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Determining the Limiting Reactant

Use molar ratios to determine which reactant will run out first in a reaction

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What's the Hold-Up?

What's the Hold-Up?

Credit: Niagara
Source: http://commons.wikimedia.org/wiki/File:Country_Fair_gas_station.jpg
License: CC BY-NC 3.0

In the 1970s, drivers were confronted with long lines of cars in front of their services stations. A shortage of oil caused by decreases in exports from Middle Eastern oil-producing countries led to a significant decrease in the supply of gasoline in the U.S. The need was still there – the same numbers of cars were on the road. However, the gasoline was not as available as it had been. The gas supply was the limiting reagent in the equation.

Why It Matters

  • In a chemical reaction, here is always a balance between using just enough reagents and having some unused material at the end of the reaction. In many cases, one starting material may be much more expensive than the other chemicals in the process. That reagent would determine how much product will be formed, so it becomes the limiting reagent.
  • Other processes also have a type of “limiting reagent”. Any time you wait in line, there is something that limits the speed of that line. It may be the availability of ticket-takers or cashiers. People in a restaurant can only be waited on based on the speed of the server. Perhaps there are only so many gates for entry to a football stadium. Whatever the reason, the line cannot move any faster than the access points or the availability of personnel to process the movement.
  • Credit: David Morris
    Source: http://www.flickr.com/photos/revdave/431123097/
    License: CC BY-NC 3.0

    Waiting in line can be a frustratingly slow experience [Figure2]

  • Watch a short video describing the gas shortage: https://www.youtube.com/watch?v=bLncjCnblMA

Show What You Know

Use the links below to learn more about limiting reagents, then answer the following questions:

  1. How does the idea of a limiting reagent help plan chemistry experiments?
  2. What was Liebig’s law of the minimum?
  3. How does Kroger speed up the grocery check-out line?
  4. How does Disney World speed up access to rides?

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Image Attributions

  1. [1]^ Credit: Niagara; Source: http://commons.wikimedia.org/wiki/File:Country_Fair_gas_station.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: David Morris; Source: http://www.flickr.com/photos/revdave/431123097/; License: CC BY-NC 3.0

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