TOPIC: Stoichiometry

Demo 040

Limiting Reactants - Mg & HCl

In this demonstration, pre-weighed amounts of magnesium (0.6g, 1.2g, and 2.4g) will be added to Erlenmeyer flasks containing 100ml of 1M HCl.  The necks of the flask are fitted with balloons.  The resulting hydrogen will fill the balloons to demonstrate each reactant as limiting.

MATERIALS

300ml 1.00M Hydrochloric acid
Magnesium metal ribbon
    in 0.6g, 1.2g, and 2.4g portions
3 125ml Erlenmeyer flasks
    labeled with the mass of each Mg portion
3 Balloons

PRESENTATION

There will be exactly 100 milliliters of 1.00M hydrochloric acid in each of the Erlenmeyer flasks.  In the neck of each flask, there will be a coil of magnesium ribbon held in the neck above the solution by the spring action of the coil.  The flask will be labeled with the masses of the metal.  A balloon will be fitted to each of the Erlenmeyer flasks.

Write the reaction on the board.  It is Mg + 2HCl --> MgCl2 + H2.  
Push the 1.2g coil of magnesium into the acid.  Observe the inflation of the balloon with hydrogen gas.  Next, ask the class to predict the size of the balloon produced when you push the 0.6g coil into the acid of the second flask.  Observe the inflation of the balloon to the predicted size, half that of the first flask.  Repeat with the 2.4g coil of magnesium.  The class will be surprised to see that the balloon does not inflate to twice the size of the 1.2g reaction, and that there is excess magnesium present in the third flask.

DISCUSSION

The reaction demonstrated here is:

Mg(s) + 2HCl(aq) --> MgCl2(aq)+ H2(g)

In the first flask, 1.2g of magnesium reacts with the HCl.  The amounts are stoichiometrically proportional.  Both the magnesium and the hydrochloric acid are used up.  In the second flask, 0.6g of magnesium reacts with the same amount of HCl.  The amount of hydrogen produced is half that of the first flask, as evidenced by the size of the balloon.  All of the magnesium has reacted, leaving excess acid.  In the third flask, 2.4g of magnesium reacts with the same amount of HCl.  To the surprise of the audience, the balloon does not inflate to twice the size of the flask with 1.2g of magnesium.  It is, in fact, the same size.  This is because all of the HCl is used up, as in the first flask.  This means only 1.2g of magnesium can react and, therefore, the same amount of hydrogen is present as in the first reaction.  This is evidenced by the presence of excess magnesium in the bottom of the flask.

    Mg     + 2 HCl  --> MgCl2 +  H2
Flask 1, 1.2g Initial Amt 
Amt Reacted
0.049mol
0.049mol
0.100mol
0.100mol
0.049mol 0.049mol
Flask 2, 0.6g Initial Amt
Amt Reacted
0.025mol
0.025mol
0.100mol
0.050mol
0.025mol 0.025mol
Flask 3, 2.4g Initial Amt
Amt Reacted
0.100mol
0.049mol
0.100mol
0.100mol
0.049mol 0.049mol

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