Stoichiometry - How the students do it.

"What are we doing to help kids achieve?"

Stoichiometry is one of the most challenging topics to learn and teach. A quick search on this site for "stoichiometry" provides a long list of resources. I believe this is for two reasons. As I already stated, it seems to be one of the more challenging concepts to teach and learn. Second, teachers try to respond to students concerns and struggles by differentiating instruction. This differentiating leads to more methods and ideas about a particular topic.

Recently, I have attempted BCA tables with my students, computer simulations like the pHet simulation "Reactants, Products and Leftovers",  demonstrations like one that Tom Kuntzleman wrote about, POGIL stoichiometry activities, stoichiometry activities involving food and even some more traditional dimensional analysis activities. As a wrap up, we did the Argument Driven Inquiry (ADI) lab that Ben Meacham wrote about in his post titled, Using Evidence to Determine the Correct Chemical Equation: A Stoichiometry Investigation. This latter activity involves four balanced chemical reactions for the decomposition of baking soda. Students have to experimentally determine which is the correct reaction.

1. NaHCO_3(s) yields NaOH(s) + CO_2(g)

2. 2NaHCO_3(s) yields Na₂CO_3(s) + CO_2(g) + H₂O_(g)

3. 2NaHCO_3(s) yields Na₂O_(s) + 2CO_2(g) + H₂O_(g)

4. NaHCO_3(s) yields NaH(s) + CO_2(g) + O_2(g)

Most students first checked for the products of the reactions. They used burning wood splints to help identifiy the gas/gases produced. They saw the splint go out and water condense on the test tube. These observations helped narrow the choice down to two reactions. In theory, here is where stoichiometry could come into play. If students decided to examine the mass of the reactant to the mass of the solid product, they could convert to moles and check which molar ratio fit, equation one or two. 95% of the students got the correct answer by doing a stoichiometric calculation that is not mentioned above. The vast majority of students found the correct answer by making mass to mass comparisons and using what they called percent composition. Let me explain....

The correct reaction is 2NaHCO_3(s) yields Na₂CO_3(s) + CO_2(g) + H₂O_(g).Here is a summary of what most students did. First they compared the amount of reactants to products based on the reaction.

1 Na₂CO_3(s) /2NaHCO_3(s)

Next, they converted moles from the balanced equation to grams. This is something we virtually NEVER did in class. My mantra has always been the exact opposite, convert grams to moles. But why listen to the teacher...?

1 Na₂CO_3(s)/2NaHCO_3(s) 105.99g/168 g

This looks much like percentages (which is exactly what students call it). Think "part over the whole". This equates to about .63. Students then compared the experimental mass before and after. Students reasoned that if it was about .63 then it must be the right reaction...and they were correct.

I was left scratching my head. Yes, it is correct. Yes, it works. Percent composition was a topic we did for about two days almost a month and a half prior to this. Despite my best efforts, it appears that somehow students are learning....but in ways that I would not have imagined or planned. I know that as a blogger I am supposed to have words of wisdom, but I am stumped. Do I start teaching my students to solve stoichiometry problems by comparing gram to gram amounts? Any thoughts would be greatly appreciated.....