Description.
    In the preparation of hydrogen, a standard quantity (2.5 mmol) of HCl is used with varying amounts of Mg(s).  The reaction is:

    The amount of HCl used will generate over 30 mL of H2(g) in reactions with excess Mg.  The exact volume depends on the temperature and atmospheric pressure.

    The amount of Mg ribbon used will vary from 0.5 cm (approximately 0.17 mmol) to 8.0 cm (approximately 3.4 mmol).  When less than 1.25 mmol Mg is used, Mg is the limiting reagent and the volume of H2(g) collected will vary in proportion to the amount of Mg used.  When the amount of Mg exceeds 1.25 mmol, HCl is the limiting reagent — and because this quantity is held constant, the volume of H2(g) will not change.  Students work in pairs and perform two measurements — one with less than 1.25 mmol of Mg and one with greater than 1.25 mmol Mg.  Experimental results from one of our trials is shown in the figure below.
 
 

Another set of experimental data and resulting chart are given at the end of this page.

General Safety Precautions.
    Always wear safety glasses.  Gases in syringes may be under pressure and could spray liquid chemicals.  Follow the instructions and only use the quantities suggested.
 

Suitability.
    This laboratory activity is suited for high school and university-level chemistry students.
 
 

Chemicals and Materials needed:²

•  60 mL plastic syringes  •  scissors
•  HCl (1.7 ? 2.0 M) •  balance
•  Latex syringe caps •  vial caps
•  Mg ribbon •  weighing dish
•  ruler to measure Mg •  disposable pipet

Before students arrive.  Determine the mass of a 25-cm strip of Mg ribbon (a typical value would be 0.20 g), then convert this to mmol/cm for use by the class.  With this value, the students can cut lengths of the Mg ribbon, measure them accurately, and convert to mmol.  Prepare a bottle of approximately 1.8 M HCl(aq) by diluting 36.0 mL concentrated HCl into 210.0 mL water to give a total of 240.0 mL.  Determine the density (g/mL) of this acid solution by measuring the mass of 100.0 mL.  Also, determine which pair of students will perform each experiment.  Include the following lengths of Mg ribbon: 0.50 cm, 1.0 cm, 1.5 cm, 2.0 cm, 2.5 cm, 3.0 cm, 3.5 cm, 4.0 cm, 5.0 cm, 6.0 cm, 7.0 cm, and 8.0 cm.  The first eight of these values (< 4 cm) represent quantities in which Mg will be the limiting reagent.  For classes with more than 5 pairs of students, other values should be included that fall between those lengths given above.
 
 

Instructions for the Students.

Perform the Experiment.  These instructions assume that students are familiar with the general procedure of gas preparation.

 
1. Measure out the two lengths of Mg that were assigned to you and your lab partner.  Use a scissors to cut the Mg.  Record the exact length of each on your Report Sheet.  Perform the experiment on one piece of Mg at a time.

2. Fold the Mg back and forth so that it fits inside the little cap.  Maintain maximum exposed surface area.  Lower the cap containing the Mg into the syringe by flotation.

3. Measure out exactly 1.50 g of the HCl(aq) into a weighing dish.

4. Draw up all of the acid into the syringe.  Add three drops water to the weighing dish to dissolve any remaining acid.  Draw these drops into the syringe and cap the syringe.

5. Read the initial volume of the syringe using the bottom of the rubber seal as the mark.  Also read the level of the acid solution — should be close to 0 mL.  The difference between these two readings is the volume of air in the syringe.  Record your data.

6. Perform the reaction by shaking the syringe.  Although the reaction is fast when Mg is the limiting reagent (the short piece), it is slow when the concentration of acid is small (such as when HCl is in small excess).  If you are using a longer piece of magnesium, continue to shake the reaction syringe for 8 ? 10 minutes; not all of the magnesium will react.  Assist the plunger from time to time by pulling it outward by a few mL.

7. (See a 1-minute movie demonstrating this step.) Oftentimes during the reaction the plunger does not move as freely as it should and erroneous final volume readings could result.  In order to eliminate this problem, draw the plunger outward to create a reduced pressure and then remove the syringe cap under water — use a large container such as an ice cream pail in order to accommodate your hands and the syringe.  Recap the syringe.  The gas pressure inside the syringe is now very close to the atmospheric pressure outside the syringe.  Be careful to not move the plunger inward or outward.  Take the final volume readings for both gas and solution as previously done in Step 5.  The difference in volumes this time is the volume of hydrogen plus the volume of air initially present.  The volume of hydrogen only is obtained by subtracting the volume of air (Step 5) from the volume of hydrogen plus air just determined.

8. Repeat the experiment with the other piece of Mg.

9. Complete the lab report sheet.  Your instructor will provide you with (a) the conversion factor so that length of Mg (cm) can be converted into mmol Mg and (b) the density and molarity of the HCl solution so that 1.50 g HCl(aq) can be converted into mmol HCl.

Laboratory Report Sheet
Trial I.

Length of Mg ribbon used:
Mass of acid used:
Volume of hydrogen collected:

Length of Mg ribbon used:
Mass of acid used:
Volume of hydrogen collected:

Laboratory Report Questions:

1. Add your data points to the graph being prepared on the chalkboard (or follow the data collection procedures given by your teacher)

2. Determine the number of mmoles of HCl used each time.

3. Convert your lengths of Mg into moles and mmoles of Mg.

4. Considering the reaction stoichiometry, how many mmoles of Mg are needed to react with the HCl used?

5. Use the answer to Question 4 to determine what length of Mg represents exactly the stoichiometric amount.

6. Inspect the class graph to determine if the value determined in Question 5 seems reasonable.
 
 

Endnotes:
1 Author to whom correspondence should be addressed.  E-mail: xenon@creighton.edu
2 The syringe and related equipment can be ordered from a variety of vendors including Educational Innovations, Flinn Scientific (US sales only), S17 Science Supplies, Micromole, Fisher Scientific, etc.  Part numbers and links to their websites are provided at our microscale gas website (Endnote 3)
3 Website:  http://mattson.creighton.edu/Microscale_Gas_Chemistry.html
4 The Chemistry of Gases, A Microscale Approach, B. M. Mattson, M. P. Anderson, Cece Schwennsen, Flinn Scientific, 1999, ISBN #1-877991-54-6.
5 Microscale Gas Chemistry, Mattson, B. M., Educational Innovations, 2000, ISBN #0-9701077-0-6.