Drill

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Drill 5/21/08
1. What does each letter represent in the above
energy diagram? 2. Is this reaction endothermic
or exothermic?
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Reaction Rate

• Chapter 17, Section 2

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What is Reaction Rate?

• The change in concentration of reactants per time.

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• In order for a reaction to occur, there must be a
  COLLISION.
• Collision Theory provides two requirements for
  reactants to yield products
• 1. Reaction needs enough energy
• 2. Colliding molecules must be oriented in a way
  to react

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• Reaction Rate changes are caused by changes in
  the amount (frequency) of collisions, changes in
  the orientation, and changes in the energy.

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Factors that affect reaction rate

• Nature of Reactants different atoms, molecules,
  or compounds react with each other in certain
  ways (ex. Na reacts vigorously with water)
• Surface Area two reactants must be in contact
  with each other to react. Gases and dissolved
  particles collide freely, so react faster.
• Temperature Increasing kinetic energy,
  increases motion collisions. Also increases
  energy.
• Concentration Increasing conc., increases
  collisions, so increases reaction rate
• Catalyst lower the activation energy so that
  reaction has enough energy.

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Rate Laws

• Equations that relates reaction rate and
  concentrations of reactions

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R kAnBm

• R reaction rate
• k specific rate constant
• A B MOLAR conc. of reactants
• n m powers to which the conc. are raised or
  ORDERS

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• Orders must be determined experimentally you
  cant figure it out from the chemical formula.
  (Unless it is stated that it is a single or one
  step mechanism)
• Ex 3NO ? N2O NO2 R kNO2
• 2NO2 ? 2NO O2 R kNO22

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• Three experiments that have identical conditions
  were performed to measure the initial rate of the
  reaction
• NO O2 ? NO2
• Conditions were identical in the 3 experiments,
  except the concentrations of reactants varied.
  The results are as follows

Experiment NO O2 Rate (M/s)
1 0.20 0.20 1.50
2 0.40 0.20 3.00
3 0.20 0.40 1.50
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• First thing you need to do is figure out the
  general rate law R kAnBm
• R kNOnO2m

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• Next, to figure out n m by comparing the
  rations of reactant concentration to the ratio of
  rates. To figure out the order for A use two
  experiments that keep B constant.

Experiment NO O2 Rate (M/s)
1 0.20 0.20 1.50
2 0.40 0.20 3.00
3 0.20 0.40 1.50
.40 M/.20 M 2 3.00 M/s / 1.50 m/s 2 If
the conc. doubles, than the rate doubles a 1 to
1 ratio. 1st Order
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• To figure out the order for B use two
  experiments that keep A constant.

Experiment NO O2 Rate (M/s)
1 0.20 0.20 1.50
2 0.40 0.20 3.00
3 0.20 0.40 1.50
.40 M/.20 M 2 1.50 M/s / 1.50 m/s 1 If
the conc. doubles, than the rate does not change
a 1 to 0 ratio. A order of zero!
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• R kNO1O20
• Now pick one experiment and fill all the values
  in to solve for k.
• k R / NO1O20
• k 1.50 / .201.200 1.50 / .20
• k 7.5

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• If a reaction is single-step then we CAN
  determine the order of concentrations from the
  balanced chemical equation. The coefficients
  tell you what power each reactant concentration
  should be raised to.
• 2X Y ? X2Y
• So R kX2Y

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• Do page 746 in the Blue Chemistry Book