy
• (Ea) (?Hact)
• The energy barrier to products

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How does an increase in temp affect the rate of
rxn

• increases effective collisions

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How

• more collisions because of more motion
• higher energy of collisions because more energy

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What rule of thumb do we use

• at room temp a 100 in temp will double a rxn
  rate

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What comprises mechanism

• A series of elementary steps. That match the
  stoichiometry of the rxn
• a series of steps a reaction goes through to get
  to productsNO2(g)CO(g) -gtNO(g) CO2(g)Rate
  kNO22
• The reaction is actually thought to proceed by a
  rather more complex mechanism than the overall
  equation indicates

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Give an example

• NO2(g) CO(g) -gt NO(g) CO2(g)
• Mechanism NO2(g) NO2(g) -gt NO3(g) NO(g)
• NO3(g) CO(g) -gt NO2(g) CO2(g)
• NO3 is an intermediate. It does not appear in the
  overall reaction.
• The two reactions add up to the original rxn
• Each of the steps has its own rate

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What is an elementary step

• Each individual step of a mechanism

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What are intermediate products

• Products within a mechanism that is not part of
  the stoichiometry of the rxn

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How does the rate-determining step control how
fast a reaction proceeds?

• The slowest step of the mechanism and therefore
  ultimately determines the rate for the rxn

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What is a rate law?

• An expression of the rate of reaction in terms of
  concentrations raised to powers (experimentally
  determined)A 2B ? 3C 2Drate k Aa Bb
  

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What influences the rate law?

• k rate constanta, b experimentally
  determinedthe exponent is the order in regard to
  each reactant and the sum of these is the overall
  order

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What effect does a catalyst have on the rate

• It lowers activation energy, but is not consumed.
  
• It uses a different mechanism

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What is equilibrium

• When the rate of the forward rxnrate of reverse
  rxn
• No net change in the system
• When it looks like the rxn has stopped
• Constancy of macroscopic prop continuation of
  micro

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Sometimes it is called dynamic, why

• Because the atoms/molecules are continuing to
  collide and react, but because they are breaking
  down at the same rate they are forming, it
  appears nothing is happening

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What is the law of mass action

• law of chemical equilibrium

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How does this work mathematically

• aA bB ? cC dD
• CcDd Keq (equilibriumAaBb
  constant)

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How do you Write an eq constant expression

• 1st you need a balanced equation
• 2 SO2 O2 ? 2SO3
• Keq SO32 SO22O2

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What are the rules for Keq expressions

• mean M(moles /liter)
• solids and pure liquids are not involved
• coefficients become exponents

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What is LeChateliers Principle

• When a stress is applied to a system at
  equilibrium the system will shift to relieve that
  stress

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What factors qualify as stresses

• temp,
• concentration,
• pressure(if gases)(you may see the term volume)

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How does this work

• you can figure it out
• or use the teeter-totter effect

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Give an example

• P4 6Cl2 ? 4PCl3 (all gases)Which way will eq
  shift if chlorine is added to the rxn
• ans shift right The increased chlorine con
  causes more collisions and therefore the forward
  rate is increased compared to the reverse rate
  ?shift right

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Give an example

• P4 6Cl2 ? 4PCl3 (all gases)P4 6Cl2
  4PCl3 ?
• If chlorine is added the left side is heavy, the
  reaction will slide right to relieve the stress.

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Does it work this way for all

• If you put temp as a term in the equation it will
  work. If not you must use a rule.
• Increase temp favors the endo rxn
• for pressure, increase P will shift to side with
  fewer moles (cant teeter totter pressure)

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What does shift left and right mean in an
equation

• If you shift right it means the con of reactants
  are decreasing and the products are increasing

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Is there anything that affects the Keq

• Only temperature

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P4 6Cl2 ? 4PCl3 12 Kcal(all gases)

• Predict the shift if the temp is decreased
• Right
• How will the concentration of chlorine change
  with temp decrease
• Go down

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P4 6Cl2 ? 4PCl3 ?H -12 kcal (all
gases)

• Predict the shift if the temp is decreased
• Right
• How will the concentration of P4 change with
  temp decrease
• Go down

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P4 6Cl2 ? 4PCl3 ?H 12 kcal (all
gases)

• Predict the shift if the temp is decreased
• Left
• How will the concentration of chlorine change
  with temp decrease
• increase

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P4 6Cl2 ? 4PCl3 ?H 12 kcal (all
gases)

• Predict the shift if the pressure is decreased
• Left
• How will the concentration of chlorine change
  with pressure decrease
• increase

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2H2 O2 ? 2H2O ?H -67 kcal (all gases)

• Predict the shift if the pressure is increased
• Right
• How will the con of water change if press is
  increased
• increase

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Is there anything that affects the Keq?

• Only a temperature change will change the value
  (size) of K
• If you change concentration or pressure the
  amounts of each substance can change but the K
  doesnot

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What is Q?

• Reaction Quotient
• You can use it to determine if a system is at
  equilibrium and/or use it to determine the
  direction an equation must run to get to
  equilibrium

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What is Q?

• Follow all the same rules as you use for writing
  Keqs except your values are initial (not
  equilibrium)This then allows us to make some
  predictions

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Using Q?

• QltK means rxn must shift to right(make more
  products)
• QgtK means rxn must shift left (make more
  reactants)
• Q K At Eq