equilibrium & le chatelier's principle

Sunday's post was mainly about Kinetics, so today I'm getting into Equilibrium.

Objectives:

1.  Define chemical equilibrium in terms of a reversible reaction.

2.  Identify physical processes and chemical reactions that go to equilibrium and those that go to completion.

3.  Describe Le Chatelier's Principle and predict changes in a chemical equilibrium system due to changes in (a) concentration, (b) temperature, (c) pressure and (d) the addition of a catalyst.

4. Explain the requirements in The Haber Process (or the Haber-Bosch process).

5.  Describe the common-ion effect.


This is a brief explanation of equilibrium with a very clear animation:

I am including this explanation as well because it will be referred to in subsequent segments:

To summarize, for a dynamic equilibrium to be established, there must be a closed system so that reactants and products do not leave but continue to react.

If the reactants or products leave the system, a reaction will continue to completion and eventually stop.  This can occur when a gas leaves a system (sometime bubbles are formed) or when a solid precipitate settles out of a solution.

Le Chatelier's Principle explains shifts in the equilibrium system due to changes in concentration, temperature, pressure and the addition of a catalyst, and the eventual establishment of a new equilibrium.  This all comes back to the idea of collision theory.

The following video continues with the previous explanation with the picture of digging and filling a hole:

It continues with the following installment:

Just to review, exothermic reactions release energy as a product (right side of the reaction), and delta H has a negative value.  The energy value is given a negative sign because it is being released or lost by the reaction.

Endothermic reactions absorb energy as a reactant (left side of the reaction), and delta H has a positive value.

Putting the energy in the reaction equation (simply as Q) makes it easier to  predict the direction of the shift if you think of the heat (Q) similarly to a reactant or product.

 One important application of Le Chatelier's principle is The Haber Process, explained in this video --  there's 1 minute of introductory credits (snore), but this clip continues right after that:

Here is another video that talks about the societal implications of The Haber Process -- this narrator gets bleeped a few times (sorry) when talking about the development and use of chlorine and mustard gas for use as a weapon during  WWI.  He also talks about current environmental impacts due to runoff of chemical fertilizers.

It is being referred to as the Haber-Bosch Process here.  BTW, the history of chemistry offers a great deal of researchable material for your students -- I'll talk about it in another post.

Another phenomenon is the Common Ion Effect -- the straight definition in textbooks can be confusing, so here is a short video with animation:

Here are a few labs where Le Chatelier's Principle is demonstrated:

This uses the colored compound CoCl2:

http://www.pleasanton.k12.ca.us/fhsweb/morris/chemlab/RatesEqu/Lab%20-%20Cobalt%20Chloride%20Equilibrium.pdf

This is a lab based on the acid-base indicator Methyl Orange:

http://www.austincc.edu/dsackett/CHEM1412/lab/1412exp20.pdf

A very simple demonstration is that of yeast being used as a catalyst to speed up the decomposition of hydrogen peroxide (H2O2) to produce water and gaseous oxygen -- if performed in a test tube, a glowing wooden splint (not flaming) will reignite when inserted into the gas.  You might want to put an index card over the end of the test tube to trap the gas.  DO NOT USE A STOPPER TO TRAP THE GAS!  The gas pressure would build up and the stopper could shoot out, maybe into someone's eye!

Here are some worksheets regarding Le Chatelier's principle:

http://www.greatneck.k12.ny.us/GNPS/SHS/dept/science/gaon/tab_3/documents/Test11handouts.pdf

http://mmsphyschem.com/lechatP.pdf

This worksheet contains multiple choice questions:

https://www.kent.edu/si/chem/upload/10060-004/worskeet_lechateliers_prin2.pdf

The National Science Teacher Association (NSTA ) annual conference is this week in Boston, Massachusetts.  For more information go to:

http://www.nsta.org/conferences/national.aspx

If you're attending the convention, please write me about it!

*I'd love to hear from you!  Your feedback would really help me to focus on your needs. You might be having a school vacation soon (spring breaks are around this time) -- take advantage of the time and write me about your classroom experiences!  There should be a "Contact" form below, or click on the "Contact" tab on the top right of this page.  

Check out my lab book "Chemistry on a Budget" at amazon.com:
http://www.amazon.com/Chemistry-Budget-Marjorie-R-Heesemann/dp/0578129159/ref=sr_1_1?s=books&ie=UTF8&qid=1389410170&sr=1-1&keywords=chemistry+on+a+budget

Each lab is presented with two possible report formats -- each lab uses the same procedure page but each has a different conclusion page -- one with 10 questions to be answered as a conclusion, the other with a full laboratory report required.  This gives the teacher the option of what type of report is desired! 

Have a good end of the week!