Laboratory Activity 2: Teacher Notes

Continued

Teacher-Student Interaction

  1. Move from group to group, insuring proper set ups and safe lighting of burners. Do not hesitate to assist in the lighting of burners at first. This process should be monitored closely.
  2. Since the crucibles will be heated for long periods of time, use this opportunity to further discuss the reactions with individual groups. Consider providing them with a list of questions you will expect them to answer after the activity so they can discuss them with each other or with you.
  3. Circulate and be sure students make good observations when they open the crucible at different points in the procedure. They will note color changes.
  4. Do not be alarmed if the final product is not pure white, as MgO should be. The gray coloration can be due to several reasons, but largely to impurities in the crucible and water. Students may note that this gray color is not the "expected" color of pure MgO. Invite them to suggest reasons. Actual laboratory data from this activity, however, usually do not indicate that this discoloration leads to any problems in the calculated results.

Anticipated Student Results

Here are actual data from one student's laboratory report.

Here are actual results from eight student groups:

* This group had continual trouble with their burner. At the close of the procedure the team noted that their Mg ribbon had hardly reacted. Do not be afraid to allow "poor" data to be collected and included in group analysis. Be prepared to lead the class in discussing what could have happened.

Answers to Implications and Applications

  1. This removes water vapor and oils, which can affect masses if the balance is sensitive enough.
  2. This is done to admit enough O2 and N2to insure that the reaction of Mg is as complete as possible.
  3. Water was added to convert magnesium nitride into magnesium oxide and ammonia, thus producing a purer MgO product.

  4. Continuing to heat and reweigh until the masses are consistent would be one way to build confidence that the reaction was as complete as possible.
  5. If the mass percent of magnesium is too high, it is likely that it did not react completely or some of the magnesium had oxidized to MgO before massing. If the mass percent is too low, it is most likely because not all of the water was evaporated. There is always a possibility of error in masses (Also see the answer to Question 8, but do not expect students to include that in their answer to this question.)
  6. If the elements in the compound were known and you could determine the mass percent composition, you could probably identify the compound by comparing the percent composition to that of known compounds.
  7. The final products would consist of both magnesium nitride and magnesium oxide. Upon close mathematical examination, one would conclude that the products should have a lower mass. Mg3N 2 has a molar mass of 100.9 g/mol, and although 3 MgO's contain the same mass of Mg it would have a molar mass of 120.9 g. Failure to add water would leave the 3 Mg in the nitride, not in the 3 MgO's. It is doubtful that most high school balances could detect this mass difference (probably < 0.05 g difference).

    (NOTE: Very few students should be expected to display this much mathematical insight in answering the question. )

    (Optional) This question may be too difficult for students if stoichiometry has not been discussed at this point in the course. If the Mg is not cleaned well enough at the start, some slight error would be introduced because the initial mass of Mg would already contain MgO. (This would lead to low results, as explained in the answer to Question 5.)

Continued


TABLE OF CONTENTS TOPIC OVERVIEW CONCEPT/SKILLS DEVELOPMENT LINKS/CONNECTIONS EXTENSIONS