Today’s lab, Simple Stoichiometry from Ian Guch’s 24 Lessons that Rocked the World, was fairly straight forward after the complexity of last weeks experiment. Stoichiometry just means that we are going to calculate the amount of product we expect to produce in a chemical reaction. For example, if you want to make 2 dozen cookies and your recipe requires 2 eggs to make 1 dozen cookies you would need 4 eggs total. But if you only have 3 eggs, then you can only make the recipe once (making 1 dozen cookies), or 1.5 times, making 18 cookies (1.5 dozen), so eggs are the limiting reagent. In this lab we start with 5.0 grams of acetic acid and 3.3 grams of sodium hydroxide, calculate how much sodium acetate we are going to produce and then measure how much we actually produce. Before we did the experiment we discussed why our measured numbers might not be equal to our prediction and one student said matter of factly, ‘Science.’ Yep, science happens, stuff splatters, and we loose some mass.
We had a bit of a back log waiting for the triple beam balance since we only have one and each group needed to measure the mass of an empty beaker, the acetic acid and the sodium hydroxide. After dissolving the sodium hydroxide pellets in the acetic acid (plus 25ml of distilled water), students placed their beakers over a butane burner or alcohol lamp to boil off the water leaving behind sodium acetate. Once the beaker cooled, they measured the mass of the beaker + sodium acetate and figured out how much they produced. All the groups ended up with more ‘sodium acetate’ then they expected and the most likely reason is that we stopped heating too soon and there was still water left in the beaker. To test that we put one beaker back on the heat for a few minutes and boiled off another gram of water.
This was a pretty straight forward lab and even though we didn’t get the best results, it was easy to see why. The chemicals involved are a bit nasty so gloves and goggles were worn while measuring and mixing, but gloves were removed around heat sources (don’t want gloves melting on your hands). Another nice thing about this lab was that the reaction is exothermic and while stirring to dissolve the sodium hydroxide pellets all the students exclaimed, ‘Its getting Hot!’ And it really does get hot, not just warm.
I asked the students to watch these videos before class: