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This presentation is about basic reaction mechanisms, intermediates, catalysts, and using (but not finding) the rate determining step. Created in March 2014 for Honors Chemistry at Hopkinton High School.
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Reaction Mechanisms

Published on Nov 18, 2015

Honors Chemistry- Reaction Mechanisms

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1.

Reaction Mechanisms

Spring 2016
This presentation is about basic reaction mechanisms, intermediates, catalysts, and using (but not finding) the rate determining step. Created in March 2014 for Honors Chemistry at Hopkinton High School.
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2.

Reaction Pathways

  • more than one collision needed
  • many steps- some fast, some slow
  • called: Reaction Mechanism
Many reactions are not simple, one-step deals!
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3.

NO2 + CO -> NO + CO2

  • NO2 + NO2 -> NO3 + NO
  • NO3 + CO -> NO2 + CO2
  • What cancels out??
If a substance appears as both a reactant and a product, you can cancel it out. This is similar to adding equations in advanced algebra.
4.

1: NO + Cl2 ->NOCl2

2: NOCl2 + NO -> 2NOCl
overall: 2NO + Cl2 -> 2NOCl

If each step is balanced, the stoichiometry carries over to the final balanced equation, so only cancel out equal quanities. Example: If you had 2 moles of NO on the left and one mole of NO on the right, only ONE of the moles of NO on the left would be cancelled out.
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5.

Intermediates

  • Was a product in an early step 
  • What was the intermediate in NO2 + CO?
  • What was the intermediate in NO + Cl2?
Intermediates START as a product and END as a reactant.
6.

Find the Intermediate

  • O3 -> O2 + O
  • O3 + O -> 2O2
  • Write the overall equation 
  • identify the intermediate
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7.

Catalysts

  • Starts as a reactant
  • Gets cancelled out as a product
  • Catalysts CHANGE the mechanism!
The steps of the reaction when a catalyst is used is different from the steps of the reaction when there is no catalyst present. This often works by making that really slow step go away and replacing it with a faster step.
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8.

Identify the Catalyst

  • H2O2 + I- -> H2O + IO-
  • H2O2 + IO- -> H2O + O2 + I-
  • What is the overall equation?
  • What is the catalyst?
  • What is the intermediate?
Remember catalysts START as a reactant and END as a product. Intermediates are the opposite!
Photo by Pacific Northwest National Laboratory - PNNL
9.

Compare Catalyst-Intermediate

10.

Rate Determining Step

  • One step is usually slower than rest
  • Only as fast as slowest step
  • Think ASSEMBLY LINE
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11.

Which Step is Rate Determining?

  • Grab one paper from each pile (20 piles)
  • Straighten papers
  • Staple
  • 3-hole punch
  • How could you speed this reaction up?
The rate is determined by the slowest step - in this case, grabbing a paper from 20 different piles. To speed this up we would need to do something to impact that slow step- having a second person grabbing half of the papers, etc.
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12.

Using the R.D.S.

  • 2NO2 -> NO3 + NO (slow)
  • NO3 + CO -> NO2 + CO2 (fast)
  • What is the net equation?
  • How could you speed up the reaction?
  • (Focus on the SLOW step!)
The rate is determined by the slow step- in order to change the rate of the reaction we have to choose something that would affect the NO2.
13.

Other Considerations

  • Complex mechanism- complex graph
  • Each step = new activation E
  • Different catalyst= whole new graph
Remember that having a multistep mechanism changes your potential energy diagram from that simple "one-hump" version to a more complex version. EACH step will have a specific activation energy!
Catalysts change the graph too- different steps, different intermediates, and different activation energies.

Devon Grilly