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Mechanisms and Potential Energy Diagrams

Explores how potential energy diagrams can describe the mechanism of a chemical reaction.

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The Yellow Flag

The Yellow Flag

Credit: Unknown
Source: https://commons.wikimedia.org/wiki/File:KennyWallace2006Car.jpg
License: CC BY-NC 3.0

Auto racing is a fast and dangerous sport. Because the cars are spread out on the track and moving at very high speeds, clear communication is needed. NASCAR uses a system of flags to signal drivers. The green flag starts the race and the black and white checkered flag indicates the end of the race. A yellow flag is used to tell drivers to slow down to a specific speed because there has been an accident or there is debris on the track.

Why It Matters

  • You are driving down the street at the accepted speed limit when you see a sign “25 miles per hour. School Crossing.” Being a good driver you slow down, because the sign tells you there are children in the area walking to school and you do not wish to endanger them. After passing through the school zone, you may resume your former speed.
  • Renewing a driver’s license can be a test of your patience. There are several desks where you can submit your paperwork, do your eye test, pay your money. Then you need your picture taken. But there is only one camera. So the line backs up and slows down while you wait, slowly moving ahead until you finally get to the front of the line for your photograph (which usually doesn’t look much like you anyway).
  • There is an interesting rate-limiting step in baseball. Unlike basketball, football, soccer, or hockey (which play timed games), there is no timing in baseball. When a ball is hit, things move swiftly. But the slow-down comes with the pitcher, who can take their time in throwing the ball. Much of the game is spent watching one person use up time.
  • Credit: Nate Steiner
    Source: http://www.flickr.com/photos/nate/2864061013/
    License: CC BY-NC 3.0

    The pitcher controls the pace of the game by deciding how long to take between pitches [Figure2]

  • Watch a video about the meanings of the different NASCAR flags at the link below:


Show What You Know

Use the links below to learn more about rate-limiting steps.  Then answer the following questions.

  1. In deriving a rate law for a multi-step reaction, which step in the reaction is used to calculate the rate constant?
  2. Why is the leaving of the bromide ion the rate-determining step in example one?
  3. What is the rate-determining step in example two?
  4. Why is the blog author talking about rate-limiting factors (RLF)?

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Image Attributions

  1. [1]^ Credit: Unknown; Source: https://commons.wikimedia.org/wiki/File:KennyWallace2006Car.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Nate Steiner; Source: http://www.flickr.com/photos/nate/2864061013/; License: CC BY-NC 3.0

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