Percent Yield, Movie Times, and the “Science Unseen”

It’s that time of year again: labs are gearing up. Drawers are being filled with new glassware, students donning lab coats are beginning to fill the halls…the ol’ machine is revving up to roll again. For me, this time of the semester means emphasizing good practices when working up data and results. I’ve written in past semesters about significant figures and some of the interesting issues that come up when teaching them—it’s about the mindset, not the rules, I swear…!

Take percent yield, a measure that has been reported with false precision by countless numbers of students across the generations. Percent yield is really interesting because the balance is one of the most precise instruments that exist in general chemistry laboratories—depending on the range and precision of the balance, measurements with five, six, and even seven significant figures are possible. Thus, it seems like percent yields (which are really just ratios of masses) should in turn have five, six, or seven significant digits. The measured mass of product points to this level of precision.

Students often struggle to understand that the precision of the balance is irrelevant—inevitable variations between runs of the reaction introduce massive uncertainty into yields. Such variations are gargantuan compared to imprecision in balance measurements and essentially render the precision of the balance meaningless. Continue reading →

The Odd Oxygen

There is a weird smattering of organic oxides whose molecules contain a foreign oxygen latched on to an otherwise familiar framework. I’ve written about DMSO before, which is essentially dimethyl sulfide with an extra oxygen atom along for the ride. N-Oxides fit into this group of compounds as well.

The structure of nitrous oxide, also known as laughing gas.

Nitrous oxide (also known as laughing gas).

Perhaps no molecule better typifies the class than nitrous oxide, N2O. Even the molecular formula sets off neural fireworks: that can’t be right. A central nitrogen flanked by N and O? Something’s wrong here. The central nitrogen seems overworked, while the oxygen seems to be missing a bond. Despite its bizarre structure, nitrous is surprisingly unreactive—I learned this recently while helping out a teacher friend with one of his students’ science fair projects. Thanks to its lack of reactivity at sane temperatures, detecting N2O is a pain.

Synthesizing nitrous, on the other hand, is quite easy. Upon heating, ammonium nitrate breaks down into N2O and two water molecules. The melting point of NH4NO3 is downright eye popping for an ionic compound: 170 ºC. Some rather unsafe methods actually produce nitrous from molten ammonium nitrate at high temperatures.

NH4NO3(l) → N2O(g) + 2 H2O(g)

One must be careful as the dissociation of ammonium nitrate into gaseous nitric acid and ammonia competes with N2O formation (“decomposition”).

NH4NO3(l) → HNO3(g) + NH3(g)

Dissociation is endothermic and decomposition exothermic, so heating can set up an interesting situation where the dissociation reaction can “quench” the heat released by decomposition. When dissociation is suppressed, the decomposition reaction can become a runaway exotherm. Although this document on the safe production of nitrous oxide from ammonium nitrate is written for chemical engineers—they even go to the trouble of writing out “gram-mole”!—it’s an illuminating read with some additional information about the synthesis of nitrous oxide. Continue reading →

Prescription versus Exploration: The Case of the Chess Master Hobo

My predecessor left a ton of books behind in my office, and among some of his old stuff I found a wonderful book called The Pupil as Scientist. The author, Rosalind Driver, makes the case that students develop scientific explanations of what they see long before they set foot into a science classroom. Children’s natural scientific curiosity is both an asset and a liability: teachers can tap into it to engage students in deeper scientific learning, but it can be a source of robust misconceptions, too.

Driver describes beautifully a tension I’ve observed teaching college freshmen laboratories. On the one hand, we want students to observe and discover scientific principles in the laboratory, and leaving procedures open ended is part of that goal. On the other hand, the impact of an experiment seems greatest when it’s done properly, according to a prescribed procedure that yields “good results.” Good data is typically a pre-requisite for grappling with complex scientific concepts, but inquiry-based labs open the door to bad data or incorrect conclusions. How can we properly balance these two opposing forces?

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Kinetics and Statistics Collide

It may not be a stretch to say that the study of reaction kinetics has claimed more hours of chemistry graduate student labor than any other enterprise. Waiting for a reaction to go to “completion” could require hours or even days, and one must keep a watchful eye on the data collection apparatus to avoid wasted runs. There’s a good chance that guy who’s reserved the NMR all night long is battening down for a kinetics run.

All of that effort, of course, leads to supposedly valuable data. The party line in introductory chemistry courses is that under pseudo-first order conditions, one can determine the order of a reactant in the rate law just by watching its concentration over time. We merely need to fit the data to each kinetic “scheme” (zero-, first-, and second-order kinetics) and see which fit looks best to ascertain the order. What could be simpler? The typical method—carried out by thousands (dare I say millions?) of chemistry students over the years—involves attempting to linearize the data by plotting [A] versus t, ln [A] versus t, and 1/[A] versus t. The transformation that leads to the largest R2 value is declared the winner, and the rate constant and order of A are pulled directly from the “winning” equation.

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