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Calculations with Equilibrium Constants

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Watch the Shift

Watch the Shift

Credit: James Emery
Source: http://www.flickr.com/photos/62126383@N00/2126342976
License: CC BY-NC 3.0

The physics lesson for the day dealt with seesaws and a simple force concept. Children of equal weight placed equally distant from the center of the seesaw will balance one another. Children of different weights in the same situation will not balance each other; the lighter child will go up and the heavier child will go down. This physics scenario deals with a static situation. We have simply put two children on a seesaw and done nothing else.

Why It Matters

  • The equilibrium on the seesaw can be temporarily altered if the heavier child pushes their feet on the ground. This action will cause that side of the seesaw to rise and the other side to go down. However, this is just a temporary change, because gravity will restore the initial equilibrium and the heavier child will once again be lower.
  • The seesaw example above illustrates one idea about equilibrium that works for some physics situations. However, in chemical systems there is no static system. In a chemical equilibrium situation, there is a constant flow of materials back and forth, from reactants to products and then back to reactants, then back to products, then ...
  • Credit: adrigu
    Source: http://www.flickr.com/photos/97793800@N00/3823605387/
    License: CC BY-NC 3.0

    Osmosis is a type of equilibrium, but not a chemical one since there is no reaction [Figure2]


  • For a system in chemical equilibrium, we can develop an equilibrium equation that allows us to calculate the effects of changes in the system. What needs to be kept in mind is that the equilibrium constant is just that – a constant. Whatever changes are made to the concentrations of any components of the reaction, the net outcome needs to be a number that equals the original constant.
  • Watch the video at the link below to learn more about Le Châtelier’s principle and equilibrium: https://www.youtube.com/watch?v=_jypU3FvS_o

Show What You Know

Use the links below to learn more about Le Châtelier’s principle and equilibrium. The answer the following questions.

  1. How are the concentrations of reactants and products expressed in the equilibrium constant?
  2. What are the units of the equilibrium constant?
  3. In the library thinkquest web site, what do the lower-case letters represent?
  4. If you lower the concentration of A in the equilibrium given on the chemguide web page, what will happen to the equilibrium?
  5. In the equilibrium between acetic acid and acetate anion, how will addition of acetic acid affect the equilibrium constant?

Image Attributions

  1. [1]^ Credit: James Emery; Source: http://www.flickr.com/photos/62126383@N00/2126342976; License: CC BY-NC 3.0
  2. [2]^ Credit: adrigu; Source: http://www.flickr.com/photos/97793800@N00/3823605387/; License: CC BY-NC 3.0


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