As chemistry teachers, we have to explain things. Complicated things, with complicated explanations. Then, these complicated explanations work their way into our students’ brains.
And sometimes things get too complicated…
In a recent laboratory activity entitled Copper to Copper,1 my Grade 11 Chemistry students enjoyed seeing copper react “full circle” using all of the reaction types covered in Grade 11: synthesis (redox, actually), double displacement, decomposition and single displacement. These reactions are carried out consecutively in the same beaker, which makes for a super-nice experience.
Photo of reaction between copper and nitric acid taken by William Barron de Burgh, a teacher at Dame Elizabeth Cadbury School, Birmingham, UK
The initial reaction in this activity is represented by
Cu(s) + HNO3 (aq) →
Cu(NO3)2(aq) + 2 NO2(g) + 2 HOH(g) (Eq. 1)
In Grade 11, I pitch this reaction as “synthesis”, rather than redox. On account of the nastiness of nitrogen dioxide gas, this reaction is demonstrated under a fume hood; students are simply given Cu(NO3)2(aq) and told to get on with the other reactions.
Students are okay with the synthesis reaction: copper is below hydrogen on the activity series; a straight-forward single displacement reaction ain’t gonna happen.
In a post-lab discussion, I asked if it would be possible to repeat this series of reactions using zinc instead of copper. That is, would a “zinc to zinc” conversion be possible, using analogous chemical reactions?
This raised the question: Since Zn is above H in the activity series, will Zn react with nitric acid in a single displacement reaction:
Zn(s) + 2 HNO3(aq) → Zn(NO3)2(aq) + H2(g) (Eq. 2)
or will zinc react with nitric acid, redox-style?
Zn(s) + HNO3(aq) →
Zn(NO3)2(aq) + 2 NO2(g) + 2 HOH(g) (Eq. 3)
I asked: “How can we know which reaction occurs?”
Best answer from a student: “Let’s try it, Sir — and see if brown NO2(g) is produced”.
So we carried out the reaction — in the fume hood, just in case.
We observed a rapid reaction with the evolution of a brown gas: NO2.
This evidence-based, empirical observation answered our question. Further, it led to (one of many) discussions:
THE TRUMP (excuse my choice of words) CARD IN SCIENCE IS EMPIRICAL EVIDENCE.
Don’t let students put chemical models or fancy explanations ahead of the evidence. There is no substitute for a properly done, properly interpreted experiment.
When your students ask questions about the polarity of this-and-that, or will the reaction of A + B be rapid or exothermic or whatever . . . your answer can always be “We’ll have to try it…”
Reference
- You really must do this lab. See, for example, Clancy et al., Chemistry 11, McGraw-Hill Ryerson, Toronto, 2011. Or look on the internet.
