Limiting Reactant: Reaction of Mg with HCl
Three 500 mL Erlenmeyer flasks each contain 100 mL of 1.0 M hydrochloric acid and some universal indicator. The color of each solution is red, indicating acidic solutions. It helps to have four flasks with the pH of the solution in each flask at pH = 3, pH = 5, pH = 7, pH = 9 Across the mouth of each flask is stretched a deflated balloon. Each balloon has a different amount of Mg in it. When the magnesium is added to the hydrochloric acid solution, the balloon will fill with hydrogen gas. The size of the inflated balloon depends on the amount of hydrogen gas produced and the amount of hydrogen gas produced is determined by the limiting reagent. There will be different amounts of magnesium left over in the bottom of the flasks when the reactions are finished. There will be different amounts of HCl consumed in each reaction. At the end of the reaction, the color of each solution will be different. Using the size of the balloons, the color of the solutions, and the quantity of magnesium un-reacted in the flask, students can determine the limiting reactant in each flask: magnesium or hydrochloric acid.
A student worksheet is available to accompany this demonstration. A series of Power Point slides, including a Clicker Question, has been developed to accompany this demonstration.
This demonstration illustrates how to apply the concept of a limiting reactant to the following chemical reaction
- Mg (s) + 2 HCl (aq) ==> H2 (g) + MgCl2 (aq)
- Mg (s) + 2 HCl (aq) ==> H2 (g) + MgCl2 (aq)
- In the first flask there is four times the stoichiometric quantity of Mg present, so the balloon inflates to a certain extent as all of the HCl reacts to form hydrogen gas; the indicator changes from red to blue, indicating that the acid was used up; and excess Mg is visible in the bottom of the flask when the reaction is finished.
- The second flask contains stoichiometrically equivalent quantities of both reactants so the balloon inflates to the same extent as the first flask as all of the HCl reacts to form hydrogen gas; most of the Mg is used up, and the indicator changes from red to peach. It takes longer for this balloon to inflate to the same extent as the first balloon because the reaction slows down considerably as the concentration of HCl and the surface area of the Mg approach zero toward the end of this reaction. Because of this effect the reaction won't truly go to completion during the class period and the indicator doesn't change as much as in the first flask.
- In the third flask there is one quarter of the stoichiometric quantity of Mg so the balloon is noticeably smaller than the other two since the Mg is used up before all of the HCl is converted to hydrogen gas and the indicator stays red, showing that there is still acid present.
- 3 500 mL Erlemeyer flasks, each with 100 mL of 1.0 M HCl and a couple of droppersful of universal indicator in it.
- 3 ring stands and clamps to hold the flasks in place
- 3 large balloons, the balloon on the first flask contains 4.8 g (0.2 mol) of Mg, the balloon on the second flask contains 1.2 g (0.05 mol) of Mg, and the balloon on the third flask contains 0.3 g (0.0125 mol) of Mg.
The Mg in the balloons is added to the hydrochloric acid solution and the reaction is allowed to run for about five minutes. Make sure all of the Mg is added to the hydrochloric acid solution. Check to see that very little of the magnesium metal doesn't get caught in the neck of the balloon. The sizes of the balloons, the colors of the solutions, and the amounts of Mg remaining in the flasks are compared. Allow about ten minutes for this demonstration.
Be sure and wear goggles in case one of the balloons pops off and spatters acid. Hydrochloric acid is corrosive.
- Skin Contact: In case of contact, immediately flush skin with plenty of water for at least 15 minutes.
- Eye Contact: Immediately flush eyes with plenty of water for at least 15 minutes, lifting lower and upper eyelids occasionally. Get medical attention immediately.
Do not prepare this demonstration the night before the presentation. The HCl vapor may react with the magnesium in the balloon and the rubber of the balloon. A small amount of extra magnesium in the middle balloon is necessary in order to drive the reaction to completion.
