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Reversible Reactions

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Are you a picky eater?

Credit: Slice of Chic
Source: http://www.flickr.com/photos/sliceofchic/4902040546/sizes/m/in/photolist-8tbeSs-8KfnNb-cjNyhC-8tbeSE-anirVD-8tEjh2-dWLNWp-7ZR4Xa-97KxWW-aecERZ-8qNM8i-8pQzY2-8zQvvQ/
License: CC BY-NC 3.0

[Figure1]

People make salads with all kinds of ingredients, much like how molecules are made with all kinds of atoms. When the chef combines the ingredients together into a salad, this is one direction of the reaction.  But when the picky eater gets his salad, he takes apart the salad and separates the ingredients into fruits and vegetables he likes and dislikes. He soon finds that it is more difficult to take apart the salad than put it together.  In this way, the salad making reaction is reversible because the salad can be put together and taken apart, much like how a molecule can be put together or taken apart.  Furthermore, the different directions the reaction can go (putting together or taking apart) are not created equally and often times one direction requires more energy than the other.

Apply this concept to discussions about alternative clean energy:  

Fuel cells are created to convert hydrogen into electrical energy and there are no pollutants created through this process.

2 H2(gas) + O2(gas) → 2 H2O + energy

This is the reverse direction of electrolysis and so instead of using energy to break apart water, energy is released by combining hydrogen and oxygen.

Creative Applications

  1. In the fuel cell example, which direction is the same as creating the salad and which direction is the same as taking apart the salad? (Which direction takes more work and which takes less work?)
  2. Come up with your own analogy to help you remember!
  3. Is backing a cake a reversible reaction?  Why or why not?

Resources:

http://fuelcellstore.com/products/heliocentris/intro.pdf

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