Chemical Kinetics

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Chemical Kinetics
Fast

• The area of chemistry that concerns reaction
  rates.
• Tells us how fast the reaction happens.
• (Not if it will happen.)

Slow
Section 17.1
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Reaction Rate

• Change in concentration (conc) of a reactant or
  product per unit time.

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Average Rate vs. Instantaneous Rate

• Average Rate between 2 points in time
• Instantaneous Rate at 1 point in time.

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Two ways to measure rate

• Breakdown of reactants
• Formation of products

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2NO2 ? 2NO O2
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Rate and StoichiometryUse the coefficients!
2Al(OH)3 6HCl ? 2AlCl3 3H2O

• HCl breaks down 3 times as fast as Al(OH)3 breaks
  down.
• Al(OH)3 breaks down as fast as AlCl3 forms.
• Water forms ½ as fast as HCl breaks down.

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Graph the following on Graph Paper

• Time (seconds) on the x-axis
• Concentration (Molarity) on the y-axis
• Title Concentration vs. Time for the
  Decomposition of N2O5
• 2N2O5 4NO2 O2

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On the same graph

• Graph the products. Use different colors
• Calculate the rate of breakdown between 600 and
  800 seconds.
• Calculate the rate of breakdown between 1600 and
  2000.
• Calculate the instantaneous rate of breakdown at
  300 seconds.
• Calculate the instantaneous rate of breakdown at
  1800 seconds.

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Collision Model

• Key Idea Molecules must collide to react.
• However, only a small fraction of collisions
  produces a reaction. Why?
• Arrhenius An activation energy must be overcome.

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Heres why.

• Collisions must have enough energy to produce the
  reaction (must equal or exceed the activation
  energy).
• Orientation of reactants must allow formation of
  new bonds.

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Change in Potential Energy
Reactants
Products
Activated Complex or Transition State
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What affects the rate?Section 17.2

• Nature of reactants
• Concentration (More conc., speed up.)
• Surface Area (More SA, speed up.)
• Temperature (Higher Temp., usually speed up.)
• Catalysts (Speed up)

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Effect of a Catalyst
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Rate Laws

• Rate k NO2n
• k rate constant
• n rate order

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Overall Reaction Order

• Sum of the order of each component in the rate
  law.
• rate k AB2C3
• The overall reaction order is 1 2 3 6.

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Method of Initial Rates

• Initial Rate the instantaneous rate just
  after the reaction begins.
• The initial rate is determined in several
  experiments using different initial
  concentrations.

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To find the order of rxn
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Answers

• 1st Order with respect to A
• 2nd Order with respect to B
• 3rd Order overall (1 2 3)
• k 2 (Use any trial and the rate law)