Wednesday, January 14, 2015

Blow the Lid Off of Magnesium Oxide Lab

Magnesium reacting with oxygen in a red-hot crucible
Photo by Julia Paneyko
Every chemistry student makes magnesium oxide in the lab in their introductory course.  My students do it twice in my lab program:  once in the study of chemical reactions (types of chemical reactions lab) and then again in the mole unit.  High school chemistry students around the world are determining the chemical formula of magnesium oxide in this classic experiment.  I use this experiment as my first "mole concept application".


The procedure is very simple:  heat magnesium in a crucible until it is fully reacted with oxygen.  Use the mass data of the magnesium before reaction and product after reaction to calculate the mole ratio of Mg and O in the compound.  This mole ratio is the experimental chemical formula of magnesium oxide.  My "go to" procedure for this lab is found in ChemTopic Labs Volume 7:  Molar Relationships & Stoichiometry by Flinn.  In Flinn's version of the lab, there are detailed instructions to heat the magnesium with the lid on, then open the lid every three minutes, and then heat it some more with the lid at a tilt.  All of this manipulation of the crucible lid invariably results in at least one dropped and broken lid each lab period.  And, the results of the lab are usually not that great, with many groups getting ratios other than the expected 1:1.

My students are heating their crucibles with lids off.
This year I decided to try the lab with no lid on the crucible during the heating process.  It seems only obvious to let the maximum amount of air into the reaction by just leaving it open for the whole reaction.  I was a little nervous that burning magnesium might pop out of the crucible, but that didn't happen all day.  The hardest part of the experiment was adjusting the bunsen burner so that it would get hot enough to start the reaction.  Most of the reactions burned at a steady rate, without a big flare up of bright light.  Once the reactions were completed without any safety issues, my students crunched the numbers.

Here's where the big payoff was obvious.  The lab results were much better this year, with most groups getting the expected 1:1 ratio without the need for sad stories about experimental errors in the conclusion.  Taking the lid off resulted in better results, more interesting observations during the reaction, and better results for the chemical formula for magnesium oxide.

7 comments:

  1. Interesting! I am happy to read this, as I can definitely relate to the broken crucible lids, and even worse--the attempt to prevent the broken crucible lid with a bare hand!

    I would have thought you would lose a lot of product as "smoke" (I should probably have an idea of what that smoke "is", but I don't....), resulting in a lower mass than expected. I take it you didn't have this issue? And how about the usual part where you add water, then heat again--did you do that? I believe that is done to remove nitrides from the mixture.

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  2. Interesting! I am happy to read this, as I can definitely relate to the broken crucible lids, and even worse--the attempt to prevent the broken crucible lid with a bare hand!

    I would have thought you would lose a lot of product as "smoke" (I should probably have an idea of what that smoke "is", but I don't....), resulting in a lower mass than expected. I take it you didn't have this issue? And how about the usual part where you add water, then heat again--did you do that? I believe that is done to remove nitrides from the mixture.

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  3. Ryan,
    I didn't notice any loss due to smoke. The reactions we did generally didn't flare up, but really smoldered so it wasn't too dramatic and not much smoke was produced. I skipped the water step altogether. Another chance for cracked crucibles when we squirt the water into them.

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  4. I wonder if perhaps you have two sources of error that are canceling each other out. You might be losing some mass via smoke, but gaining mass via nitrides??

    It would be "fun" (Ok, teacher fun) to try doing lid-less heating with water, then lidded heating without water, and seeing if you could see a difference.

    We have started doing this lab by giving everyone a different amount of Mg, intentionally giving a nice wide range. Then everyone plots their data (mass of Mg vs. Mass of MgO) on a shared Google spreadsheet. The points come out in a nice straight line, leading to the law of constant proportions. Got that little upgrade from a co-worker and its a fantastic addition.

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  5. Ryan, you have made me very curious about the off-setting sources of error in the lab. I agree with you that it would be fun to run it twice to highlight each of the possible errors to see if they are indeed just canceling each other out when done without the lid on. I LOVE you suggestion for starting with different masses of Mg. What a great way to extend the outcome of this experiment! I wish I could do it again this week to try it out with this different approach.

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  6. Hi! I read this post prior to completing the MgO lab this year. I too in the past have had various results.
    I used the Flinn ChemTopic version of the lab, and changed three aspects: (1) no crucible lid, (2) varying lengths of Mg, and (3) a more in depth data table (easier to score for me!).
    Each of 4 groups was assigned a different length and they completed as a group only one trial. They then gathered the data from the 3 other groups. They were to show all calculations for their Trial, but complete the table for the other three groups as well.
    I would be happy to send you my completed data table. We got great results!
    Thank you for the suggestions! lblack@gastonday.org

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