Limiting Reactant

Three 500 mL Erlenmeyer flasks each contain 100mL of 1.0 M hydrochloric acid and some universal indicator. Across the mouth of each flask is stretched a deflated balloon. Each balloon has a different amount of Mg in it. When the Mg is dumped into the acid, the balloons will fill with different amounts of hydrogen and there will be different amounts of Mg left over in the bottom of the flasks when the reactions are finished. These results indicate the effect of limiting amounts of either Mg or acid. 

Curriculum Notes 

This demonstration is usually used in a General Chemistry course when the concept of limiting reactant is being introduced. Allow about ten minutes for this demo.

Lead Time 
One day of lead time is required for this project.
  • 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 dumped into the acid and the reaction is allowed to run for about five minutes. Make sure that all of the Mg gets into the flask and 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. 

Safety Precautions 

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.
Prep. Notes 

Don't let this demo stand overnight with the balloons on. The HCl vapor may react with the Mg and the rubber of the balloon. You might want to fudge and put a little extra Mg in the middle balloon to drive the reaction to completion.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group