Mg + HCl Limiting Reactant Demo
Many of us present stoichiometry as a relationship between only two substances in a chemical
reaction. The assumption is made that there were always enough reactants so that none were left
over and all reactions always go to completion. Use this demonstration as inquiry to show your
students that in real life chemistry the quantities of all reactants must be taken into account when
determining how much product will be produced.
Equipment

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
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3 Florence flasks or Erlenmeyer flasks or large test tubes.
3 different colored 9.5 inch balloons.
Wide stem funnel
100 mL graduated cylinder
Chemicals (You must adjust quantities as needed to fit your glassware!)
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300 mL of 1.0 M HCl
Mg turnings (0.60 g, 1.20 g, and 2.40 g samples)
Procedure
1. Add 100 mL of the HCl solution to each of the Florence flasks.
2. Use the funnel to fill one of the balloons with 0.60 g of Mg. Place the balloon around the
mouth of one of the Florence flasks (or whatever container you decide to use) by stretching
the end of the balloon over it. Make sure the balloon hangs limp off to the side. Add a little
sign that tells the students the mass of the Mg in the flask. Repeat this step for the other two
balloons and flasks.
3. You might be concerned about the air in the balloon. To remove the air, pull a small part of
the balloon away from the container and squeeze out excess air from the balloon. The
problem with this is that it will be somewhat difficult to get the Mg into the flask. (See next
step). Alternatively, leave the air in the balloon and don’t worry about it. The quantity of air
in each balloon is probably the same and will therefore be negligible when the balloons
inflate with hydrogen gas.
4. To add the Mg to the HCl, simply lift up the balloon and the Mg will fall into the flask. The
flask that has the 2.40 g of Mg will be most vigorous. It is possible that HCl may enter the
balloon at this time. Don’t be alarmed but be cautious! Use a larger container and/or smaller
quantities of Mg and HCl to prevent this from happening. Be sure to practice, practice,
practice so you know what to expect!
5. As the reactions proceed be sure to dialogue with the kids. Have them write out the reaction.
Predict the products. Make observations about the size of the balloons and whether or not Mg
is used or left over. You might even want to have someone note the warmth of the flask.
(Caution! The flasks do become quite hot!)
6.
Summarize the observations as shown below.
Summary of Demo
Blue Balloon flask
Red Balloon flask
Yellow Balloon flask
100 mL 1.0 M HCl
0.60 g Mg
100 mL 1.0 M HCl
1.20 g Mg
100 mL 1.0 M HCl
2.40 g Mg
No Mg left in flask
Balloon is ½ as big as Red
and Yellow balloons.
No Mg left in flask
Balloon is equal in size to
Yellow balloons and twice
as big as Blue balloon.
Mg left in flask.
Balloon is equal in size to
Red balloons and twice as
big as Blue balloon.
A typical student observation might be ”O.K, the Red balloon is twice as big as the Blue balloon
because you used twice as much magnesium and twice as much hydrogen should be produced. But
how can the Yellow balloon be the same size as the Red balloon? What’s going on, here?”
Challenge your students to explain the observations as shown above using their knowledge of
stoichiometry. You can totally leave it up to them or give them some direction. For example, have
them determine the moles of hydrogen gas which can be used to explain the volumes of hydrogen
gas collected. You can also have them calculate the exact amount of Mg needed for each reaction. A
really good problem is for them to determine how much Mg is left over in the 3rd flask.
Following are some factor-label calculations to give you a starting point.
Mg +
2 HCl (aq) 
MgCl2(aq) + H2 (g)
1. 100 mL of 1.0 M HCl would have 0.1 mol HCl present.
0.1 L x 1.0 mole HCl
1 L HCl
=
0.1 mol HCl
2. From the balanced equation, we can do a calculation that will show 0.1 mole HCl requires
exactly 1.2 g Mg to completely react:
0.1 mol HCl
x 1 mole Mg
2 mol HCl
x
24.3 g Mg
1 mole Mg
=
1.2 g Mg
This is the same amount of Mg in the flask with the Red balloon! Therefore, both the HCl and the
Mg in the 2nd flask are completely consumed! Cool!
3. How much H2 should be produced in the Red balloon? (Emphasize that the size of the balloon
can be tied to the number of moles of gas produced. I haven’t taught molar volume of a gas yet
to my students so I don’t bring that in to the calculations. You can do what you want, however!)
1.20 g Mg
x 1 mole Mg
24.3 g Mg
x
1 mole H2
1 mole Mg
=
0.050 mol H2 (Red balloon)
4. How much hydrogen should be produced by the Blue balloon flask? Students might be able to
reason this out but make them show the stoichiometry! This part introduces the idea of limiting
reactants.
Since there was 0.60 Mg in the blue balloon flask, the Mg will limit the amount of hydrogen
produced. Therefore, use the amount of Mg to determine the amount of H2 produced.
0.60 g Mg
x 1 mole Mg
24.3 g Mg
x
1 mole H2 =
1 mole Mg
0.025 mol H2 (Blue balloon)
This is half the amount of gas formed in the red balloon. This explains why the blue balloon is half
the size of the red balloon.
5. How much HCl would be needed to react with the 0.60 g Mg? The kids might be able to reason
this out but have them show the math!
0.60 g Mg
x 1 mole Mg
24.3 g Mg
x
2 mole HCl
1 mole Mg
x ___1 L___ = 50 mL!
1 mole HCl
That means 50 mL of HCl are left over un-reacted in the 1st flask!
6. What about the 3rd flask with the Yellow balloon? You can do similar calculations to those
shown previously.
The 3rd flask requires 200 mL of HCl to react the 2.4 g of Mg completely. This means the Mg is in
excess because there is not enough HCl to completely react the Mg! This is why left over Mg is
readily visible at the bottom of the 3rd flask. Have the students calculate the amount of Mg left over.
(If 1.2 g reacts completely with 100 mL of 1.0M HCl as shown in calculation #2, then there should
be 1.2 g left over in the 3rd flask.)
The 3rd flask will only produce 0.050 mol H2 because only 1.2 g of the Mg in the 3rd flask will
react! This is the same as the amount used in the 2nd flask. Thus, the sizes of the balloons in the 2nd
and 3rd flasks will be the same.
Tips
Another way to use the Mg is to use Mg ribbon and coil it so that it is held in the neck of the flask by
friction. Now, squeeze out the air in the balloon and attach it to the container. To add the Mg to the
HCl, lift the balloon and poke through it to push the coil of Mg into the acid.
You can download this demo from www.chemwest.org. Go to the ‘Teaching Tips’ area and click on
the link. It is in ‘Word’ format.
Reference
I first saw this demonstrated at the Summer Institute for Science and Mathematics held at Fermilab
in 1989. I saw another reference to it in a text called Chemistry: The Study of Matter. Sorry I don’t
remember the author of either.
Submitted by:
Michael Bachrodt (mbachrodt@d211.org)
William Fremd High School
1000 South Quentin Road Palatine, Il. 60102
847.755.2816