Because of this, many of them remembered that they could identify the presence of CO 2 and O 2 based on what happened when a lit splint was placed into the test tube. 3) Application of qualitative evidenceĭuring our reactions unit, they had learned about testing for certain gases using a flame test. The use of stoichiometry to generate sufficient evidence that will support their eventual conclusion will be the meat of their argument. Though some of the products can be easily determined qualitatively, stoichiometry will need to be applied when trying to identify the solid product that remains. Since we were nearing the end of our stoichiometry unit, this was a perfect application. Even though I could have given them a brief explanation as to why something like this would not decompose in this manner, I did not need to since it is not even offered as a potential equation-away with bicarbonate! 2) Application of stoichiometry As absurd as this seems to you and me, it seems plausible to many of them. You know their gut instinct will be to suggest it decomposes into sodium and bicarbonate.
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As novices, they have no idea how to confidently predict the products of such a reaction.
To the students, this reactant is a complete curveball. We had just finished our reactions unit, so they were all familiar with the generalized pattern that a decomposition reaction follows. 1) Their lack of prior knowledge makes all four options appear plausible. At the same time, their lack of knowledge prevents them from confidently knowing the correct reaction prior to investigation.Īfter thinking about it a bit, there were at least five distinct features that convinced me to pursue this lab. They know just enough to successfully accomplish their task, even if they do not immediately start making connections to content already learned. While they might be lacking in these areas, they are not completely clueless. They lack the content knowledge and they most certainly lack the laboratory skills to easily generate a plan for arriving at an evidence-based answer. Even if we did need to do the investigation ourselves to determine the right equation, our experience in the lab and overall scientific literacy allows us to easily come up with a plan and identify exactly what we should be looking for. As chemistry teachers, the depth of our content knowledge allows us to systematically rule out three of the reactions without even performing the experiment. Their task: Figure out which balanced chemical equation accurately represents the decomposition of sodium bicarbonate.Īt first glance, the ingenuity of this challenge was not completely obvious to me. Option 4: NaHCO 3 (s) à NaH (s) + CO (g) + O 2 (g) Option 2: 2NaHCO 3 à Na 2CO 3 (s) + CO 2 (g) + H 2O (g) Option 1: NaHCO 3 (s) à NaOH (s) + CO 2 (g) The students are provided with four different balanced chemical equations that could explain how the atoms are rearranged during this decomposition. Though I had used a version of the decomposition of sodium bicarbonate lab in our stoichiometry unit for years, with consistent results, what the ADI book provided was a surprisingly different and more creative approach. Which Balanced Chemical Equation Best Represents the Thermal Decomposition of Sodium Bicarbonate? 1 To achieve this, I opened my Argument-Driven Inquiry in Chemistry (ADI) book and happened to find a wonderful example. Many of the stoichiometry labs I had done in the past followed more of a traditional structure involving something like, “ here is the reaction…predict how much…do the reaction…compare to prediction…determine % yield.” While merit for such a lab can be argued for, I really wanted to immerse my students in an actual investigation that more accurately reflected the scientific skills I try to advocate for- experimental design, data collection, analysis, creating an argument from evidence, engaging in argument, etc. During our stoichiometry unit, I wanted my students to take part in an engaging investigation.