Are you the Limiting Reactant?

The Limiting Reactant Demo

No one wants to think of himself as the limiting reactant, but is it any better to be considered the excess? 

Before the reactions started.

This week I pulled out the baking soda and vinegar again for a great visual demonstration of the limiting reactant concept.  In this scenario, I measured out a varying amount of baking soda (6 g, 12g, 18 g, 24 g, 30 g, and 36 g) to pour into six balloons.  The balloons were attached to 1 liter plastic bottles, each containing 250 mL of vinegar.  I actually used a 1 molar solution of acetic acid because I was out of vinegar.  This substitution turned out to be a bonus because we could very easily calculate the moles of acetic acid in each bottle without getting bogged down in the molarity calculation.

How much more gas will be produced?

I asked for six volunteers, one for each balloon.  The kids flipped up their balloons, starting the famous fizzing reaction that produces carbon dioxide gas.  Each balloon inflated quickly.  Everyone wanted to have the balloon that exploded, but to everyone’s disappointment, they all stopped reacting before popping.  However, they noticed that the size of the balloons was different for some and the same for others.

Everyone is concentrating on their reaction.

The first three balloons each inflated more, which makes sense because of the increasing amount of baking soda.  But the second set of three balloons appeared to be roughly the same size.  How could that be, they each contained increasing amounts of baking soda too.  Ah Ha!  The limiting reactant switched from the baking soda to the vinegar somewhere between the pink balloon (18 g of baking soda) and the red balloon (24 g of baking soda).  If you look carefully, you can see a white film at the bottom of the jars in the three big balloons.  That’s the left over baking soda that did not react. 

You can see the three on the left get bigger, but the there three on the right are the same size.

I couldn’t get enough of the balloons, as you can tell by the many photos.  I love this demo because it gives the students a visual to refer back to throughout the stoichiometry unit.   So the bottom line is that sometimes you’re the limiting reactant and sometimes you’re the excess, it all depends on who else shows up.