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. |
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.
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.
Thanks, used this with my students this week.
ReplyDeleteHow did it go? I'm doing it tomorrow. This year I have introduced molarity just before doing this demo so the can run the calculations with the solid and the solution in groups. I'm hoping that this will be a good lead into titrations.
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