Friday, March 21, 2014

When the Results Matter!

Setting up the gas collection apparatus.
Making a prediction!
I ended the winter term with a short unit on gas laws.  The culminating event for the term was the Target Gas Law Lab (Publication 91654 by Flinn Scientific).  In this fun experiment, I give each group a piece of zinc metal.  They have to use the ideal gas law to predict how much hydrogen gas will be produced when their piece of zinc is reacted with hydrochloric acid.  I love this experiment for many reason:  the kids use a 1-liter graduated cylinder to collect the hydrogen gas (my favorite piece of glassware), they have to apply the ideal gas law to a real problem, and the results actually matter (part of their grade comes from how accurate their prediction is).  They use a paperclip on the zinc metal to suspend it above the acid with a neodymium magnet until they are confident of their prediction. They tape an arrow on the graduated cylinder with their prediction, then pull off the magnet and watch.

Here's a group running their reaction.

Waiting to see if the hydrogen will fill to the prediction mark.
What do you need to know to make an accurate prediction?  The mass of the zinc piece, the temperature of the water bath, atmospheric pressure, a value of the ideal gas constant to match your units,  and the stoichiometric relationship between zinc reacted and hydrogen produced (a nice 1:1 ratio in this reaction).  In the Flinn procedure, there is a "pressure correction factor" step in which you multiply your predicted volume by 9% to account for the vapor pressure of water in the hydrogen gas.  Most of my students used this correction factor, however one group used the direct approach and corrected for the vapor pressure of water by subtracting it off the atmospheric pressure.  Either method ended up with a "ball park" answer, but none of my groups got better then 9/10 for results.  All of the groups predicted a larger volume of gas than was actually produced.  I'm not sure what measurement was inaccurate, but my guess is that the atmospheric pressure was the trouble spot.  But it is possible the the zinc metal I used has some impurities in it, but there was no residue left in the flask at the end of the reaction.  More tinkering is needed to figure out if there was a systematic error, or just nervous students making careless mistakes..

And here's what we did with the hydrogen gas at the end of the experiment.