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Chemical Reaction Rate

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Crash!

Crash!

Credit: Rian Castillo
Source: http://www.flickr.com/photos/digitizedchaos/5913304617/
License: CC BY-NC 3.0

When cars moving at a high speed collide with one another, the result can be a particularly dramatic change. A high energy collision can convert a functional car into a pile of scrap metal and spare parts. On the other hand, a slower, low energy collision might just cause a scratch or a minor dent.

Not all collisions are purely destructive. You are probably familiar with the phrase, “You can’t make an omelet without breaking some eggs.” Similarly, you can’t cause a chemical reaction without breaking some bonds. Most chemical reactions occur only when reactants collide with one another. However, not every collision leads to a chemical change. In order for a reaction to occur, the reactants must collide in a certain relative orientation with enough energy to break one or more chemical bonds. Based on this simple model, we can predict which factors will speed up or slow down a chemical reaction.

News You Can Use

  • Conditions that cause more frequent collisions between reactants speed up a reaction, because there are more opportunities for a chemical change to occur.
  • Conditions that make each collision between reactants more energetic or more effective also increase the rate of a reaction, because the percentage of collisions that lead to a change will be increased.
  • Five factors that increase the rate of a reaction, and maybe your chances of getting a date to prom, are highlighted in the following video: 

http://ed.ted.com/lessons/how-to-speed-up-chemical-reactions-and-get-a-date

Show What You Know

With the links below, learn more about chemical reaction rates. Then answer the following questions.

  1. A chemical reaction is taking place between two reactants dissolved in a liter of water. If an additional liter of pure water is added to the reaction mixture, what will be the effect on the reaction rate?
  2. We often want to decrease the rates of certain reactions rather than speeding them up. For example, to prolong the shelf lives of certain foods, the chemical reactions by which they spoil must be slowed down. How can this be accomplished by using the factors introduced in the video above?
  3. Grain dust is highly explosive, as seen in the link above. Before they are ground up into flour, whole grains like wheat are flammable, but not explosive. What is causing the grain to react so much faster after it has been milled?
  4. An aqueous reaction is taking place in a liter of water that is being stirred in a 5-liter flask. If the entire reaction was transferred to a 2-liter flask, what would be the effect on the overall reaction rate?

Image Attributions

  1. [1]^ Credit: Rian Castillo; Source: http://www.flickr.com/photos/digitizedchaos/5913304617/; License: CC BY-NC 3.0

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